The author investigates in a nonperturbative way the effects of Rashba interaction and electromagnetic field on the edge states of a two-dimensional topological insulator. He shows that the electron dynamics is equivalent to a problem of massless Dirac fermions propagating with an inhomogeneous velocity, unveiling interesting consequences. The Rashba coupling always enhances the electron velocity and, despite the inelastic and time-reversal breaking processes induced by the electromagnetic field, no backscattering occurs without interaction. Furthermore, when the photoexcitation occurs far from the Rashba region, the latter effectively acts as a “superluminal gate”, boosting the photoexcited wavepacket outside the light-cone determined by the bare Fermi velocity ${v}_{F}$. In contrast, for an electric pulse overlapping the Rashba region, the emerging wave packets are squeezed in a manner that depends on the overlap area. The author also discusses the effects of electron-electron interaction, for both intraspin and interspin density-density coupling. The results suggest that Rashba interaction, often considered as an unwanted disorder effect, may be exploited to tailor the shape and the propagation time of photoexcited spin-polarized wave packets.

[Phys. Rev. B 95, 085434] Published Fri Feb 24, 2017

]]>The authors present a method for characterizing the hyperuniformity (the suppression of density fluctuations at long wavelengths) for quasicrystals (and other structures whose diffraction pattern includes a dense set of Bragg peaks) based on determining the behavior of the integrated spectral intensity $Z$($k$) for small wavenumber $k$. Surprisingly, we find that quasicrystals with peaks at the same wavenumbers $k$ can have different behavior for $Z$($k$) and, hence, qualitatively different degrees of hyperuniformity. This effect has never been explored in the laboratory. The figure shows a one-dimensional example: two “sidewalks” with different widths (pitched at an incommensurate angle) built on a lawn with a square crystal pattern of spots. The construction rule is that, every time the sidewalk crosses a spot, the sidewalk slab is cut and a new one begins. This rule guarantees both sidewalks are quasicrystalline. But the upper one (a Fibonacci sidewalk) has a different degree of hyperuniformity than the lower one.

[Phys. Rev. B 95, 054119] Published Thu Feb 23, 2017

]]>The time evolution of a quantum system after a sudden change of one of its parameters may show characteristic nonanalyticities, provided that this quench crosses a quantum phase transition. The authors investigate the relation between such dynamical quantum phase transitions and the universal properties of the corresponding quantum critical point.

[Phys. Rev. B 95, 075143] Published Thu Feb 23, 2017

]]>It has been argued that the itinerant magnetism in the simplest of all iron arsenide systems, the monoarsenide FeAs, may be related to the magnetic ground states of iron-based superconductors. Here, the authors provide new insight into the magnetism of FeAs through a combination of resonant x-ray scattering measurements and electronic structure calculations. They report measurements of the magnetic anisotropy of the spin helices by using polarized x-rays tuned to the resonant absorption edge of iron. They elucidate details of the magnetic structure, showing that the ratio of ellipticity of the spin helix is significantly larger than previously thought, and discover both a right-handed chirality and an out-of-plane component of the magnetic moments in the spin helix. The electronic structure calculations demonstrate how the origin of this spin canting effect may be accounted for by considering the spin-orbit coupling at work in the system.

[Phys. Rev. B 95, 064424] Published Wed Feb 22, 2017

]]>Understanding the spread of quantum entanglement and scrambling of information across a quantum many-body system is a fundamental problem in quantum dynamics. A necessary part of the spread of the entanglement is the growth in time of Heisenberg operators of initially localized operators. This operator growth can be diagnosed by studying the space-time structure of commutators of well-separated operators. Previous studies in this area focused on translation invariant models, but the effects of disorder, which can radically alter the motion of heat and charge, had not been studied. Here, the authors study the growth of operators in interacting systems with quenched disorder. In the localized phase, they find that operator sizes grow logarithmically in time similar to other measures of entanglement spreading. In the disordered metal, they find that operator sizes grow linearly with time, i.e., ballistically, in contrast to the diffusive motion of charge and heat. The ballistic growth of operators is quantified by the butterfly velocity which the authors relate to the charge diffusion constant and the interaction-induced inelastic scattering rate. When the diffusion of charge is slow, the resulting butterfly velocity is much smaller than the maximum speed allowed by microscopic causality constraints.

[Phys. Rev. B 95, 060201(R)] Published Tue Feb 21, 2017

]]>Recently, there have been extensive efforts to extend the physics of the two-dimensional (2D) graphene to three-dimensional (3D) semimetals with point/line nodes. Although it has been known that certain crystalline symmetries play an important role in protecting band degeneracy, a general recipe for stabilizing the degeneracy, especially in the presence of spin-orbit coupling, is still lacking. Here, the authors show that a class of novel topological semimetals with point/line nodes can emerge in the presence of an off-centered rotation/mirror symmetry whose symmetry line/plane is displaced from the center of other symmorphic symmetries in nonsymmorphic crystals. Due to the partial translation perpendicular to the rotation axis/mirror plane, an off-centered rotation/mirror symmetry always forces two energy bands to stick together and form a doublet pair in the relevant invariant line/plane in momentum space. Such a doublet pair provides a basic building block for emerging topological semimetals with point/line nodes in systems with strong spin-orbit coupling. When an external magnetic field is applied to these semimetals, a Dirac-type point/line node with four-fold degeneracy splits into two Weyl-type point/line nodes with two-fold degeneracy, with emergent surface states connecting the split nodes.

[Phys. Rev. B 95, 075135] Published Tue Feb 21, 2017

]]>In recent years, physicists have made great strides toward better understanding isolated many-body quantum systems. Already, two distinct phases are well known: systems that are ergodic and thermalize completely, and, by contrast, systems that exhibit many-body localization (MBL) due to a strong disorder potential, which prevents thermalization. It is tempting to wonder whether a phase of matter could exist between the extremes of full thermalization and MBL in a generic, i.e., nonintegrable, isolated many-body quantum system. Here, the authors provide evidence that such a nonergodic phase can be realized in a translationally invariant itinerant electron model, with both charge and spin degrees of freedom. Exact diagonalization calculations on finite system sizes reveal that there exists a “spin band” of nonthermal eigenstates. These states have entanglement entropy that scales as an overall “volume law,” but after a partial measurement of the spin on each site the scaling is modified to an “area law,” thus suggesting the states are not ergodic. These properties and others suggest that this model realizes a new nonthermal phase of matter, known as a quantum disentangled liquid (QDL). The putative existence of this phase has striking implications for the foundations of quantum statistical mechanics.

[Phys. Rev. B 95, 054204] Published Fri Feb 17, 2017

]]>Laser beams carrying orbital angular momentum (OAM), first proposed by Allen et al. in 1992, is one of hottest research subjects in modern optics. Such a beam, called an optical vortex, is used to realize superresolution microscopes, chiral optical ablation, optical tweezers, and so on. More recently, electron beams with nonzero OAM are also proposed and experimentally realized. In spite of the growing attention towards such “vortex beams”, their applications for condensed matter physics are almost untapped so far, especially for controlling microscopic electronic and magnetic degrees of freedom in solids. Here, the authors show theoretically that vortex beams can be useful for controlling magnetic properties of solids. In particular, they focus on chiral ferromagnets and antiferromagnets where topologically stable magnetic defects (skyrmions) appear. On the basis of numerical simulations using the stochastic Landau-Lifshitz-Gilbert equation, they show that via the laser-beam-induced spatially nonuniform temperature changes, the characteristic donut-shaped spatial profile of vortex beams can be faithfully transferred to magnets as a family of magnetic defects (n$\pi $ vortices), which include skyrmions and skyrmioniums.

[Phys. Rev. B 95, 054421] Published Thu Feb 16, 2017

]]>The rare-earth nickelate perovskites ($R$NiO${}_{3}$, $R$ = rare-earth element) are an archetypal class of correlated electron materials. They order antiferromagnetically at a composition-dependent “Néel” temperature. While it is known that the magnetic structure has a period of four Ni spins along each (pseudo)cubic crystal axis, the arrangement of these spins is still under debate, especially in view of the conjecture of ferroelectricity in the antiferromagnetic phase. Here, the authors report the experimental discovery of an unexpected second magnetic phase transition in TlNiO${}_{3}$, a material closely resembling the analogous rare-earth compounds. Results of nuclear magnetic resonance and muon spin rotation experiments reveal the presence of two distinct magnetic phase transitions. The new phase is suppressed by magnetic fields on the order of at most 1 T.

[Phys. Rev. B 95, 060411(R)] Published Thu Feb 16, 2017

]]>Helium nanodroplets are considered ideal model systems to explore quantum hydrodynamics in self-contained, isolated superfluids. Rotation of a superfluid droplet is manifested as a collection of quantum vortices. Here, the authors use single-shot femtosecond X-ray coherent diffractive imaging to investigate the shapes and vorticity of rotating superfluid ${}^{4}$He droplets. They find that most of the diffraction contours exhibit noticeable ellipticity. A smaller but still significant number of distorted diffraction contours are marked by regions of high intensity concentrated along the direction of the long axis in the diffraction image, indicating the presence of extremely deformed ${}^{4}$He droplets. Forward modeling of the diffraction images shows that the shapes of superfluid ${}^{4}$He droplets are very similar to their classical counterparts. In particular, the existence of both axially symmetric (oblate) and, for higher angular momenta, two-lobed (triaxial prolate) droplets is revealed. The latter shapes are strongly deformed compared to the spheroidal bodies that are expected for smaller rotational speeds. The observed shapes provide direct access to the droplet angular momenta, angular velocities, and estimated number of quantum vortices inside the droplets.

[Phys. Rev. B 95, 064510] Published Thu Feb 16, 2017

]]>The simultaneous presence of sizable spin-orbit interactions and electron correlations in iridates has led to predictions of novel electronic ground states, including Dirac semimetals, Kitaev spin liquids, and superconductivity. Electron and hole-doping studies of Sr${}_{2}$IrO${}_{4}$ are one of the venues being pursued to attain such novel quantum states, driven in part by the extensive parallels with the La${}_{2}$CuO${}_{4}$ parent compound of high-${T}_{c}$ cuprates. In particular, Rh doping has been intensively pursued but the mechanism of electronic doping and its impact on the evolution of magnetic and transport properties has been under scrutiny. Using x-ray absorption spectroscopy at the Rh $K$, $L$, and Ir $L$ edges, the authors reveal unusual charge partitioning between Rh and Ir sites. The resultant anomalous doping introduces holes into the ${J}_{\text{eff}}$=1/2 band at low Rh content only for these holes to be removed at higher Rh content. This anomalous doping provides a natural explanation for the reentrant insulating phase in the phase diagram of Rh-doped Sr${}_{2}$IrO${}_{4}$ and should be taken into account when searching for novel electronic phases in 5$d$ iridates doped with 4$d$ or 3$d$ elements where anomalous charge partitioning is expected.

[Phys. Rev. B 95, 060407(R)] Published Wed Feb 15, 2017

]]>Solitons are self-preserving traveling waves of great interest in nonlinear physics, offering many interesting application such as high-bandwidth optical fiber communication. Solitons can also appear in ultrafast acoustics, and several observations suggest acoustic solitons as short as 200 fs. Here, the authors designed an experimental setup to observe and characterize acoustic solitons traveling through a GaAs(001) substrate. The experimental signal is explained well with the help of the Korteweg–de Vries equation, demonstrating the particlelike nature and the unique properties of the solitons. Moreover, the temporal distribution of the solitons is also analyzed with the help of the inverse scattering method. Such investigations provide a new tool to probe transient properties of highly excited matter through the study of the solitons due to the acoustic pulse emitted after laser excitation.

[Phys. Rev. B 95, 064306] Published Tue Feb 14, 2017

]]>The spin Hall effect is of great technological interest because it can produce efficient spin-transfer torques with the potential to advance magnetic memory technologies. Here, the authors demonstrate that the spin Hall effect can be enhanced by the presence of $f$-orbital states near the Fermi level. The authors compare measurements of spin Hall torques generated by the rare-earth metals Gd, Dy, Ho, and Lu with the results of first-principles calculations of intrinsic spin Hall conductivity with and without $f$-orbital participation. They find that contributions from the $f$-orbital states are required to explain the experimental trend in spin Hall conductivity as a function of $f$-orbital filling. The observed trend is consistent in both sign and magnitude with a simple Hund’s rules picture of $f$-orbital filling, similar to the trend observed in the 4$d$ and 5$d$ transition metals as a function of $d$-orbital filling. These results suggest that rare earths are a promising research direction for creating and tailoring materials with large spin-orbit torques.

[Phys. Rev. B 95, 064412] Published Tue Feb 14, 2017

]]>The investigation of topological transitions in quantum systems is a thriving area of modern research. The authors recently predicted that multiterminal Josephson junctions realize a novel type of topological matter. Weyl singularities may appear in the Andreev bound-state spectrum of junctions with four superconducting terminals, giving rise to topological transitions as the superconducting phases are tuned. These transitions manifest themselves in a quantization of the transconductance between two voltage-biased terminals in fundamental units of 4${e}^{2}$2/$h$, where $e$ is the electric charge and $h$ is the Planck constant. The present work addresses the observability of this effect. The quantized transconductance is associated with adiabatic transport at fixed occupations of the Andreev states. On the other hand, a bias voltage leads to multiple Andreev reflections, where a quasiparticle is dissipatively transferred from the occupied states below the superconducting gap to the empty states above the superconducting gap. By computing the currents in the presence of weak inelastic relaxation, the authors establish the voltage threshold below which the equilibrium occupations of the Andreev states are restored, and the transconductances reach their quantized values. The results are an important step towards experimental verification of the topological properties of multiterminal Josephson junctions.

[Phys. Rev. B 95, 075417] Published Tue Feb 14, 2017

]]>The out-of-time-ordered (OTO) correlation is unconventional. It is a key quantity for quantifying quantum chaoticity and has been recently used in the investigation of quantum holography. On the other hand, it is difficult to reveal and understand the special features of many-body localization (MBL) by conventional quantities and methods. Here, the authors demonstrate that OTO correlation can provide a pertinent and comprehensive description for MBL. It readily reveals the special logarithmic dynamics of MBL, distinguishes MBL sharply from Anderson localization as well as ergodic states, and allows accurate extraction of localization lengths for MBL. The divergence of the localization length at a finite interaction predicts an ergodic-MBL transition. Furthermore, the infinite-time “thermal” fluctuation of the OTO correlation is zero (finite) in the ergodic (MBL) phase and thus can be considered as an order parameter for the ergodic-MBL transition, through which the transition can be identified and characterized. Specifically, the critical point and related critical exponents can be calculated.

[Phys. Rev. B 95, 054201] Published Fri Feb 10, 2017

]]>Many semiconductor nanostructures are made from planar two-dimensional heterojunction systems. With a view to merging semiconductor physics, spintronics, and quantum computing, strongly spin-orbit coupled systems such as semiconductor holes have generated burgeoning research interest. While recent experiments in various low-dimensional hole systems have shown great promise in achieving electrical spin control, a thorough understanding of spin-orbit interactions in holes is lacking. Owing to the intricate interactions between holes and their environment, calculations on two-dimensional holes in semiconductor heterojunctions have hitherto always been numerical and material specific, and thus are not easily generalizable to other material systems. In this work, the authors exploit the variational method, k.p perturbation theory, and the theory of invariants to derive general, semianalytical expressions for the spin-orbit interaction in semiconductor heterojunctions. In particular, the authors systematically evaluate the dependence of the Rashba and Dresselhaus spin-orbit interactions on experimental parameters such as material, hole density, and background dopant type. The authors show that the semianalytical approach is easily generalizable to various materials, and present results for common semiconductors GaAs, Ge, InSb, InAs, and Si. Furthermore, the semianalytical results yield good agreement with existing numerical as well as experimental findings.

[Phys. Rev. B 95, 075305] Published Fri Feb 10, 2017

]]>The dynamics in the vicinity of quantum phase transitions entails a variety of important and universal phenomena, such as critical slowing down or the Kibble-Zurek mechanism, which are associated with the low-energy and long-time properties at criticality. Here, the author reveals that the opposite regime of transient nonequilibrium time evolution at criticality can similarly host universal temporal behavior. By strongly perturbing a quantum critical state through quenching the associated order parameter, the system experiences a dynamical quantum phase transition with physical quantities becoming nonanalytic at critical times. These dynamical quantum phase transitions are caused by superextensive energy fluctuations induced by the particular protocol, which turn out to have profound consequences for quantum speed limits and potential restricted thermalization despite the nonintegrability of the system.

[Phys. Rev. B 95, 060504(R)] Published Thu Feb 09, 2017

]]>In the conventional theory of superconductivity the critical temperature Tc is determined by the electron-phonon coupling constant and the phonon cut-off frequency. The hallmark experiments of McMillan and Rowell demonstrated that bosons (phonons) responsible for pairing can be observed through the frequency dependence of the gap parameter. Determination of the electron-boson coupling strength in high-${T}_{c}$ cuprates is, however, not an easy task. One of the promising ways is to measure the energy relaxation rate of photoexcited carriers by using femtosecond real-time techniques. Here, considering the electron relaxation process within the conduction band, it is commonly assumed that the underlying Eliashberg electron-boson coupling function is independent of electron excess energy. Conversely, studies of light-induced suppression of superconductivity in Pr${}_{1.85}$Ce${}_{0.15}$CuO${}_{4}$ reported here imply a strong variation of the electron-boson coupling function on electron energy. Considering the competing scenarios of superconductivity being mediated by either phonons or magnetic excitations, the results suggest that high-energy electrons strongly couple either to phonons or magnetic modes, while the situation is reversed when considering pairing of low-energy electrons.

[Phys. Rev. B 95, 085106] Published Fri Feb 03, 2017

]]>Topological crystalline insulators are insulating in the bulk, but exhibit conducting surface states protected by crystal symmetries. Here, the authors show that the surface states of crystalline topological insulators come in two different varieties: (i) as standard Dirac cones with pointlike Fermi surfaces (type-I) and (ii) as tilted Dirac cones that appear at the contact of electron and hole pockets. They call these new tilted surface states “type-II Dirac states” in analogy to the three-dimensional type-II Weyl points that have been recently discovered in WTe${}_{2}$. These type-II Dirac states can exist only at the surface of topological crystalline insulators, but are absent in ordinary topological insulators, where they are forbidden by symmetry. The two types of Dirac surface states have very different physical properties, in particular with regards to their thermodynamics and magnetotransport. The authors predict that the antiperovskites ${A}_{3}\phantom{\rule{0}{0ex}}E$O are an example of a crystalline topological insulator that hosts the type-II Dirac surface states.

[Phys. Rev. B 95, 035151] Published Mon Jan 30, 2017

]]>Light waves propagating in optical fibers or other guides are usually vulnerable to defects or channel deformations, such as twisting and bending. These perturbations invariably generate undesired reflections and scattering and typically imply a power penalty. Here, building on an analogy with electronics and with the spin Hall effect, the author unveils a general theoretical solution for this dilemma. It is shown that there is a wide class of three-dimensional metamaterial platforms protected by a particular symmetry – a combination of a geometrical operation with other more subtle symmetries of the materials response – that guarantees bidirectional transport of light totally free of reflections, independent of the specific geometry of the propagation channel and other imperfections. Crucially, this scattering anomaly only requires the symmetry protection in the frequency range of interest, and hence relies on much weaker assumptions than topological theories.

[Phys. Rev. B 95, 035153] Published Mon Jan 30, 2017

]]>Tamm resonators are an alternate type of cavities, in which the light-matter interaction may be significantly enhanced, as required for many applications in photonics or optoelectronics. A Tamm resonator comprises at least one metal layer as mirror, in the present case combined with a distributed Bragg reflector. The metal layer may turn out to be useful, for example, for injecting carriers or applying an electric field. Here, the authors use photon echoes in the exciton four-wave mixing signal to demonstrate that the studied Tamm resonator, which contains quantum dots as the optically active medium, allows one to perform coherent manipulations on quantum-dot excitons at power levels more than one order of magnitude lower than those required for bare quantum dots. This occurs despite the moderate quality factor of about 100. Moreover, it speeds up the radiative decay of the excitons by a factor of two (the so-called “Purcell effect”). On the other hand, the coherence of the quantum dot exciton, which may become compromised by plasmon excitations in the metal, is hardly shortened. These results demonstrate that Tamm resonators are indeed prospective candidates for performing coherent optical manipulations.

[Phys. Rev. B 95, 035312] Published Mon Jan 30, 2017

]]>Here, the authors extend the study of semimetallic WTe${}_{2}$ to the 2D limit, utilizing new fabrication schemes that preserve sample quality to create high-mobility electronic transport devices from few-layer thick crystals, previously found to be electrically insulating. The large magnetoresistance persists in these samples and can be turned off by electrostatically tuning the system to a simple-metal (noncompensated) regime. Furthermore, the semimetallic magnetoresistance is found to follow a density-independent subquadratic power law, a topic for future study. Finally, quantum oscillations are also analyzed as a function of total carrier density, providing the first insights into the band structure of ultrathin WTe${}_{2}$.

[Phys. Rev. B 95, 041410(R)] Published Mon Jan 30, 2017

]]>What happens to the edge states when two edges intersect? The authors answer this question for the case in which the topologically protected edge states are the quasi-one-dimensional bound states that propagate unidirectionally under the two-dimensional Dirac equation, if it includes an inhomogeneous potential term that changes sign across a zero line. These “zero line modes” are bound to the line along which the potential vanishes, and so if the potential has a “checkerboard” form, such that two zero lines cross, then the question becomes: what happens to the zero line modes at the intersection? By finding an exact integral representation for the two-dimensional Dirac energy eigenstates, the authors show that the eigenstates take surprisingly narrow “T-junction” forms, so that incoming wave packets split coherently at the intersection into two outgoing packets, while remaining tightly bound to the zero lines. The intersection thereby acts as a beam splitter for zero line modes.

[Phys. Rev. B 95, 045430] Published Mon Jan 30, 2017

]]>Phase transitions to absorbing states are among the simplest examples of critical phenomena out of equilibrium. The characteristic feature of these models is the presence of a fluctuationless configuration that the dynamics cannot leave, which has proved a rather stringent requirement in experiments. Recently, a proposal to seek such transitions in highly tunable systems of cold atomic gases offers to probe this physics and, at the same time, to investigate the robustness of these transitions to quantum coherent effects. Here, the authors specifically focus on the interplay between classical and quantum fluctuations in a simple driven open quantum model which, in the classical limit, reproduces a contact process, which is known to undergo a continuous transition in the “directed percolation” universality class. The authors derive an effective long-wavelength field theory for the present class of open spin systems and show that, due to quantum fluctuations, the nature of the transition changes from second to first order, passing through a critical point which appears to belong instead to the “tricritical directed percolation” class.

[Phys. Rev. B 95, 014308] Published Fri Jan 27, 2017

]]>In the field of geometrically frustrated magnetism, the very nature of the ground state of Tb${}_{2}$Ti${}_{2}$O${}_{7}$ has remained a long-standing conundrum. In this pyrochlore material, no conventional spin-ice or long-range magnetic order is stabilized, even at very low temperatures. Quantum fluctuations are suspected of being at the origin of such an exotic quantum phase, yet so far there is no conclusive evidence. Here, high-resolution synchrotron-based terahertz spectroscopy probes the lowest energy excitations of Tb${}_{2}$Ti${}_{2}$O${}_{7}$. By exciting transitions within the crystal electric field and crystal lattice, it is revealed that a double hybridization of crystal-field phonon modes is present across a broad temperature range. This unique vibronic process triggers coupling between several degrees of freedom that provides a crucial path for quantum spin-flip fluctuations to inhibit the stabilization of conventional magnetic states. While providing evidence for the unique quantum phase in Tb${}_{2}$Ti${}_{2}$O${}_{7}$, the result also highlights the powerful and complementary nature of terahertz spectroscopy as a probe in the study of exotic magnetic and frustrated phases.

[Phys. Rev. B 95, 020415(R)] Published Fri Jan 27, 2017

]]>The Dzyaloshinskii-Moriya interaction in the cubic chiral magnet MnSi stabilizes a magnetic helix - a periodic one-dimensional modulation of the magnetization. The orientation of this helix is determined by weak magnetocrystalline anisotropies, but it can be reoriented by applying a magnetic field. Here, the authors have studied this reorientation process by means of small-angle neutron scattering and susceptibility measurements. Their results are in excellent agreement with predictions of an effective mean-field theory taking into account the precise symmetries of the crystal structure. Measurements of the magnetization and ac susceptibility provide evidence that the reorientation of helimagnetic domains is associated with large relaxation times exceeding seconds. In addition, hysteresis at the Ising transitions indicates, within the same theoretical framework, the formation of an abundance of plastic deformations of the helical spin order. These deformations comprise topologically nontrivial disclinations, reminiscent of the skyrmions discovered recently in the same class of materials.

[Phys. Rev. B 95, 024429] Published Wed Jan 25, 2017

]]>Engineering topological degeneracy within an experimentally accessible platform has been an active area of research with applications towards the pursuit of universal topological quantum computation. In this work, the authors consider Abelian fractional quantum spin Hall states with multiply connected gapped boundaries. Such a system manifests topological degeneracy (also dubbed “boundary degeneracy”) and in principle can realize universal quantum computation with no superconducting proximity required. The authors construct an exactly soluble model manifesting this topological degeneracy and solve it within Hamiltonian formalism, which is a natural framework to quantify the Hilbert space structure associated with this degenerate subspace. Furthermore, this platform is accessible within experiments and can be created by applying a depletion gate to the quantum spin Hall material and using a spin-mixing process realized by generic magnetic impurities that gap out the edge modes.

[Phys. Rev. B 95, 045309] Published Wed Jan 25, 2017

]]>Measuring the magnetization vector field of magnetically ordered materials is the ultimate goal of magnetic imaging. The authors propose a technical realization of quantitative magneto-optical Kerr microscopy, which makes it possible to obtain the continuously changing vector field of complete magnetization processes on arbitrarily oriented surfaces. It is based on a pulsed light emitting diode (LED) light source that allows for a computer controlled, real-time contrast selection of all three magnetization components of the domain patterns. As parasitic Faraday contributions in the microscope optics are eliminated and by using monochrome LED light, the accuracy and signal-to-noise ratio of the analysis is significantly enhanced compared to preceding solutions. The approach to vector imaging thus exceeds previous limits of the experimental instrumentation for magneto-optical domain investigation. The potential of the technique was demonstrated by the quantitative analysis of a remagnetization cycle of a Permalloy film element, and by the vectorial analysis of a domain structure in a GaMnAs thin semiconducting film with extremely low Kerr response.

[Phys. Rev. B 95, 014426] Published Tue Jan 24, 2017

]]>While it is mostly well-known for its excellent thermoelectric properties at high temperatures, Yb${}_{14}$MnSb${}_{11}$ has also come to be characterized as a ferromagnetic semiconductor. Such systems support ferromagnetic correlations similar to metallic compounds, although with semiconducting electronic configurations. As such, they are promising materials for use in spintronic devices. The authors here use neutron scattering techniques to examine the distribution of the magnetization density and the energy scale of the magnetic exchange interactions in the Yb${}_{14}$MnSb${}_{11}$ compound. They find that there is a large ferromagnetic moment localized to the Mn site in the crystal structure with a ‘Kondo cloud’ of negative moments distributed over the other atomic sites. The results provide long sought answers to questions regarding the valence, magnetization distribution, and energy scales of the magnetic interactions in this compound. The ferromagnetic spin-wave spectrum supports this premise and is well characterized by inclusion of nearest and next-nearest neighbor exchange interactions.

[Phys. Rev. B 95, 020412(R)] Published Mon Jan 23, 2017

]]>The three-dimensional (3D) Dirac materials represent a dimensional extension of the two-dimensional physics seen in graphene. Recently, two types of 3D Dirac materials have been proposed. The first type are the symmetry protected 3D Dirac materials with the Cd${}_{3}$As${}_{2}$ as the representative material. The second type are materials with accidental touching of the conduction and valence bands in a single point. In this paper, the authors are reporting bulk quantum oscillations in Pb${}_{1-x}$Sn${}_{x}$Se, a latter type of material, which has also been recently identified as a topological crystalline insulator with a topological phase transition predicted to take place at $x$=0.17. The authors have used the quantum oscillations in resistivity and magnetization to identify the phase of the oscillation and prove that this material has the 3D Dirac linear dispersion for $x$=0.17. Combining the oscillations in the resistivity and magnetization, the authors provide the consistent indexing method for drawing the Landau level diagrams that are used in identifying the Berry phase.

[Phys. Rev. B 95, 035208] Published Fri Jan 20, 2017

]]>In the presence of strong disorder and weak interactions, closed quantum systems can enter a many-body localized phase wherein the system does not conduct heat or charge, does not equilibrate even for arbitrarily long times, and robustly violates quantum statistical mechanics. While this is well established in closed quantum systems, the interplay of such systems with delocalized degrees of freedom is much less understood. Here, the authors consider a model for which, in the noninteracting limit, some degrees of freedom are localized while others remain delocalized. Such a system can be viewed as a model for a many-body localized system brought into contact with a small bath of a comparable number of degrees of freedom. The authors numerically and analytically study the effect of interactions on this system and find that in certain parameter regimes, results are consistent with interaction-induced localization of the entire system.

[Phys. Rev. B 95, 035132] Published Thu Jan 19, 2017

]]>The authors identify a new class of optically controllable, semiconductor-based defect spin that is formed from the $d$-orbital electrons of chromium ions in silicon carbide and gallium nitride. These ions possess a simple lambda optical structure that couples only weakly to phonons and lattice strain. Therefore, even though they probe an ensemble of many ions at once with varying strain environments, the optical transitions they observe are exceptionally narrow and possess a high radiative efficiency. These properties allow the authors to individually interrogate the magnetic sublevels of the ground-state spin using resonant optical excitation, enabling ensemble optical spin polarization as well as optically detected magnetic resonance in the time domain. Each ion species emits the majority of its luminescence within a near-infrared zero-phonon line, suggesting a capacity for efficient photonic integration. Additionally, as magnetically active $d$-orbital states, the spins of these ions possess a number of degrees of design freedom not available to other common defect spin species such as those based on vacancy complexes. The authors therefore expect that these studies will broaden the range of opportunities available to semiconductor-based quantum device engineering, and will motivate further explorations into the use of transition metal ions as optically active qubit states.

[Phys. Rev. B 95, 035207] Published Thu Jan 19, 2017

]]>Due to the one-dimensional nature of single-wall carbon nanotubes (SWCNTs), electron-electron interactions are of great importance because the lack of screening enhances the effective Coulomb interactions between electrons, and individual SWCNTs have been considered as an ideal realization of a Tomonaga-Luttinger liquid (TLL). In an actual SWCNT sample, SWCNTs usually pack together closely and form a bundle structure, in which intertube effects may alter the TLL state significantly. Here, the authors study the TLL effect in bundles of metallic SWCNTs using ${}^{13}$C nuclear magnetic resonance techniques to understand how the intertube effects alter the electron-electron interaction in the bundle form. They find a modified charge Luttinger parameter for the bundled metallic SWCNTs. Their findings give direct evidence that bundling reduces the effective Coulomb interactions via intertube interactions within bundled metallic SWCNTs.

[Phys. Rev. B 95, 035128] Published Wed Jan 18, 2017

]]>Fraunhofer interference is a paradigmatic phenomena arising due to phase coherence in diverse systems from optics to superconducting junctions. In Josephson junctions, coupling two superconductors through a weak link, such patterns arise in the maximal dissipationless current the system can sustain, oscillating as function of a perpendicular applied magnetic field. By investigating this effect in a recently realized material system epitaxially coupling a thin superconductor to a semiconducting region, the authors here discover novel effects arising due to a combination of magnetic field screening, spin physics, and disorder. In an appropriately aligned in-plane magnetic field, they find that due to screening by the superconducting leads, a flux dipole develops in the semiconducting region leading to an effective confinement of the superconducting states to edges of the intervening semiconducting region. When the out-of-plane field is swept in the presence of an in-plane field, striking asymmetries in the Fraunhofer pattern are observed. By analyzing the underlying theoretical symmetries of the system, they demonstrate that such an effect arises as a result of an intricate interplay between disorder in the junction, splitting of the spin states in the applied field, and coupling between the momentum of the electrons and their spin.

[Phys. Rev. B 95, 035307] Published Wed Jan 18, 2017

]]>Quantum dots are promising building blocks for future optoelectronics or quantum information applications. Many of their properties derive from the quantum state of the hole trapped in the dot. “Light” and “heavy” hole states differ by the anisotropic character of their spin and of the electric dipole they form with a trapped electron. Light holes are of particular interest as they offer extended opportunities for optical manipulation of single carriers, or the electrical manipulation of magnetic objects. Here, the authors demonstrate that the hole ground state can be engineered through a proper design of the strain built in the dot when inserted inside a nanowire of a different lattice parameter. Two complementary techniques provide evidence of a light hole state: polarization-resolved Fourier imaging, which is sensitive to the nature of the electron-hole electric dipole, and magneto-optical spectroscopy, which probes the hole spin state through its coupling to magnetic atoms.

[Phys. Rev. B 95, 035305] Published Tue Jan 17, 2017

]]>Monolayer two-dimensional transition metal dichalcogenide (TMDCs) exhibit exceptionally strong spin-orbit and electron-electron interaction effects and have provided a rich new playground for the exploration of exciton physics. Recent experiments have demonstrated that in the presence of excess charge carriers the prominent excitonic features in optical absorption split into two separate peaks. The appearance of the additional peak is usually attributed to the presence of trions, charged fermionic quasiparticles formed by binding two electrons to one hole or two holes to one electron. The authors here argue that in the density range for which amplitudes of two peaks are comparable three-particle physics is of importance, and the appropriate picture is one of excitons interacting with the Fermi sea formed by additional charge carriers. These interactions result in the dressing of excitons into exciton-polarons. The exciton spectrum splits into a lower energy attractive exciton-polaron branch, normally identified as a trion branch, and a higher energy repulsive exciton-polaron branch, normally identified as an exciton branch. The authors provide a complete theory of absorption, which incorporates both static and dynamic effects of Fermi sea, and analyze frequency and doping dependence of optical conductivity in detail. The calculated density dependence of peak splitting and their amplitudes and widths are in agreement with recent experiments.

[Phys. Rev. B 95, 035417] Published Tue Jan 17, 2017

]]>The interplay between symmetry and topology is one of the most exciting avenues of modern condensed matter research. Here, the authors propose a new and unified approach for describing the low-energy physics of two-dimensional spin-singlet superconductors, placing them in a class of topologically ordered states akin to quantum Hall fluids and spin liquids. Starting from a microscopic model of two-dimensional paired fermions with a dynamical electromagnetism (that is also confined to two spatial dimensions), the authors derive Chern-Simons theories for all spin-singlet gapped superconductors, including $s$-wave and chiral states. The topological field theories constructed here thus pave the way towards understanding superconductors as symmetry-enriched topological phases of matter.

[Phys. Rev. B 95, 014508] Published Fri Jan 13, 2017

]]>The band-unfolding method is now widely used to calculate the effective band structures of a disordered system from its supercell model. The unfolded band structures are obtained as if the disordered system really has the crystallographic symmetry of its underlying structure. However, it has still been difficult to decompose the unfolded band structures into the modes based on the crystallographic symmetry of the underlying structure. This has prohibited detailed analyses of, e.g., selection rules and avoided band crossings for the unfolded band structures. The authors here develop a procedure to decompose the unfolded band structures according to the small representations (SRs) of the little groups. The decomposition is performed using the projection operators for SRs derived from the group representation theory. Using this procedure, the authors investigate the phonon band structure of disordered face-centered-cubic Cu${}_{0.75}$Au${}_{0.25}$, which has large variations of atomic masses and force constants among the atomic sites due to the chemical disorder. In the unfolded phonon band structure, several peculiar behaviors such as discontinuous and split branches are found in the decomposed modes corresponding to specific SRs.

[Phys. Rev. B 95, 024305] Published Fri Jan 13, 2017

]]>The discovery of topological matter should be an opportunity for revolutionizing electronic devices. Nevertheless, for all their unique properties, to date there are limited feasible devices or applications based on topological materials. The work here recognizes that topological Weyl semimetal materials, through their protected gap closing and spin-orbit locking, could be used to detect low-frequency infrared radiation. Detecting infrared light electrically beyond a ten-micron wavelength is rather difficult with few ready made commercial solutions. The authors find that Weyl semimetals could provide an ideal solution for detecting mid- and far-infrared light even at room temperature. In this work, they describe the conditions of the materials to be effective photodetectors (noncentrosymmetry, doping levels etc.) and provide a detailed analysis of this photovoltaic effect.

[Phys. Rev. B 95, 041104(R)] Published Fri Jan 13, 2017

]]>The authors present a systematic density functional theory GGA characterization of the ${R}_{4}$Fe${}_{2}$As${}_{2}$Te${}_{1-x}$O${}_{4-y}$F${}_{y}$ family of compounds (called “42214”) as a function of a set of key tuning parameters: rare earth ($R$ = Pr, Sm, and Gd), Te content, oxygen to fluorine substitutional doping, and external pressure. The analysis is based on an unfolding procedure that allows observation of the behavior of hole and electron pockets of the Fermi surface and of the nesting function. The present study includes characterization of a related hypothetical compound having Se substituting for Te. Results show that this peculiar compound family offers very good opportunities to properly harness material properties. Nesting, Fermi surface (topology and size), and magnetism can be easily modified by changing chemical composition: such tuning may lead to improved superconducting properties.

[Phys. Rev. B 95, 014415] Published Thu Jan 12, 2017

]]>As the temperature lowers in the gapless region, quantum quasi-1D spin systems show one-dimensional (1D) short-range ordering (SRO) corresponding to a Tomonaga-Luttinger liquid (TLL) by the interaction within chains, and then at lower temperatures where the small interchain interaction cannot be ignored, they have a three-dimensional (3D) long- range ordered (LRO, BEC) phase. This generally accepted scenario for the dimensional crossover of quasi-1D spin ordering, however, is debatable around the critical field, because the 1D SRO (TLL) regime has a linear boundary on the $H$-$T$ phase diagram, while the 3D LRO transition temperature has a convex shape. To this dimensionality paradox around the critical field, the authors have given the solution, experimentally by detailed analyses of the specific heat and ac susceptibility for bond-alternating antiferromagnetic chains in pentafluorophenyl nitronyl nitroxide (F${}_{5}$PNN). They reveal the existence of a definite field region near a critical field, where 3D LRO (BEC) directly occurs without passing through 1D SRO region at higher temperatures, even if the interchain interaction is sufficiently small compared to the intrachain one.

[Phys. Rev. B 95, 020408(R)] Published Thu Jan 12, 2017

]]>The authors investigate the properties of 378 half-Heusler compounds using density functional theory with the goal of identifying promising candidates for spintronic applications, e.g. half-metals. Although DFT has often been applied to the search for half-metals, this study may be the most comprehensive attempt to identify which of the compounds predicted by DFT to be half-metals are likely to be fabricated. The calculated formation energy of each of the 378 potential half Heuslers was compared to that of all competing phases and combination of phases in the Open Quantum Materials Database. Those semiconductors, half-metals, and near half-metals within an empirically determined 0.1 eV/atom hull distance margin for neglected effects were deemed of interest for further experimental investigation.

[Phys. Rev. B 95, 024411] Published Wed Jan 11, 2017

]]>Materials physicists are increasingly coming to understand that domain walls in complex materials often have a life of their own. Far from being unwanted “defects”, they may have rich and interesting physics not available in the bulk material. Here, the authors use first-principles theoretical methods to consider ferroelectric domain walls in a promising family of corundum-derivative materials. Beyond studying issues of domain wall mobility and clarifying the factors that allow polarization reversal, the authors also identify an unusual set of domain-wall-specific couplings between polarization, magnetization, and chirality that they suggest may lead to new functionalities.

[Phys. Rev. B 95, 014105] Published Tue Jan 10, 2017

]]>A Cr atom in a semiconductor host carries a localized spin with an intrinsic large spin to strain coupling particularly promising for the development of hybrid spin-mechanical systems. The authors demonstrate here that the spin of an individual Cr atom can be controlled optically. To optically access to the spin of an individual atom, they insert it in a cadmium telluride quantum dot. With a single Cr atom introduced in the quantum dot, the energy and polarization of the photon emitted or absorbed by the dot depends on the spin state of the magnetic atom. The authors first show that excitation with a laser beam tuned to the wavelength of one of these optical transitions can be used to initialize the state of the Cr spin and to probe its dynamics optically: the Cr behaves like an optically addressable spin-based memory. Under optical excitation exactly resonant with an absorption transition one can also enter the strong coupling regime where hybrid states of matter and light are created. The spin-dependent strong coupling with the laser field is exploited to optically tune the Cr atom’s energy levels.

[Phys. Rev. B 95, 035303] Published Tue Jan 10, 2017

]]>Yttrium iron garnet (YIG) holds a special place among all magnetic materials since it shows an unprecedentedly small magnetic damping, resulting in the narrowest known line in ferromagnetic resonance and enabling propagation of spin waves over long distances. Due to these unique characteristics, YIG films have been recently considered as a promising material for spintronic and magnonic applications. By combining YIG and a single layer of graphene in a bilayer, one obtains a unique model system for investigations of magnetic interfacial effects. Here, the authors demonstrate experimentally that the system shows a sizable chiral charge pumping, which could arise from an imbalance of screw dislocations. The effect, which cannot be attributed to the ordinary spin pumping, reveals itself in the creation of a dc electric field/voltage in graphene as a response to the dynamic magnetic excitations (spin waves) in an adjacent out-of-plane magnetized YIG film. The authors show that the induced voltage changes its sign when the orientation of the static magnetization is reversed, clearly indicating the broken mirror reflection symmetry about the planes normal to the graphene/YIG interface. The strength of effect shows a nonmonotonic dependence on the spin-wave frequency, in agreement with the proposed theoretical model.

[Phys. Rev. B 95, 024408] Published Mon Jan 09, 2017

]]>Quantum oscillations are the fundamental measurement made when studying metals and semimetals. They provide information about the Fermi surface by measuring interference of particles drifting around it in a magnetic field. So far it was only possible to measure them by applying an external magnetic field. Here, the authors argue that the deformation of the Dirac/Weyl semimetal can produce a strong enough pseudo-magnetic field to measure quantum oscillations in the strength of the strain. They argue that the effect of the strain is identical to the effect of the real magnetic field at energies close to the Dirac/Weyl points. They propose a setup in which the strain can be tuned continuously, and in which the quantum oscillations can be first observed in the complete absence of a magnetic field.

[Phys. Rev. B 95, 041201(R)] Published Mon Jan 09, 2017

]]>Can one investigate more efficiently quantum critical behavior by employing a local order-parameter pinning field, which explicitly breaks the symmetry of the model under investigation? To answer this question, the authors consider the two-dimensional square-lattice bilayer quantum Heisenberg model using a world-line quantum Monte Carlo method. The pinning-field approach is found to accurately locate the quantum critical point over a wide range of pinning-field strengths. However, the identification of the quantum critical scaling behavior is found to be complicated by the fact that the pinning field introduces strong corrections to scaling. A renormalization group analysis exhibits important analogies to surface critical phenomena, and a crossover effect to an infinite pinning-field fixed point, which the authors study in detail by simulations of an improved classical lattice model in the three-dimensional Ising universality class. At the infinite pinning-field fixed point, the short-distance expansion of the order-parameter profile exhibits a new universal critical exponent, which characterizes the experimentally relevant critical adsorption on a line defect. This result also implies the presence of slowly decaying scaling corrections, which are analyzed here in detail.

[Phys. Rev. B 95, 014401] Published Tue Jan 03, 2017

]]>The NiCl${}_{2}$-4SC(NH${}_{2}$)${}_{2}$ spin-chain compound, also called “DTN”, is one of the most studied archetype materials that at low temperature presents the magnetic-field-induced ordered phase that is described as a Bose-Einstein condensate (BEC). The paper here presents an NMR study of the static properties of this phase, where, for the first time, the exact absolute value of the order parameter has been determined and compared to theoretical predictions. The precise phase boundary of the BEC phase close to its upper critical field is found to be in perfect agreement with theory, and used to define and discuss the range of validity of the Hartree-Fock-Popov approximation. DTN is a unique quasi-1D BEC-type material for which the sub-dominant 3D exchange couplings are known from independent measurements, so that theoretical descriptions are completely constrained. Therefore, precise state-of-the-art numerical methods could be used to critically discuss the applicability of approximate 1D-based descriptions (e.g., Tomonaga-Luttinger liquid $+$ mean field) to the case when the quasi-1D character is of intermediate strength. The NMR results presented here and the corresponding theoretical descriptions can be considered as reference data and analysis for BEC-type compounds, which have been missing for a long time in this field of research.

[Phys. Rev. B 95, 020404(R)] Published Tue Jan 03, 2017

]]>The presence of few-body bound states is an important issue in the study of interacting bosonic systems due to the instability it introduces into the condensation. The problem is more interesting in two dimensions because the system can host only a finite number of bound states, in contrast to the Efimov effect in three dimensions. The authors considered a two-component bosonic system in two dimensions, with repulsive intra- and attractive inter-species interactions. They found that there is only one three-particle bound state and the binding energies for various scattering lengths fall on an universal curve, where the microscopic details are of no relevance. Their conclusion is pertinent for various systems such as excitons in quantum well structures and bosonic dipoles in bilayer geometries. They developed an efficient renormalization group type method, capable of handling both short- and long- range physics and free of numerical instabilities. This method also makes use of the Hopf coordinates and running basis to reduce the proliferation of hyperspherical harmonics appearing in alternative approaches, yielding significant saving in numerical implementations.

[Phys. Rev. B 95, 045401] Published Tue Jan 03, 2017

]]>WTe${}_{2}$ is well known for its manifestation of anomalously large magnetoresistance. It has recently attracted much attention because it is theoretically predicted to be the first material candidate that may realize the “type-II” Weyl state. This work reports combined experimental and theoretical investigations on the electronic structure of WTe${}_{2}$. Taking advantage of the latest-generation laser-based angle-resolved photoemission (ARPES) system with superior instrumental resolution, a complete picture of the electronic structure of WTe${}_{2}$ is revealed. The existence of a surface state that connects the bulk electron and hole pockets is identified. High-temperature ARPES measurements make it possible to reveal electronic states above the Fermi level where the Weyl points are predicted to be located. The observed connection of the surface state with the bulk bands, its momentum evolution, and its momentum and energy locations, are all in good agreement with the calculated band structures. These results provide key information to understand the anomalous transport properties of WTe${}_{2}$. They also provide electronic signatures that are consistent with the type-II Weyl state in WTe${}_{2}$ and lay a foundation for further investigations on its topological nature.

[Phys. Rev. B 94, 241119(R)] Published Fri Dec 30, 2016

]]>The authors propose a new method for realizing mechanical ground-state cooling that could generate quantum entanglement at convenient temperatures (1 K). They consider a diamond beam bending above a magnetic tip (the beam is clamped at one end and free to move at the other). Near the oscillating end, they insert a lattice defect, specifically a silicon-vacancy center. As the beam bends, the defect experiences a varying magnetic field which may flip its spin by absorbing mechanical energy from the low-frequency flexural motion of the beam. At the same time, the center is coupled to high-frequency compression modes, which locally distort the surrounding lattice and lead to a fast relaxation of the center’s electronic states. By stimulating the defect with microwave fields, these two coupling mechanisms can be connected and an energy flow from the bending motion into the high-frequency phonon reservoir is induced. According to predictions made here, the mechanical temperature of the beam – how much it bends – can then be tuned via optimizing these alternating absorption and reemission processes in the same way the temperature of atoms can be controlled with light.

[Phys. Rev. B 94, 214115] Published Thu Dec 29, 2016

]]>The excitation of a ferromagnetic film by a femtosecond laser pulse causes an unexpectedly fast quenching of the film’s magnetization on subpicosecond time scales. The microscopic physical mechanisms responsible for this remain a scientific puzzle. The authors employ femtosecond extreme ultraviolet pulses produced by high harmonic generation to follow how the magnetization of a thin cobalt film evolves after the excitation by a 40-fs laser pulse. By measuring the time-, energy-, and angle-resolved magneto-optical response of the Co films across the ${M}_{2,3}$ absorption edge, they obtain a set of time-lapsed magnetic asymmetry spectra, which contain a wealth of information about the different mechanisms at work. When combined with advanced ab initio magneto-optical calculations, they identify two dominant contributions: first, a transient reduction of exchange splitting, and second, magnon excitation. This work thus distinguishes between two fundamental models of magnetism, the Stoner and Heisenberg models, which ascribe magnetization dynamics to an exchange splitting reduction and spin wave excitations, respectively.

[Phys. Rev. B 94, 220408(R)] Published Wed Dec 28, 2016

]]>Miniaturized magnetic tunnel junctions enable single-shot time-resolved studies of magnetization reversal driven by spin currents that can test basic physical models, particularly as recent technological advances enable the study of sub-100 nm diameter junctions. The junctions consist of two magnetic layers with uniaxial magnetic anisotropy separated by a thin insulating layer that enables electron tunneling and thus transfer of spin-angular momentum in the tunneling process. One magnetic layer is free to reverse in response to the spin current while the other is fixed. Pulsed current measurements show the stochastic nature of the reversal process on nanosecond time scales and provide evidence for nonuniform magnetization reversal processes in junctions that are larger than 50 nm in diameter.

[Phys. Rev. B 94, 214432] Published Tue Dec 27, 2016

]]>The authors have developed a new method for exploring and controlling matter at the molecular scale. Their technique, called “electro-CARS” (for “Coherent anti-Stokes Raman scattering under electric field stimulation”), consists of exposing the sample to an electric field and laser radiation simultaneously. Under these conditions, the molecules in the sample organize themselves and start vibrating, while returning a tiny part of the received light energy. Although very weak, this reemitted light provides a lot of chemical information about the sample, with a better sensitivity and signal-to-noise ratio in comparison to existing spectroscopy tools. Electro-CARS could have a major impact in the field of electrochemotherapy, a promising method for cancer treatment. It could enable investigation of cell membrane modifications thoroughly and monitoring of anticancer drug delivery into cells without damaging them.

[Phys. Rev. B 94, 245136] Published Tue Dec 27, 2016

]]>A disordered quantum system can be driven, e.g., by increasing disorder, through the Anderson transition between a delocalized and localized phase. Recently, Anderson localization on treelike graphs has attracted much attention in view of its connections with problems of many-body localization. One of the central questions in this context is that of ergodicity of eigenstates. The authors of this paper perform a numerical study of Anderson transition on random regular graphs (RRGs) with diagonal disorder. The problem can be described as a tight-binding model on a lattice with $N$ sites that is locally a tree with constant connectivity. Focusing on the delocalized side of the transition, they show that the data can be interpreted in terms of the finite-size crossover from a small ($N\ll {N}_{c}$) to a large ($N\gg {N}_{c}$) system, where ${N}_{c}$ is the correlation volume diverging exponentially at the transition. A distinct feature of this crossover is a nonmonotonicity of the spectral and wave-function statistics, which is related to properties of the critical phase in the studied model and renders the finite-size analysis highly nontrivial. The results support an analytical prediction that states in the delocalized phase (and at $N\gg {N}_{c}$) are ergodic.

[Phys. Rev. B 94, 220203(R)] Published Thu Dec 22, 2016

]]>Strong electron correlations in solids are at the heart of fascinating phenomena such as high temperature superconductivity, whose understanding remains a prominent open problem in Physics. A central prediction of dynamical mean field theory (DMFT) is the breaking of Landau’s Fermi liquid, which describes extremely well simple metals like copper, into itinerant heavy quasiparticles and localized Mott-Hubbard states. To test it, electronic structure calculations based on DMFT are usually benchmarked against the photoemission spectra of SrVO${}_{3}$, a cubic perovskite with one $d$ electron per unit cell that is considered the gold standard in this field. However, the present study shows that the UV synchrotron radiation used in the photoemission experiments creates oxygen vacancies, resulting in a strong peak of defect states essentially on top of the SrVO${}_{3}$ Hubbard band. As recent works found that the same peak of vacancy states occurs in noncorrelated wide-gap ${d}^{0}$insulators, such as SrTiO${}_{3}$, an unavoidable worry emerges. What if the putative Hubbard bands observed in SrVO${}_{3}$ were just a mirage? What if this strongly correlated effect, a hallmark prediction of DMFT for the past 25 years was just not there? This study thoroughly explores those crucial issues both experimentally and theoretically.

[Phys. Rev. B 94, 241110(R)] Published Mon Dec 19, 2016

]]>In quantum magnetism, the kagome antiferromagnet is one of the most widely studied spin-liquid candidate systems, in which geometric frustration inhibits the formation of conventional magnetic order. In this paper, the authors employ a recently developed pseudofermion functional renormalization group approach that reveals the competition of magnetic orders in the kagome system and a three-dimensional generalization, the hyperkagome lattice. Including couplings beyond the nearest-neighbor exchange, the authors identify a plethora of magnetically ordered states as well as extended spin-liquid regimes.

[Phys. Rev. B 94, 235138] Published Thu Dec 15, 2016

]]>Interactions between aligned dipoles in periodic model systems are often considered spurious and removed by partitioning the supercell in isolated slabs separated by vacuum layers. One popular and efficient screening approach is the so-called dipole correction. In this work, the authors present an alternative finite-field based method which retains all electrostatic interactions and dispenses with vacuum layers. Adapting the constant-$D$ approach developed by Stengel, Spaldin, and Vanderbilt for the study of ferroelectric nanocapacitors, the new scheme here is intended for molecular dynamics simulations of the interface between an insulator carrying a specifically absorbed surface charge and an ionic conductor (the electrolyte) at finite temperature. The authors show that the dipole correction scheme is equivalent to $D$=0 electric boundary conditions with or without vacuum layers. Thus, both global polarization $P$ of the heterogeneous system and corresponding average macroscopic electric field $E$ are finite. The authors then continue with the investigation of the variation of $P$ with $E$ or $D$ and validate expressions based on $P$ for computing the electric double-layer capacitance using atomistic simulations.

[Phys. Rev. B 94, 245309] Published Thu Dec 15, 2016

]]>The boson peak is an excess in the vibrational spectrum of amorphous solids, which was recently conjectured to be simply related to features already inherent in the vibrations of the corresponding crystal. In this work, the authors reveal that this is not the case, at least for metallic systems. Using atomic-scale computer simulations of a four-component alloy in different structural states, either as a metallic glass or as a high-entropy alloy, they show that the boson peak does not occur in a defect-free lattice, independent of its density. This disproves, at least for the case of amorphous metals, the recent notion that the vibrational spectra of glasses are not distinct from that of crystals, but simply shifted because of the reduced density that always accompanies the amorphous state. Instead, additional modes arise in softened regions of the material, which are connected to structural defects. These defects can take the form of ”soft spots” in glasses or even a high concentration of lattice defects in crystals. The boson peak is therefore, in fact, a signature of structural disorder.

[Phys. Rev. B 94, 224203] Published Wed Dec 14, 2016

]]>Pump-probe Faraday/Kerr rotation spectroscopy is a powerful tool for exploring the coherent spin dynamics with picosecond temporal resolution. However, this has so far been possible only in time ranges up to a few nanoseconds. In this work, the technique is extended to address time ranges orders of magnitude longer by employing a tailored pump and probe pulse sequence, while maintaining picosecond time resolution. The submicrosecond electron spin dynamics in $n$-type GaAs with doping concentrations close to the metal-insulator transition can be directly accessed thereby with high precision, rendering the electron spin dephasing time ${T}_{2}^{*}$ and the longitudinal spin relaxation time ${T}_{1}$. Both approach 250 ns in weak magnetic fields. The Larmor spin precession in high magnetic fields applied in the Voigt geometry shows nonmonotonic dynamics deviating strongly from a mono-exponential decay and revealing $g$-factor variations on the level of 10${}^{-4}$, demonstrating the high precision of the method. The possibility of varying the pump pulse train composition from single to multiple pulses provides detailed insight into the synchronization of the spin precession in the electron gas when subject to periodic laser excitation.

[Phys. Rev. B 94, 241202(R)] Published Wed Dec 14, 2016

]]>The hyperfine field ${B}_{\text{hf}}$ and the magnetic properties of the BaFe${}_{2}$As${}_{2}$ family are studied using the fully relativistic Dirac formalism for different types of substitution. Relativistic contributions were found to have a significantly stronger impact for the iron pnictides when compared to bulk Fe. Based on the calculations here and the measured hyperfine field from ${}^{57}$Fe Mössbauer spectroscopy for undoped BaFe${}_{2}$As${}_{2}$, we can estimate a magnetic moment of roughly 0.7–1.0 ${\mu}_{\text{B}}$, which is more consistent with neutron diffraction reporting 0.87 ${\mu}_{\text{B}}$. This solves the longstanding puzzle of different reports for the magnetic moment of BaFe${}_{2}$As${}_{2}$ based on either ${}^{57}$Fe Mössbauer spectroscopy or neutron diffraction. However, for substituted iron pnictides, the behavior of ${B}_{\text{hf}}$ with the concentration $x$ is clearly unpredictable and might lead to wrong conclusions. Thus, relating the hyperfine fields ${B}_{\text{hf}}$ of Fe obtained via ${}^{57}$Fe Mössbauer spectroscopy to the magnetic moments is not sensible for substituted iron pnictides.

[Phys. Rev. B 94, 214508] Published Mon Dec 12, 2016

]]>One of the fundamental prerequisites for the formation of exotic anyonic bound states in superconducting quantum wires is that their normal-state band structure exhibits a so-called helical gap. Besides application of an external magnetic field, such a helical gap can also arise as a consequence of strong electron-electron interactions. The authors show that one possible realization of such an interaction-driven helical gap can be found in wires with transverse subbands and spin-orbit coupling. This state can be described in terms of quasiparticles with fractional charge $e$/2. These fractional charges in turn lead to important modifications of the threshold behavior of measurable dynamic response functions, such as the spectral function and the density structure factor. These experimentally accessible quantities can therefore be used to discriminate between interaction-induced helical gaps, and the more mundane gaps opened by an applied magnetic field.

[Phys. Rev. B 94, 245414] Published Mon Dec 12, 2016

]]>Electron spin relaxation in bulk cubic GaN is investigated by time-resolved magneto-optical spectroscopy over a wide range of temperatures, external magnetic fields, and doping densities. Very long spin relaxation times in the nanosecond range are found even up to room temperature, making cubic GaN a highly interesting material for spintronics. These results on a system with relatively weak spin-orbit coupling differ significantly from previous results for bulk III-V semiconductors such as GaAs. There are several possible reasons for the observed discrepancy, including the role of charged dislocations and a phase mixture of cubic GaN with inclusions of hexagonal GaN. Spin relaxation in such disordered materials is still poorly understood, and so this work should stimulate further research on this relevant topic.

[Phys. Rev. B 94, 235202] Published Fri Dec 09, 2016

]]>Samples of few-layer phosphorene can be produced by mechanical or liquid exfoliation. However, it remains challenging to produce flakes with large sizes. Optical spectroscopy performed on phosphorene has revealed a peculiar gap dependence that is roughly inversely proportional to the layer number. This dependence has been poorly understood to date. Because of their small size, these samples have been out of reach of powerful electronic structure characterization techniques such as angle-resolved photoemission (ARPES). The present work shows how to get a handle on the experimental band structure of few-layer black phosphorus. The authors conjecture that the bulk black phosphorous crystal already provides all the necessary information regarding the electronic structure of few-layer phosphorene. They perform ARPES measurements of the bulk crystal to fit to an experimental tight-binding model. Then, they apply the zone-folding method to the three-dimensional electronic dispersion. By quantizing the electron wavevector component perpendicular to the layers, they are able to not only extract reliable band structure data for few-layer phosphorene from the bulk band structure but also to fully explain the peculiar band-gap dependence. In particular, the band gaps as predicted by zone-folding are in excellent agreement with optical experiments and $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ calculations.

[Phys. Rev. B 94, 245410] Published Wed Dec 07, 2016

]]>Weyl nodes, the essential low-energy theory of Weyl semimetals, are theoretically predicted to show a novel type of disorder-driven quantum phase transition. At a critical disorder strength, the system passes from a semimetal phase to a diffusive metal phase. Crucially, these theoretical predictions rest on the assumption that the disorder is of potential type, i.e., a spatially modulated energy offset for the two bands crossing at the Weyl node. However, there is no symmetry reason restricting realistic disorder to the potential case. In this Rapid Communication, the authors generalize the above setup to include generic spatially dependent couplings of the two bands, which can be interpreted as a random vector potential. Using a variety of analytical and numerical methods, the authors discover a phase diagram controlled by the number of independent components of the disorder vector. Surprisingly, no phase transition into a diffusive metal phase could be found for disorder of the single Pauli-matrix type. In contrast, all other forms of vector disorder are capable of driving a transition.

[Phys. Rev. B 94, 220202(R)] Published Mon Dec 05, 2016

]]>A quantum vortex liquid theory for composite Fermi liquid (CFL) states in the lowest Landau level is proposed. Contrary to the traditional Halperin-Lee-Read approach, the composite fermions in this formulation are vortices carrying no physical charge. Furthermore, a Berry phase ${\varphi}_{B}$=-2$\pi \phantom{\rule{0}{0ex}}\nu $ is enclosed by the composite Fermi surface, where $\nu $ is the total filling fraction. This vortex liquid theory is closely related to Read’s formulation of bosonic CFL at $\nu =1$ in the 1990’s, and to Son’s recent formulation of the half-filled Landau level in terms of Dirac composite fermions. Various consequences of this new picture are studied, including some nontrivial predictions for transport properties, and an emergent particle-hole symmetry for bosons at $\nu =1$.

[Phys. Rev. B 94, 245107] Published Mon Dec 05, 2016

]]>Exciton-polariton condensates have been realized by illuminating a microcavity system containing 2D quantum wells. When the polariton scattering rates exceed polariton lifetimes, the polariton condensate states can be described microscopically using equilibrium statistical mechanics. This work describes a fully microscopic mean-field theory of equilibrium polariton condensates that treats quantum well band states explicitly and goes beyond the commonly used model in which excitons are treated as Bose particles that are coupled to cavity photons. The authors compare their results with the predictions of simplified bosonic theory, and explain the quantitative differences in condensate properties. Their theory predicts that effective polariton-polariton interaction strengths are weaker and that exciton-fractions in the condensate are smaller than in the simplified exciton-photon model, and that the effective Rabi coupling strengths depends on detuning. The quasiparticle bands that appear in their theory are dressed by both electron-electron and electron-photon interactions, with the electron-electron contribution dominating even when the photon fraction of the condensate is relatively large. The properties of the dressed bands suggest novel mechanisms for the electrical manipulation of polariton condensates.

[Phys. Rev. B 94, 235302] Published Fri Dec 02, 2016

]]>In the early 1950s, Bohm and Pines (BP) developed the random-phase approximation (RPA) theory and revealed the plasmon as an emergent collective mode in metals. In this paper, for the first time ever since the original BP theory, a novel collective mode is predicted to emerge in the dilute electron liquid. The electronic compressibility of this mode is negative and thus dielectric catastrophe appears, predicted by a theory that goes far beyond RPA and fully satisfies various sum rules, constraints, and asymptotic behavior. While the plasmon occurs by the coherent excitations of mutually uncorrelated electrons and holes, the new mode is associated with the excitations of a large collection of excitons (or tightly bound electron-hole pairs). Mathematically, this excitonic collective mode is intimately connected with a newly discovered singularity in the dielectric function in the electron liquid. This collective mode should add a new dimension to plasmonics in nanotechnology in the future.

[Phys. Rev. B 94, 245106] Published Fri Dec 02, 2016

]]>The Kondo effect refers to a localized spin impurity that is screened by conduction electrons at low temperatures, leading to entanglement and nontrivial transport properties. Connecting zero-energy Majorana bound states of a charge-quantized superconducting island to $M$ normal metallic leads realizes exotic SO($M$) Kondo models with mesoscopic systems. In the Coulomb-blockade regime, the Majorana fermions lead to a family of non-Fermi-liquid fixed points and symmetric correlated transport between the leads, dubbed the topological Kondo effect (TKE). This work brings light to the resonant limit, where the charging energy selects two consecutive macroscopic charge states. The interplay between the charge states and the Majorana fermions allows for an exact mapping to the interacting multichannel Kondo model (MCKM). The quantum Brownian motion analogy proves the existence of a continuously varying line of fixed points between the MCKM and the TKE, characterized by fractional conductance, depending on the electron-electron interactions in the leads.

[Phys. Rev. B 94, 235102] Published Thu Dec 01, 2016

]]>Multiferroic hexaferrites have been extensively investigated because they exhibit ferroelectricity associated with noncollinear magnetic orders in relatively low magnetic field and high temperature regions. To understand the static and dynamical spin-polarization coupling in such systems, it is necessary to determine the magnetic structures and spin-wave modes. This is impossible for polycrystalline samples. Here, the authors have successfully grown single crystals of BaSrCo${}_{2}$Fe${}_{12}$AlO${}_{22}$ by means of a laser floating zone technique. They perform magnetization, electric polarization, and neutron diffraction measurements to establish the magnetic phase diagram. Several interesting features are seen and the new understanding should provide a useful guide for developing multiferroic-based functional devices working at room temperature.

[Phys. Rev. B 94, 195154] Published Wed Nov 30, 2016

]]>The optical properties of graphene are among its remarkable characteristics. Measurements have found that the opacity of a single graphene layer for infrared and visible light is simply equal to $\pi $ times the fine-structure constant. Explanations of this beautiful relation have tended to view it as a consequence of the material’s Dirac-like band structure. However, in this paper the authors emphasize that the interband absorption rate must be treated by using the underlying nonrelativistic Hamiltonian, not the quasimomentum state label of the bands. They employ both a straightforward model and a tight-binding calculation to show that the opacity of graphene follows not from its celebrated linear dispersion relation but from the fact that it is a two-dimensional material. The same characteristic is therefore present in semiconductor membranes (as revealed by recent experiments) and other two-dimensional systems. This work focuses attention on the universality that reduced dimensionality bestows on optical absorption.

[Phys. Rev. B 94, 205439] Published Wed Nov 30, 2016

]]>Spiral spin orders often produce various fascinating phenomena via interaction with electronic structures. Examples include, but are not limited to, multiferroicity and skyrmion formation. Among them, a deformed spiral spin structure, which can be viewed as a periodic sequence of 2$\pi $ spin-rotation kinks, can exhibit the functionality of information transmission owing to its phase coherence. This is the chiral magnetic soliton lattice (CSL). Here, the authors have succeeded in building a spin spiral in a thin film of the chiral magnet FeGe, and revealed its characteristic magnetic responses. Magnetic anisotropy and boundary conditions inherent to the epitaxial thin film secure the alignment of the spiral winding direction along the film normal, even under external magnetic fields. This constraint leads to the anisotropic deformation of the spin spiral with respect to the magnetic field direction. Especially, the CSL, which is realized under an in-plane magnetic field, transforms with field between states with different numbers of kinks. The authors clearly demonstrate the deformation by direct observation of the spiral propagation vector by use of the small-angle neutron scattering technique.

[Phys. Rev. B 94, 184432] Published Mon Nov 28, 2016

]]>The already overcrowded phase diagram of cuprate superconductors has recently accommodated a region characterized by incommensurate charge density waves (CDW), partly overlapping with superconductivity, and included in the pseudogap regime. Although charge order is strengthened by high magnetic fields and competes with superconductivity, its role in Cooper pairing is still unassessed, calling for more complete characterization in different families of cuprates. The authors present an extensive x-ray scattering study of charge order in single layer La-doped Bi2201, and they show it extends up to optimal-doping (OP33K, $p$=0.16) in this family too. The CDW peak sharpens at ${T}_{c}$ and disappears above the pseudogap onset at temperature ${T}^{*}$. They also demonstrate that in the underdoped sample (OD15K, $p$=0.115) the charge density wave has no out-of-plane correlation and has dominant unidirectional nature within the planes.

[Phys. Rev. B 94, 184511] Published Mon Nov 28, 2016

]]>The “parity” anomaly is a potential violation of time-reversal and reflection symmetry that can occur for relativistic fermions coupled to gauge fields and/or gravity in 2+1 dimensional spacetime. It has played a role in condensed matter physics since the work of Haldane and others in the 1980’s. Only recently, as a result of developments in the theory of topological superconductors, has it been understood that the extension of this anomaly to an unorientable spacetime is significant. In the present paper, a systematic description is given of the “parity” anomaly on an unorientable spacetime. This is used to get a fuller understanding of gapped symmetry-preserving boundary states of a topological superconductor and to resolve some questions about the world-volume path integral of M-theory membranes.

[Phys. Rev. B 94, 195150] Published Mon Nov 28, 2016

]]>Slow light has been exploited in optoelectronics to enhance the light-matter interaction, including light absorption and emission, nonlinear processes such as sum-frequency generation and phase modulation, and lasing. Modulated structures that modify the propagation of light can also be tailored to alter the propagation of sound. The question then arises: how can slow high-frequency acoustic waves and slow light in the same device perform with a perspective in optomechanics? This question becomes particularly relevant in view of recent reports of photon lifetimes extended to the millisecond range (lifetimes that are characteristic of phonons) by introducing slow-light effects in microresonators. With this application in mind, this work studies DBR-based GaAs/AlAs microcavities as structures that present both light confinement and slowing-down, depending on whether the laser is tuned resonantly to the cavity or stop-band edge modes. Interestingly, the authors show that for these kind of devices precisely the same happens for acoustic phonons, thus providing a rich playground to investigate confined and slow optomechanical effects. Time-resolved coherent phonon generation experiments using picosecond lasers are reported, showing a strong enhancement of the optomechanical coupling using both confined and also properly designed slow photon and phonon modes. The prospects for the use of these optoelectronic devices in confined and slow optomechanics is addressed.

[Phys. Rev. B 94, 205308] Published Mon Nov 28, 2016

]]>Surface codes represent a promising route towards universal fault-tolerant quantum computation, allowing for the protection of stored quantum states against many types of errors. This theoretical work shows that, in addition, a significantly enhanced versatility in quantum information processing is expected when qubits are encoded into the Majorana bound states in topological superconductor-semiconductor heterostructures. In view of recent experimental evidence for Majorana states in such heterostructures, a two-dimensional network of interacting Majorana states thus may feature key advantages towards the long-term goal of a functional quantum computer. The authors discuss how topologically protected logical qubits in this Majorana surface code architecture can be defined, initialized, manipulated, and read out. The physical ingredients needed to implement such operations are familiar from topologically trivial quantum devices. In particular, by employing quantum interference terms in conductance measurements, composite single-electron pumping protocols, and gate-tunable tunnel barriers, the full set of gates required for universal quantum computation can be implemented.

[Phys. Rev. B 94, 174514] Published Wed Nov 23, 2016

]]>Lead telluride (PbTe) is a widely known thermoelectric material and a narrow-gap semiconductor. The introduction of impurity dopants in this compound has shown to significantly increase the thermoelectric figure of merit, and for certain dopants, to even lead the material to a superconducting ground state. Understanding the physical origin of these enhanced properties and their dependence on the choice of dopant chemistry requires a detailed knowledge of the electronic structure, in particular its evolution with carrier concentrations. With this in mind, the authors of this paper present a combined experimental and first-principles study of the evolution of the low-temperature Fermi surface of Na-doped PbTe. Their Shubnikov–de Haas measurements allowed the determination of the full morphology of the Fermi surface for hole concentrations one order of magnitude higher than other previous studies in this material, showing that the band dispersion is consistent with a nonparabolic single-band Kane-model dispersion, in which the hole-pockets are ellipsoids of fixed anisotropy throughout the band, but the effective masses depend strongly on Fermi energy. Additionally, the authors show that standard density functional calculations underestimate the energy difference between the first two valence-band maxima, since their experimental data is consistent with a single-band Fermi surface over the entire doping range studied.

[Phys. Rev. B 94, 195141] Published Wed Nov 23, 2016

]]>Ferroelectric materials encode information through the displacement of atoms within the unit cell, but the ultimate speed limits governing how fast the polarization can change and how it responds to electric fields remain largely unknown. These processes in turn fundamentally determine the functionality of ferroelectric materials within devices. Here, the authors show that single-cycle light pulses at terahertz frequencies can be used as an ultrafast electric field to drive large amplitude rotations of the ferroelectric polarization on picosecond time scales within the prototypical ferroelectric BaTiO${}_{3}$, probed by femtosecond x-ray scattering to directly resolve the motions of the atoms. These measurements are connected directly to first-principles molecular dynamics simulations of ultrafast electric-field-driven effects in ferroelectrics with good agreement obtained between experiment and theory.

[Phys. Rev. B 94, 180104(R)] Published Tue Nov 22, 2016

]]>Ferroelectric control of the electronic and magnetic properties of a correlated oxide provides new opportunities for fundamental science and practical device applications. However, the exploding interest in ferroelectric control of magnetic interfaces, which typically happens in a few nanometers, has been inhibited by the lack of appropriate characterization techniques. Here, the authors have used polarized neutron reflectivity (PNR), a nondestructive yet powerful technique, to directly probe the evolution of the interfacial magnetism at the interface between ferromagnetic La${}_{0.8}$Sr${}_{0.2}$MnO${}_{3}$ and ferroelectric PbZr${}_{0.2}$Ti${}_{0.8}$O${}_{3}$. Using PNR, the authors find that the orientation of the ferroelectric polarization of the ferroelectric layer critically determines the interfacial magnetism, which occurs within a few nanometers of the interface. When the polarization is oriented towards the manganite layer, it is capable of enhancing the interfacial magnetization above the bulk region of the film. This finding not only proves the ferroelectric field effect control of magnetism, but also emphasizes the necessity of separating bulk properties from interfacial phenomena in magnetoelectric materials.

[Phys. Rev. B 94, 174432] Published Mon Nov 21, 2016

]]>The spinel aluminates ($A$Al${}_{2}$O${}_{4}$) are physical realizations of the diamond-lattice antiferromagnet, a model system wherein competing nearest- (${J}_{1}$) and next-nearest- (${J}_{2}$) exchange interactions frustrate order and can lead to novel magnetic ground states, including a “spiral spin liquid” (SSL). CoAl${}_{2}$O${}_{4}$ has been studied intensely as a SSL candidate, and exhibits unusual low-temperature short-range correlations that have been interpreted as arising from unconventional spin-glass behavior or antiferromagnetism, possibly with SSL correlations, but with no clear consensus to date. It has been suggested that the tendency towards “cation inversion” disorder inhibits long-range order. Here, the authors provide a comparative neutron scattering study of single crystals of the two spinel aluminates CoAl${}_{2}$O${}_{4}$ and MnAl${}_{2}$O${}_{4}$. Despite the fact that MnAl${}_{2}$O${}_{4}$ has greater cation inversion, it clearly behaves as a classical Néel antiferromagnet. In contrast, CoAl${}_{2}$O${}_{4}$ exhibits a freezing transition at ${T}^{*}$ = 6.5 K into a phase characterized by finite-sized domains separated by a series of sharp walls. The results are consistent with kinetically inhibited domain growth, which can arise naturally in a frustrated magnet containing a first-order phase transition. The ratio ${J}_{2}$/${J}_{1}$ is close to the value where theory predicts such a transition. The contrast with MnAl${}_{2}$O${}_{4}$ shows that frustration arising from competing interactions, and not disorder, is the driving factor in determining the low-temperature magnetic state in CoAl${}_{2}$O${}_{4}$.

[Phys. Rev. B 94, 184422] Published Thu Nov 17, 2016

]]>Quantum transport in nanosystems is often characterized by strong coupling between electronic and vibrational degrees of freedom. Examples include single-molecule junctions, nanoelectromechanical systems, and suspended carbon nanotubes. Electronic-vibrational coupling manifests itself in vibronic structures in the transport characteristics and results in a multitude of nonequilibrium phenomena, such as current-induced local heating and cooling, multistability, switching, hysteresis, and decoherence. The theoretical study of quantum transport, in particular in the strong-coupling regime, requires nonperturbative approaches that can be systematically converged, i.e., numerically exact methods. In this work, the authors outline how the hierarchical quantum master equation approach, which generalizes perturbative master equation methods by including higher-order contributions as well as non-Markovian memory, can be used in this context. The results show that vibrational nonequilibrium effects are important in a broad spectrum of scenarios, which range from the nonadiabatic to the adiabatic regime and include both resonant and off-resonant transport.

[Phys. Rev. B 94, 201407(R)] Published Tue Nov 15, 2016

]]>Recently, a number of experimental methods have been found by which magnetic domains can be controlled and made to propagate through a host material. Theoretical schemes have also been devised to cleverly manipulate magnetic domains in space in order to enable new logic and memory devices with improved performance. Efforts to better understand the physics of domain motion have revealed that the interactions involved in driving this phenomenon are rich and numerous. The magnetic domains studied in this work form in a GaAs quantum well 2D electron system in a nonequilibrium regime of a correlated state of matter called a fractional quantum Hall liquid. In this exotic medium, ferromagnetic and nonmagnetic domains can be created and made to propagate when the system is excited by a current. This propagation is imaged here in real time using photoluminescence microscopy. The speed of the domains is measured and found to depend on several factors, including the strength of nuclear polarization in the system, as probed by nuclear magnetic resonance. This finding reveals the surprisingly significant impact of the hyperfine interaction on magnetic domain dynamics in a semiconductor, in which noticeable hyperfine effects are uncommon.

[Phys. Rev. B 94, 201408(R)] Published Tue Nov 15, 2016

]]>Recently, there is widespread interest in odd-parity (e.g., $p$-wave) superconductivity in strongly spin-orbit-coupled materials, such as doped topological insulators. Cu${}_{x}$Bi${}_{2}$Se${}_{3}$ is a prime example. A series of recent experiments, including NMR, specific heat, magnetoresistance, and torque magnetometry, has provided mounting evidence of odd-parity spin-triplet superconductivity in Cu${}_{x}$Bi${}_{2}$Se${}_{3}$ as well as Sr${}_{x}$Bi${}_{2}$Se${}_{3}$ and Nb${}_{x}$Bi${}_{2}$Se${}_{3}$. Here, the authors construct a general theory of such degenerate superconducting components and the competition between chiral and nematic states. They derive a general criterion to establish which state is energetically favored, which highlights the crucial role of spin-orbit coupling. In particular, when applied to the specific case of Cu${}_{x}$Bi${}_{2}$Se${}_{3}$, the theory is consistent with three key experimental findings: a full pairing gap, triplet pairing, and spontaneous rotational symmetry breaking. Remarkably, both nematic and chiral superconductors with odd-parity pairing symmetry are topological superconductors, belonging respectively to time-reversal-invariant (class DIII) and time-reversal-breaking (class D) categories of the topological classification. The authors show that due to the nonunitary nature of chiral pairing in spin-orbit coupled materials, spin nondegenerate point nodes realize Majorana fermion quasiparticles in three dimensions.

[Phys. Rev. B 94, 180504(R)] Published Thu Nov 10, 2016

]]>Logarithmic negativity is a proper measure of quantum entanglement in mixed states. Originally proposed in the context of quantum information, in recent years it is attracting interest in quantum field theory, condensed matter physics, and statistical physics. The key object to construct the negativity is the partial transpose of the reduced density matrix. Here, focusing on two adjacent intervals, the authors provide a complete study of the distribution of the eigenvalues of the partial transpose (negativity spectrum) for gapless one-dimensional systems described by a conformal field theory (CFT). Similar to the entanglement spectrum, the distribution of the negativity spectrum is universal, and it depends only on the central charge of the underlying CFT. The precise form of the spectrum depends on whether the two intervals are in a pure or mixed state, and in both cases, a dependence on the sign of the eigenvalues is found. This dependence is weak for bulk eigenvalues, whereas it is strong at the spectrum edges. The authors also investigate the scaling of the smallest (negative) and largest (positive) eigenvalues of the partial transpose. The analytical results are thoroughly checked against numerical DMRG simulations as well as in exactly solvable models.

[Phys. Rev. B 94, 195121] Published Thu Nov 10, 2016

]]>The spectral functions of strongly correlated metals, with the cubic perovskite SrVO${}_{3}$ as a prototype, are characterized by the presence of satellite structures below and above the main quasiparticle peak. These satellites have been explained as Hubbard bands, whose energy separation is determined by the Hubbard $U$. Here, the authors show that this commonly accepted interpretation, based largely on LDA+DMFT calculations, has to be reconsidered. An important ingredient missing in these calculations is the nonlocal screening and the nonlocal self-energy. Using a truly first-principles and fully self-consistent $G\phantom{\rule{0}{0ex}}W$+extended dynamical mean-field theory calculation, the authors find that the screening from nonlocal Coulomb interactions substantially reduces the effective local Coulomb repulsion, while vertex corrections beyond $G\phantom{\rule{0}{0ex}}W$ provided by DMFT lead to strong plasmonic effects and a reduction in the plasmon energy. The resulting effective local interactions are too weak to produce pronounced Hubbard bands. Instead, prominent plasmon satellites appear at energies corresponding to the experimentally observed sidebands. The new results demonstrate the important role of nonlocal interactions and dynamical screening in determining the effective interaction strength of correlated compounds and point to the need of revising the current view on the electronic structure of strongly correlated metals.

[Phys. Rev. B 94, 201106(R)] Published Thu Nov 10, 2016

]]>Due to their small mass and large mechanical stiffness, free-standing carbon nanotubes turn out to be superb high-frequency mechanical resonators that could find applications in mass and force sensing. With resonance frequencies up to the gigahertz regime, one can envision cooling the mechanical motion of such nanotubes to its quantum mechanical ground state at cryogenic temperatures. This opens the possibility to generate highly nonclassical states of the mechanical motion, similar to those created several years ago with electromagnetic radiation in superconducting microwave resonators. Such states are interesting from a fundamental physics point of view, but potentially also for quantum information and sensing applications. While classical activation of a harmonic oscillator will always produce “quasiclassical” coherent states, a necessary ingredient for the production of nonclassical states such as Fock states is a nonlinear element, e.g., superconducting qubits in the case of microwaves. This work predicts that for mechanical oscillations, the spin-orbit coupling of the mechanical motion to a single localized electron spin can do the trick by acting as a nonlinear element for phonons.

[Phys. Rev. B 94, 205413] Published Thu Nov 10, 2016

]]>Experimental demonstration of a truly quantum mechanical solid-state heat engine or refrigerator remains elusive up to now. Here, a four-stroke quantum refrigerator that can be realized using a superconducting qubit coupled alternately to two resonators with embedded reservoirs is analyzed theoretically. Besides being a practically feasible thermal machine, this concept presents a number of fundamentally interesting features. At high frequencies, the system exhibits nearly coherent dynamics, leading to quantum oscillations in cooling power versus operation frequency. At intermediate frequencies, an ideal Otto cycle can be realized. In the nearly adiabatic regime, the cooling power is quadratic in frequency and can be separated to classical and quantum contributions. Interestingly, it is shown here that quantum coherence leads always to performance that is inferior as compared to that arising from purely classical dynamics. Furthermore, in the nearly adiabatic regime the efficiency of the refrigerator exceeds clearly that of the standard Otto cycle.

[Phys. Rev. B 94, 184503] Published Wed Nov 09, 2016

]]>In regular semiconductors each electronic state is occupied by spin-up and spin-down electrons. It is well known that inversion symmetry breaking at a surface may lift this degeneracy by spin-orbit coupling (the Rashba effect). As already proposed by Rashba, bulk inversion symmetry breaking may cause a similar degeneracy lifting of bulk states. Ferroelectric materials provide such an inversion symmetry breaking. Thin ferroelectric $\alpha $-GeTe(111) films grown on Si(111) show a stable ferroelectric polarization with a polarization vector perpendicular to the surface that can be switched by an electric field into a metastable state with opposite polarization. Spin-resolved time-of-flight momentum microscopy reveals a Rashba-type spin splitting of the valence band caused by the inner electric field. Due to orbital polarization, the outer Rashba branch has a larger polarization than the inner branch. The corresponding net spin polarization provides new functionalities in spintronic devices, e.g. it may strongly affect the conversion process of spin into current via the spin-galvanic effect.

[Phys. Rev. B 94, 201403(R)] Published Wed Nov 09, 2016

]]>Complex interactions in quantum magnets lead to a variety of phases with manifold correlations, which are measurable at temperatures very low compared to the energy scale of the system. The common expectation is that quantum effects are just suppressed upon increasing temperature due to the additional thermal fluctuations. However, recent inelastic neutron scattering investigations in one- and three-dimensional compounds indicate that interplay between quantum and thermal effects is much more complicated. By combining high-resolution neutron resonance spin-echo measurements with diagrammatic perturbation theory, the authors show that anomalous decoherence effects can also be observed in the time domain, the natural domain for decaying correlations. This offers a complementary view on this phenomenon, allowing for a direct comparison between experimental data and theoretical prediction without the need for intensive data analysis. The specific form of the echo amplitude, in combination with the temperature dependence of the spectral weight, provides a smoking gun signature for nontrivial scattering processes of hard-core bosonic excitations.

[Phys. Rev. B 94, 180404(R)] Published Tue Nov 08, 2016

]]>Due to their unique balance of energy scales, correlated iridates have proven a fruitful materials platform for both the prediction and discovery of numerous novel electronic phases and ground states. The comparable strengths of spin-orbit coupling, on-site Coulomb interactions, and local crystal-field splitting inherent to the iridium cations hosted within various lattice geometries has led to the realization of a novel form of spin-orbit assisted Mott insulating ground state, where the iridium’s spin and orbital angular momenta become entangled into a ${J}_{\mathbf{\text{eff}}}$=1/2 wave function. In this paper, the stability of this state with respect to chemical pressure is explored in the prototypical spin-orbit assisted Mott insulator Sr2IrO${}_{4}$. This study reveals that isovalent substitution of smaller Ca${}^{2+}$ onto Sr${}^{2+}$ sites in this system drives the lattice to evolve through three distinct structural phases with increasing Ca${}^{2+}$ content: evolving from the parent quasi-two-dimensional $n$=1 Ruddlesden-Popper structure, to a novel three dimensional corner sharing IrO${}_{6}$ octahedral network to a quasi-one dimensional IrO${}_{6}$ edge-sharing chain structure. All three phases remain insulators reliant on the inherently strong spin-orbit coupling of Ir. However, their magnetic properties evolve dramatically across the phase diagram. The work here provides a global view of the evolution of the structural and electronic properties of Sr2IrO${}_{4}$ as its lattice is destabilized via chemical pressure.

[Phys. Rev. B 94, 195115] Published Tue Nov 08, 2016

]]>The transmission electron microscope (TEM) is a powerful tool for structural or chemical characterization of materials with atomic, or even subatomic, spatial resolution. Recent developments in producing electron vortex beams carrying orbital angular momentum, spin polarization technology, and aberration correction technology have opened up new doors to image magnetism with high resolution in the TEM. The development of such methods requires a thorough understanding of the effects of magnetic interactions in electron scattering processes. The authors of this paper use a full-fledged description of elastic electron scattering, including magnetic effects, to computationally identify possible magnetic signals from an electron beam scattered through a magnetic solid. Three types of magnetic fingerprints, namely one originating from the orbital angular momentum of electron vortex beams, one from spin polarized beams, and one from the nontrivial phase distributions of aberrated electron beams, are found. The possibility of atomic resolution mapping of magnetism by scanning TEM is investigated and deemed in principle possible, but experimentally challenging, while nanometer resolution magnetic imaging using electron vortex beams is proposed as a realistic goal for future experimental work.

[Phys. Rev. B 94, 174414] Published Mon Nov 07, 2016

]]>Quantum transport of electrons through ultraclean two-dimensional cavities depends sensitively on the geometry. In scanning gate microscopy, the charged tip is a valuable tool to tune the shape of a cavity while measuring its conductance, and thus to address questions of quantum chaos, such as the relevance of the underlying classical electron dynamics. For the tip placed in the center of an integrable cavity of circular shape, a remarkable nonmonotonic variation of the conductance is measured as a function of the voltage applied to the tip. Numerical simulations yield conductance oscillations with tip strength that agree well with the experimental findings. A semiclassical analysis of the conductance, based on the classical trajectories transmitted through the cavity, reproduces the conductance oscillations, proving that the effect is of classical origin. Remarkably, only particular classes of short trajectories are responsible for the effect within this semiclassical analysis. When the strength of the tip-induced potential is increased, the character of the dominant trajectories alternates between transmitted and reflected ones, thereby providing an unusually simple explanation for the observed phenomenon.

[Phys. Rev. B 94, 195304] Published Mon Nov 07, 2016

]]>The layered transition-metal dichalcogenide MoS${}_{2}$ as well as related compounds, such as WS${}_{2}$ and WSe${}_{2}$, undergo a transition from an indirect to a direct band-gap semiconductor when thinned down to a single layer. This unexpected observation, which goes along with substantial changes in electronic and optical properties, has stimulated numerous recent studies on this compound class, including pump-probe time-domain studies addressing fundamental aspects of hot carrier relaxation. So far, mainly all-optical methods have been applied to probe the ultrafast dynamics in a rather comprehensive manner. Due to intermixing of different processes and a lack of momentum-resolution, the interpretation of such data is, however, not necessarily straightforward. In order to complement these studies, the authors present here results of a time- and angle-resolved photoemission (trARPES) experiment on bulk MoS${}_{2}$, a technique that provides the most direct view onto hot electron dynamics in energy and momentum space. Surprisingly, the new data agree well with past results of monolayer MoS${}_{2}$ as opposed to bulk MoS${}_{2}$. The authors interpret this finding in terms of the high density of surface defects in MoS${}_{2}$ samples, which substantially affect hot carrier relaxation due to trapping. In contrast to all optical studies, trARPES is very efficient in probing such processes owing to the extreme surface sensitivity of photoemission.

[Phys. Rev. B 94, 205406] Published Mon Nov 07, 2016

]]>GdFeCo ferrimagnetic films have the unique property of responding with a full reversal of the magnetization following irradiation of a single femtosecond laser pulse. The switching of the magnetization happens within a picosecond. Referred to as “all-optical switching,” the mechanism has been attributed to the rapid heating of electrons to extremely high peak temperatures and to the existence of two different magnetic sublattices. In this work, the authors demonstrate that all-optical switching is possible for pulses up to 15 ps in duration, where the electron peak temperatures are much more modest, barely reaching the Curie temperature. This result was obtained by using static and time-resolved experiments to access the critical switching fluences and the magnetization dynamics for various pulse durations and initial temperatures.

[Phys. Rev. B 94, 184406] Published Fri Nov 04, 2016

]]>First proposed in the context of elementary particle physics, supersymmetry is an exotic symmetry that relates bosons and fermions but has not yet been observed in nature. While the traditional expectation is that supersymmetry would be first observed in a high-energy physics experiment, an alternative possibility stressed recently is that supersymmetry could be realized as an emergent symmetry at long wavelengths and low energies in interacting many-particle systems. More specifically, it was suggested that supersymmetry emerges at the quantum phase transition between the gapless Dirac semimetal and a gapped superconductor on the surface of a 3D topological insulator. The authors of this paper provide additional support for the conjecture by performing a three-loop renormalization group calculation and showing that the supersymmetric fixed point in 2+1 dimensions remains stable to that order in perturbation theory.

[Phys. Rev. B 94, 205106] Published Thu Nov 03, 2016

]]>The Ruddlesden Popper series Sr${}_{n+1}$Ru${}_{n}$O${}_{3+n+1}$ contains highly interesting physical phenomena depending on the number of RuO${}_{2}$ layers in the unit cell, spanning from unconventional superconductivity for the $n$=1 member Sr${}_{2}$RuO${}_{4}$ to itinerant ferromagnetism in the infinite-layer system SrRuO${}_{3}$. Obviously, the unique interplay between electronic, spin, and lattice degrees of freedom in this group of correlated itinerant electron systems leads to a tremendous change of the physical properties. This is also demonstrated by anisotropic and orbital-dependent magnetism and a strong spin-phonon coupling found in many members within the series. This paper focusses on triple-layer Sr${}_{4}$Ru${}_{3}$O${}_{10}$, which becomes ferromagnetic at ${T}_{c}$$\sim $105 K and shows a metamagnetic transition below ${T}^{*}$$\sim $50 K for a magnetic field along the $a\phantom{\rule{0}{0ex}}b$ plane. The authors investigate the magnetoelastic properties of this compound as a function of temperature and magnetic field. The high-resolution thermal expansion and magnetostriction measurements indicate a highly anisotropic response with a specific rearrangement of the crystal lattice in fields parallel and perpendicular to the $c$ axis. Finally, they reconstruct the $T$-$H$ phase diagram, which shows a new feature suggestive of the multiple-orbital character of Sr${}_{4}$Ru${}_{3}$O${}_{10}$, where both the ferromagnetic (${d}_{x\phantom{\rule{0}{0ex}}y}$) and the metamagnetic orbitals (${d}_{x\phantom{\rule{0}{0ex}}z;y\phantom{\rule{0}{0ex}}z}$) coexist and interact.

[Phys. Rev. B 94, 155154] Published Mon Oct 31, 2016

]]>Angle-resolved photoemission spectroscopy (ARPES) is widely used to investigate topological states and their spin textures in topological materials. Here, the authors examine the role of the ARPES matrix element in laser-ARPES spectra, and delineate how imprints of the Dirac states and their spin textures are encoded in these spectra in terms of the character and symmetry of the underlying initial and final states. For this purpose, they collect laser-ARPES spectra from the exemplar topological insulator material Bi${}_{2}$Te${}_{3}$ over a wide range of momenta in the ${k}_{x}$-k${}_{y}$ plane at energies ranging from 5.57–6.70 eV for two different linear polarizations of the incident light. The experimental results are analyzed via first-principles fully relativistic calculations of ARPES intensities within the framework of the one-step model of photoemission. These include effects of the ARPES matrix element from a semi-infinite solid surface. The authors obtain new insight into the nature of the laser-ARPES spectra from topological materials, and show how these spectra contain fingerprints of the initial state ${k}_{z}$ dispersions and spin textures of the Dirac-cone states, and how laser-ARPES could open a previously unrecognized window on the presence of delicate gaps in the final-state spectra.

[Phys. Rev. B 94, 155144] Published Thu Oct 27, 2016

]]>Given the extreme faintness of Hawking radiation from astrophysical black holes, researchers are actively pursuing the experimental quest of its analogs in condensed-matter and optical systems. This work reports a detailed theoretical study of different aspects of analog Hawking physics in a flowing fluid of exciton-polaritons in a semiconductor microcavity. Previous experimental work has demonstrated the relative ease to generate a black-hole horizon for collective sound waves in this system. Here, the authors propose realistic schemes to experimentally observe stimulated and spontaneous Hawking radiation effects, and they characterize their efficiency in view of forthcoming experiments. In particular, the study illustrates how complex shapes of the effective analog metric are naturally realized in this system and how this results in peculiar new features in the Hawking emission. The study also pinpoints the potential of quantum fluids of light for future studies and highlights the novelties and the experimental advantages of using driven-dissipative systems over conservative ultracold atomic gases.

[Phys. Rev. B 94, 144518] Published Wed Oct 26, 2016

]]>Electrons in low-dimensional materials sometimes reorganize themselves with a new translational symmetry, which gives rise to an energetically more favorable ground state. This phenomenon, called a charge density wave (CDW) phase, is often the result of the coupling of electrons to phonons. In the quasi-two-dimensional material TiSe${}_{2}$, the CDW occurring at low temperature is unusual, since it appears to be driven by strong electron-hole correlations, which in turn couple to phonons. In this work, the authors use time- and angle-resolved photoemission spectroscopy to investigate the peculiar nature of the CDW phase in TiSe${}_{2}$.Depending on the temperature at which measurements are performed, they observe that the response of the material is either strongly coupled to the lattice or dominated by its electronic degrees of freedom. This selective response is enhanced by tuning the pump pulse wavelength to specific optical resonances. They conclude that their measurements do indeed confirm a combined scenario, for which both the electron-hole correlations and the coupling to phonons are essential to the origin of the CDW phase in TiSe${}_{2}$.

[Phys. Rev. B 94, 165165] Published Tue Oct 25, 2016

]]>Incommensurate compounds consist of two or more interpenetrating sublattices with lattice periods incommensurate along one or more crystal axes. Intriguing lattice dynamics have been predicted for these compounds and are relevant to not only the thermal properties but also electronic, optical, and magnetic properties via coupling between different energy excitations. Here, the authors report inelastic neutron scattering measurements of phonon and magnon dispersions in Sr${}_{14}$Cu${}_{24}$O${}_{41}$, which contains incommensurate chain and ladder substructures. Two distinct acoustic phononlike modes, corresponding to the sliding motion of one sublattice against the other, are observed for atomic motions polarized along the incommensurate axis. In the long-wavelength limit, it is found that the sliding mode shows a remarkably small energy gap, indicating very weak interactions between the two incommensurate sublattices. The measurements also reveal an appreciable contribution from gapped magnons to the low-temperature specific heat, as well as a gapped and steep linear magnon dispersion of the ladders. The high group velocity of this magnon branch and weak coupling with acoustic phonons can explain the large magnon thermal conductivity in Sr${}_{14}$Cu${}_{24}$O${}_{41}$ crystals. These findings offer new insights into the phonon and magnon dynamics and thermal transport properties of incommensurate magnetic crystals that contain low-dimensional substructures.

[Phys. Rev. B 94, 134309] Published Fri Oct 21, 2016

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