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

]]>Lattice models are widely used to represent the statistical mechanics of condensed phases for the study of alloy thermodynamics, phase transitions, magnetic properties, fluid mechanics, and other applications. A longstanding problem has been the identification of the true global ground states of a lattice model, which is challenging for general models, and only a limited number of solutions for highly simplified systems are known. Here, a practical and general algorithm is presented that allows to determine provable periodically constrained ground states of complex lattice models up to a given unit cell size and in most cases is able to prove global optimality over all possible choices of unit cells. This universal algorithm is developed by reformulating the ground-state problem in terms of maximum satisfiability (MAX-SAT) and convex optimization, which results in both provable accuracy and sufficient numerical efficiency to handle systems of physically relevant complexity. Considering that lattice-model ground states are conventionally solved using simulated annealing or genetic algorithms, techniques that are often unable to find the true global minimum and provide no handle to prove the optimality of their results, this work promises to resolve the longstanding uncertainties in lattice models of important physical phenomena.

[Phys. Rev. B 94, 134424] Published Fri Oct 21, 2016

]]>Topological insulators have been attracting much attention as potential candidates for spintronic applications. Spin-momentum-locked surface states can lead to spin current naturally accompanied by electric charge current. As a way to control the current, circularly polarized photon injection has been widely used as the photon can excite spin-polarized carriers selectively depending on the photon’s helicity. Here, the authors investigate the helicity-dependent photocarrier dynamics in an unbiased Bi${}_{2}$Se${}_{3}$ thin film by monitoring the terahertz pulse emitted from the sample after illumination by femtosecond laser pulses. Although all the measurements were performed on an unbiased sample with no electrical contacts, the authors observe three-fold periodic helicity-dependent terahertz emission, and attribute such circular anisotropy in the photocurrent to the circular photon drag effect, namely linear and angular momentum transfer from photons to photocarriers.

[Phys. Rev. B 94, 161405(R)] Published Thu Oct 20, 2016

]]>Classification of symmetry-protected topological phases such as the topological superconductor (TSC) is well known in the free fermion picture. Recently, it was observed that including many-body interactions that preserves symmetry gives rise to novel surface phases and breaks down the classification scheme. A complete understanding of the microscopic theory for such phases is missing. This paper is an attempt to understand the interaction that gives rise to these phases in the case of the three-dimensional TSC. The authors build a coupled wire model Hamiltonian for the gapless surface and show that the addition of a time-reversal symmetric two-body interaction, constructed in this paper, opens an energy gap in the spectrum. They also show this gapped surface phase is topologically nontrivial and is classified differently compared to the noninteracting case.

[Phys. Rev. B 94, 165142] Published Wed Oct 19, 2016

]]>Weyl semimetals make up a novel three-dimensional topological phase of matter. Among their most fascinating properties is the existence of a very peculiar type of surface bound state. Contrary to common topological materials, the surface Fermi surface of a Weyl system consists of disconnected arcs – the so-called Fermi arcs – instead of a closed surface. These Fermi arcs connect the projected position of band crossings in their bulk band structure. The Weyl phase typically arises from a symmetry-protected topological insulator upon spontaneously breaking one of the protecting symmetries. In the case of the pyrochlores, the magnetic momenta inside the constituting Ir tetrahedra break time-reversal symmetry, so the pyrochlores are considered magnetic Weyl semimetals. Here, the authors show theoretically that the nontrivial spin polarization of the surface Fermi arcs of a magnetic Weyl semimetal provide characteristic feature observable via spin-resolved scanning tunneling microscopy (STM). They analyze the Fourier transform (FT) of Friedel oscillations in the local density of states occurring due to scattering by localized magnetic and nonmagnetic impurities. They note that the FT features inherit traces of the disconnected nature of the Fermi surface and the spin rotation happening on the Fermi arcs.

[Phys. Rev. B 94, 165146] Published Wed Oct 19, 2016

]]>Magnetic insulators are widely used as prototypes for the study of quantum criticality. Among the more exotic experimental realizations is the $z=1$ Heisenberg quantum critical point (QCP). It hosts asymptotic freedom of quasiparticles, unconventional scaling properties, and excitations analogous to the Higgs boson in high-energy physics. An interesting question is how this quantum phase transition is affected by disorder. Theory predicts a destruction of the quantum critical point and the emergence of a quantum Griffiths phase. In the present work, this issue is addressed experimentally by studying one of the precious few materials that host a pressure-induced Heisenberg QCP, namely the organometallic quantum magnet piperazine hexachlorodicuprate (PHCC). Disorder is introduced by chemical substitution of bromine for chlorine on a nonmagnetic site. Magnetic ordering at low temperatures is then studied as a function of applied hydrostatic pressure for a range of impurity concentrations. The tool of choice is the highly sensitive muon-spin rotation technique. The authors uncover a dramatic effect of disorder on the magnetic phase diagram. In agreement with theoretical expectations, they detect strong magnetic inhomogeneities already for small impurity concentrations, followed by the destruction of pressure-induced magnetic order at high concentrations of impurities.

[Phys. Rev. B 94, 144418] Published Fri Oct 14, 2016

]]>The authors present a new method for variationally optimizing projected entangled-pair states (PEPS) in the thermodynamic limit based on the global energy functional and novel tensor contraction strategies. It is successfully benchmarked using Ising and Heisenberg Hamiltonians. The method is completely independent from previous PEPS implementations and improves on the variational energies and order parameters as compared to other state-of-the-art PEPS simulations. The algorithm presented here should prove valuable in simulating ground-state properties of strongly correlated quantum spin systems, can be used with other variational ansatzes, and can be applied to tensor network states in general.

[Phys. Rev. B 94, 155123] Published Fri Oct 14, 2016

]]>The electronic structure of crystalline materials reflects the influence of underlying discrete symmetries of the atomic lattice. When these symmetries are broken, associated degeneracies in the spectrum of states often split, with important consequences for potential electronic and spintronic applications. The mathematical language of group theory can be used to capture the effect of these symmetries and provide insight into the origins of spectrum features obtained from essentially empirical ab initio numerical calculations such as density functional theory. A salient example comes from the unusual electronic structure of atomically thin two-dimensional “four-six-ene” semiconductors such as tin sulfide, germanium telluride, etc. (related to group-V phosphorene but formed from group IV and VI). In this case, group theoretic methods provide a straightforward framework for understanding the consequences of inversion symmetry breaking due to inequivalent sublattice atomic identity. In particular, the quantum states at the edge of the fundamental band gap – relevant to all transport, optoelectronic, and spintronic properties – are shown to directly inherit their character from nearby points of high symmetry in the reciprocal lattice, where the form of allowable energetic interactions is constrained.

[Phys. Rev. B 94, 155124] Published Fri Oct 14, 2016

]]>Double quantum dots play an important role in the field of semiconductor and carbon nanotube quantum dots. They have resulted in, for example, correlated electron physics beyond the single spin-1/2 Kondo system and the realization of qubits for quantum computation. Using organic molecules to create a double quantum dot can be beneficial due to the larger energy scales present as compared to its more extended counterparts. Furthermore, they open up the possibility to investigate interactions between spins and perform coherent experiments on electrons in organic molecules. In this article, the authors experimentally investigate charge transport through a three-terminal single-molecule junction. The data shows features in transport that arise from the broken conjugation in dihydro-anthracene, which localizes π electrons and creates a set of nearly degenerate orbitals. The ferromagnetic and antiferromagnetic interactions are of the same order of magnitude, so that the singlet and triplet states are close in energy. These findings thus demonstrate a double quantum dot programmed in a single molecule due to its broken conjugation. This research is a starting point for further experiments, whereby with chemical means the interaction between the two dots can be lowered to observe Pauli spin-blockade and investigate spin coherent effects in single molecules.

[Phys. Rev. B 94, 165414] Published Fri Oct 14, 2016

]]>In the helical edge states of a quantum spin Hall (QSH) bar, the phenomenon of electron photoexcitation is quite different from systems commonly used in optoelectronics. The vertical electric dipole transitions typically occurring between valence and conduction bands of conventional semiconductor-based devices are forbidden in QSH edge states due to a selection rule. Also, electrons in QSH edge states are described by a massless Dirac fermion theory rather than the conventional Schrödinger-like parabolic band, so their response to an electromagnetic field is intrinsically affected by a peculiar effect known as the chiral anomaly. Here, the authors analyze this problem in the mesoscopic regime. They show that, when a spatially localized electric pulse is applied at the edge of a QSH system, helical electron wave packets can be photoexcited by purely intrabranch electrical transitions, without invoking the magnetic Zeeman coupling or transitions to bulk states. At the end of the pulse, the wave packets counter propagate with opposite spin orientations, keeping their space profile unaltered. Notably, the authors find that the space profile of the photoexcited electron wave packets is independent of the temperature and chemical potential of the initial equilibrium state. Instead, the profile depends purely on the applied electric pulse, in a linear manner, as a signature of the chiral anomaly. Then authors discuss how the wave packet profile can be tailored by the electric pulse parameters, for both low and finite frequencies.

[Phys. Rev. B 94, 165412] Published Thu Oct 13, 2016

]]>The well known spin-orbit coupling is a peculiar type of interaction that links the direction of electron motion to its intrinsic magnetic moment, called spin. The role of the spin orbit is believed to be particularly important in giving rise to exotic states of matter, such as topological insulators and topological superconductors, that could host, for example, Majorana fermions. Despite several methods of measuring the spin-orbit constant having already been developed, given the rapid progress in the discovery and possible applications of new materials with strong spin-orbit coupling, it is important to provide experimentalists with novel and general approaches to unequivocally measure the value of this coupling. Here, the authors show theoretically that the spin-orbit coupling in two-dimensional and one-dimensional systems, both superconducting and metallic, can be read off directly and unambiguously via spin-resolved scanning tunneling microscopy (STM). The authors analyze the Friedel oscillations in the local density of state occurring due to scattering by localized magnetic impurities. In particular, they focus on the Fourier transform (FT) of the Friedel oscillations and note that the FT features observed in non-spin-polarized measurements split when using spin-polarized STM, and the splitting is exactly equal to twice the inverse spin-orbit length. Moreover, the superconducting systems exhibit extra high-intensity FT features with respect to the metallic systems, at a wavevector given by twice the inverse spin-orbit length. Such features provide a direct measure of the spin-orbit coupling in these materials.

[Phys. Rev. B 94, 134511] Published Wed Oct 12, 2016

]]>Type-II multiferroics –- materials where magnetic order induces ferroelectricty -– offer great potential for novel electronic devices due to the strong intrinsic coupling between order parameters. Such coupling is found in CuFeO${}_{2}$, where ferroelectricity is driven by proper-screw “helical” magnetic ordering for applied magnetic fields between 7.5 and 13 T at low temperatures. Here, the authors report a polarization memory effect in CuFeO${}_{2}$, whereby the nonpolar antiferromagnetic ground state retains a strong memory of the high-magnetic-field ferroelectric phase. The dependence of the polarization on the magnetothermal history is investigated, and it is found that the direction and almost the full polarization magnitude is “remembered” after cycling through the zero-field nonpolar phase. This polarization memory persists in samples in which multiple crystallographic domains are suppressed by applied stress, ruling out crystallographic boundaries as the cause of the observed memory. Instead, the authors propose that the memory is carried by Bloch-type antiferromagnetic domain walls with definite helicity. As similar memory effects have been also been reported for other multiferroics, such as MnWO${}_{4}$ and CuO, the proposed mechanism could be a general feature of phase transitions between collinear-antiferromagnetic and incommensurate magnetic phases.

[Phys. Rev. B 94, 144411] Published Wed Oct 12, 2016

]]>Recent developments in density matrix renormalization group/matrix product state techniques allow today for a quantitative computation of dynamical correlations at finite temperature in low-dimensional quantum spin systems. By combining numerical results and field theoretical asymptotic methods, e.g., bosonization, it is now possible to give a full description of the dynamical quantity one is interested in for the range of temperatures experimentally accessible. This enhances of course the possibility of one-to-one comparisons with experiments without having to resort to uncontrolled approximations. Here, the authors focus on the temperature dependence of the NMR spin-lattice relaxation rate $1/{T}_{1}$ in spin-$\frac{1}{2}$ chain systems. This quantity is directly related to local, dynamical spin-spin correlation functions. The results agree in the low-temperature regime with bosonization predictions, and show interesting deviations from this behavior at intermediate temperatures.

[Phys. Rev. B 94, 144408] Published Tue Oct 11, 2016

]]>Quasi-one-dimensional (1D) antiferromagnets are good candidates to realize a Tomonaga-Luttinger liquid (TLL), describing 1D interacting quantum systems, provided 3D coupling remains small as compared to temperature. In systems such as NiCl${}_{2}$-4SC(NH${}_{2}$)${}_{2}$ (DTN), made of weakly coupled spin $S=1$ chains, interchain coupling plays an important role at low temperature as it leads to antiferromagnetic long-range order below $1$ K, corresponding to a Bose-Einstein condensate of magnons. Using time-dependent numerical simulations at finite temperature, the authors compute here the nuclear magnetic resonance (NMR) relaxation rate $1/{T}_{1}$, a key experimental quantity, and address the question of the low-temperature crossover between TLL predictions and the higher-temperature regime in purely $1$D quantum spins chains. They also comment on the experimental observation of the TLL regime, possibly masked by $3$D long-range order in materials such as DTN.

[Phys. Rev. B 94, 144409] Published Tue Oct 11, 2016

]]>Layered molecular-intercalated iron selenides (FeSe) show great promise as a class of high-${T}_{\mathrm{c}}$ unconventional superconductors, with a ${T}_{\mathrm{c}}$ as high as 45 K reported so far. The FeSe layers in these materials also show structural and electronic similarities to monolayer FeSe on SrTiO${}_{3}$, where ${T}_{\mathrm{c}}$ values up to 65 K have been observed. Information on the superconducting properties of these materials may provide vital insight into the unconventional mechanism of high-temperature superconductivity in iron-based compounds. One property of particular interest is the symmetry of the superconducting gap function on the Fermi surface, which is a key prediction for any theory of superconductivity. However, a precise experimental determination of this has proven elusive in the iron-based superconductors. In this paper, the authors study the LiOD-intercalated FeSe derivative Li${}_{1-x}$Fe${}_{x}$ODFe${}_{1-y}$Se via inelastic neutron scattering, detecting a spin resonance which appears in the superconducting state below the pair breaking energy 2$\mathrm{\Delta}$. This shows that superconductivity in this compound is unconventional and, in conjunction with previous ARPES and STS results, strongly constrains the possible superconducting gap symmetries in this material.

[Phys. Rev. B 94, 144503] Published Mon Oct 10, 2016

]]>Since the discovery of graphene, there has been great interest in finding other nodal semimetals in two and three dimensions. Over the past few years, numerous examples of these materials have been proposed or observed. However, the explanations for the symmetry and topology mechanisms protecting nodal features in these materials have generally been provided on a case-by-case basis. In this paper, we use a mathematical consideration of crystal lattices on flat manifolds like Mquotobius strips and Klein bottles, most recently revived by Watanabe, Po, Vishwanath, and Zaletel, to develop a framework for discussing all possible two-dimensional and quasi-two-dimensional semimetals in systems with strong spin-orbit interactions. Using this framework, we draw connections to known nodal features in two- and three-dimensional semimetals like SrIrO${}_{3}$, and we predict new ones, including an exotic “Cat’s Cradle” Weyl fermion feature.

[Phys. Rev. B 94, 155108] Published Thu Oct 06, 2016

]]>The magnetic ratchet effect is optical rectification in two-dimensional systems – such as bilayer graphene – whereby a steady in-plane magnetic field and the alternating electric field of a laser produce a dc electric current. It occurs in systems with broken inversion symmetry due, say, to an inhomogeneous distribution of impurities, leading to increased electronic scattering on the upper layer of the bilayer. For a given direction of electric field, electrons are driven downwards by the Lorentz force towards the lower layer where scattering is low whereas, when the electric field switches direction, electrons are driven towards the upper layer where scattering is high. Such asymmetry in scattering leads to a nonzero dc current. By deriving linear-in-magnetic-field terms in the low-energy electronic Hamiltonian, the authors predict a very large ratchet effect in bilayer graphene. They compare symmetry breaking due to impurities with interlayer asymmetry due to an external gate, the latter allowing for a tuneable magnetic ratchet.

[Phys. Rev. B 94, 165404] Published Thu Oct 06, 2016

]]>Magnetic resonance spectroscopy is one of the most important tools used across the physical and life sciences, but typically requires macroscopic samples. A key challenge is therefore to develop methods suitable for the nanometric regime. This paper investigates a new approach whereby quantum relaxation of a single nitrogen-vacancy center spin in diamond is monitored to construct a spectrum of the surrounding electron and nuclear spin environment within a nanometric volume. In contrast to existing decoherence-based techniques, the technique is all optical, requiring no complex quantum control, and intrinsically broadband, which means it can probe both electron and nuclear spin resonances. As an illustration, the authors report broadband nanoscale magnetic resonance spectroscopy of nitrogen impurities located within the diamond. This work provides a new route towards magnetic resonance spectroscopy of electrons and nuclear spins at the single molecule level.

[Phys. Rev. B 94, 155402] Published Wed Oct 05, 2016

]]>The $G\phantom{\rule{0}{0ex}}W$ method is the standard method for predicting quasiparticle energies as measured in experimental photoemission spectroscopy. Until recently, routine $G\phantom{\rule{0}{0ex}}W$ calculations have been restricted to fairly small systems and few $k$ points for sampling the Brillouin zone, since the computational demand usually scales quartic with the number of atoms and quadratic with the number of $k$ points. The original work of Lars Hedin suggests that one can do better by calculating the self-energy as $\mathrm{\Sigma}$(1,2)=$i\phantom{\rule{0}{0ex}}G$(1,2)$W$(1,2) – the equation that coined the phrase ”$G\phantom{\rule{0}{0ex}}W$”. Here, $G$ is the one-particle Green’s function and $W$ the screened interaction between the electrons. Using this relation directly in computations brings the scaling down to linear in the number of $k$ points and cubic in the number of atoms. Although this approach has been used in the past by others, in this work its full potential is unveiled by implementing it in a massively parallel code and adopting it to the projector augmented wave method. This makes $G\phantom{\rule{0}{0ex}}W$ calculations as convenient as calculations based on local density functionals and allows for calculations for hundred atoms in few hours.

[Phys. Rev. B 94, 165109] Published Wed Oct 05, 2016

]]>Good progress has been made in electronic structure calculations for real materials as well as for model Hamiltonians. However with an eye on $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ calculations, it is still very challenging to accurately calculate, e.g., the dynamical response of small molecules or atom clusters. At present the method of choice is the GW-BSE method. Its notorious drawback is that solving a computationally demanding self-consistency equation is required. Thus motivated, the author explore an alternative idea replacing the self-consistency problem by the functional renormalization group (FRG). The traditional FRG was formulated for systems with translational symmetry. We investigate an alternative ($\epsilon $FRG) that is also applicable to inhomogeneous matter, such as disordered metals or molecules. It accounts for Fermi-liquid corrections to quasiparticle energies and particle-hole excitations. In principle it can go beyond the state of the art approximation, GW-BSE, by solving the Bethe-Salpeter equation (BSE) self-consistently We present an efficient implementation of $\epsilon $FRG together with performance tests on the spinless disordered Hubbard model.

[Phys. Rev. B 94, 155102] Published Mon Oct 03, 2016

]]>Among others, pyrochlore iridates with strong spin-orbit coupling have received attention recently due to the prediction of Weyl semimetal states in these materials. Despite tremendous efforts, there exists no direct measurement of the spin structure at the iridium site by means of neutron diffraction. This might be caused by the small ordered Ir moment and also by the neutron absorbing properties of the Ir ions. Up to now, only basic indirect evidence for the spin structure at the iridium site exists. From the observation here of the full magnetic structure of Nd${}_{2}$Ir${}_{2}$O${}_{7}$ via neutron diffraction experiments, the authors obtain direct insight into unknown aspects of the physics of the iridium ions in pyrochlore iridates.

[Phys. Rev. B 94, 161102(R)] Published Mon Oct 03, 2016

]]>Wannier functions provide a useful description of the electronic states in a crystal, providing a chemical picture for the bonding structure of a material, in addition to applications in computational materials science. Topological insulators are an interesting class of materials with spin-polarized conducting surface states alongside an insulating bulk. To date, constructing Wannier functions for topological insulators has proven difficult, relying on an educated guess for the trial orbitals or requiring the system to be parameterized in order to tune between the normal and topological phase. The authors have developed an automated approach within the optimized projection functions method for constructing localized Wannier functions for topologically nontrivial bands. They demonstrate the method on a two-dimensional tight-binding model, a strong three-dimensional topological insulator, and finally, the prototypical topological insulating material Bi${}_{2}$Se${}_{3}$. They find that the Wannier functions contain large imaginary components and are more extended than those in the topologically trivial phase.

[Phys. Rev. B 94, 125151] Published Fri Sep 30, 2016

]]>Superfluidity in two dimensions relies on the stability of a vortex imposed by external rotation of the fluid. It is lost if the flow around the test vortex is screened by spontaneously generated free vortices. In thermal equilibrium, this occurs through a Kosterlitz-Thouless phase transition, where vortex-antivortex pairs bound by Coulomb-like forces unbind only above the critical temperature. Recent experiments with exciton-polariton fluids and other driven dissipative systems raise the question of how this physics changes away from thermal equilibrium. In this paper, the authors generalize the electrostatic duality, which represents vortices as Coulomb charges, to a full electrodynamic description of the nonequilibrium system. The unbinding of vortices is analyzed within this theory using a renormalization group framework. In contrast to the equilibrium case, it is found that vortices always unbind beyond a large emergent length scale due to nonlinearities in the field dynamics. Hence, there is no superfluidity in a truly infinite driven system, while a finite system may appear as a superfluid. The heuristic derivation of the dual electrodynamics presented in this paper is supplemented by a systematic one starting from a microscopic lattice theory in a companion paper.

[Phys. Rev. B 94, 104520] Published Tue Sep 27, 2016

]]>Duality transformations have a long history in physics, and recently saw a surge of renewed interest in the context of strongly correlated fermion systems. A prime example for such a transformation was established to describe the phase transition in two-dimensional superfluids in thermal equilibrium. Here, Kosterlitz and Thouless developed a dual representation of the superfluid, which maps the vortices in the latter to charges in a Coulomb gas. In this framework, the dissociation of vortex-antivortex pairs at the critical temperature corresponds to the formation of a plasma of free charges. How can such a framework be leveraged over to nonequilibrium situations, relevant to the understanding of driven open fluids of light such as exciton-polariton systems? In this work, the authors make a crucial step in this direction by deriving a transformation that maps the stochastic equation of motion for the phase of a driven open condensate — the compact Kardar-Parisi-Zhang (cKPZ) equation — to a dual electrodynamic theory. This results in modified, and in particular, nonlinear Maxwell equations for the electromagnetic fields, and a diffusion equation for the charges representing vortices in the cKPZ equation. In a companion paper, the authors apply this theoretical framework to the study of nonequilibrium vortex unbinding.

[Phys. Rev. B 94, 104521] Published Tue Sep 27, 2016

]]>The nematic state, where a system is translationally invariant but breaks rotational symmetry, has drawn great attention recently due to the experimental observations of such a state in high-${T}_{c}$ superconductors. For iron-based superconductors, the nematic state intertwines strongly with the antiferromagnetic order. Hence, clear experimental delineation of the nematic state has always been challenging. Here, the authors study the electronic structure of a multilayer FeSe film using angle-resolved photoemission spectroscopy. The band reconstruction in the nematic state is clearly delineated. They find that the energy splitting between ${d}_{x\phantom{\rule{0}{0ex}}z}$ and ${d}_{y\phantom{\rule{0}{0ex}}z}$ bands shows a nonmonotonic distribution in momentum space. The energy splitting was also observed on the ${d}_{x\phantom{\rule{0}{0ex}}y}$ bands with an energy scale around 45 meV. The momentum dependence of the ${d}_{x\phantom{\rule{0}{0ex}}z}$ and ${d}_{y\phantom{\rule{0}{0ex}}z}$ energy splitting and the reconstruction of ${d}_{x\phantom{\rule{0}{0ex}}y}$ exclude the simple on-site ferro-orbital ordering as a driving force of nematicity. Instead, strong anisotropy exists in the hopping of all ${d}_{x\phantom{\rule{0}{0ex}}z}$, ${d}_{y\phantom{\rule{0}{0ex}}z}$, and ${d}_{x\phantom{\rule{0}{0ex}}y}$ orbitals, the origin of which holds the key to a microscopic understanding of the nematicity in iron-based superconductors.

[Phys. Rev. B 94, 115153] Published Mon Sep 26, 2016

]]>Materials with strong spin-orbit coupling often host a unique electronic structure, both in the bulk and on the surface. In particular, in systems that break inversion symmetry, spin-orbit coupling facilitates the Rashba-Dresselhaus effect, leading to a lifting of spin degeneracy in the bulk and intricate spin textures of the Bloch wave functions. Such materials could see potential applications in spintronics and quantum information technology, but these applications would benefit from a broader suite of materials, with a greater range of properties, and in particularly from very strong spin-orbit splitting. This paper’s angular-resolved photoemission and low-temperature scanning tunneling microscopy experiments, together with relativistic first-principles band structure calculations, identify several Dirac surface states in BiPd, one of which exhibits an extremely large spin splitting. Unlike the surface states in inversion-symmetric systems, the Dirac surface states of BiPd have completely different properties at opposite faces of the crystal and are not trivially linked by symmetry. The spin splitting of the surface states exhibits a strong anisotropy by itself, which can be linked to the low in-plane symmetry of the surface termination. Both may be useful to implement new functionalities for spintronic applications.

[Phys. Rev. B 94, 121407(R)] Published Thu Sep 22, 2016

]]>Although NaFe(WO${}_{4}$)${}_{2}$ is not multiferroic itself, the magnetic order in this $S$=5/2 chain material is a reference for many multiferroic materials, such as MnWO${}_{4}$ and $R$MnO${}_{3}$, because NaFe(WO${}_{4}$)${}_{2}$ exhibits a transition from an incommensurate spiral order to a commensurate up-up-down-down structure. Here, the authors perform a very extensive study of the magnetic properties in the this double tungstate compound at low temperatures and high magnetic fields. A complex $B$-$T$ phase diagram is unveiled. Strong magnetoelastic anomalies with a relative contraction of the $b$ lattice parameter of up to 2.6 $\times $ 10${}^{-4}$ are associated with the up-up-down-down ordering, which appears in the commensurate and also in the anharmonic incommensurate structure. In contrast, the transition to the magnetically ordered phase, with a purely harmonic incommensurate spiral modulation, does not cause resolvable anomalies in the thermal expansion. Thus, the strong magnetoelastic anomaly coupling can be attributed to the lifting of magnetic frustration in the up-up-down-down structure, where neighboring moments align either parallel or antiparallel, although all these bonds are equivalent in the paramagnetic state.

[Phys. Rev. B 94, 104423] Published Wed Sep 21, 2016

]]>Superconducting qubits are among the most promising elements for the implementation of the concept of quantum computing. Quasiparticles are an intrinsic sources of qubit decoherence, and are more generally detrimental to the operation of superconducting devices, e.g., Cooper pair pumps. Experiments reveal that quasiparticles fail to equilibrate and their density remains high even at low temperatures. Planting normal-metal traps on a superconducting device offers a way to reduce the quasiparticle density: once a quasiparticle tunnels into the normal metal and relaxes to subgap energy via inelastic processes, it cannot return to the superconductor. This paper presents a theoretical model for the time-resolved dynamics of quasiparticles injected into a qubit, and experiments with transmon qubits validating the model. The authors show that, contrary to expectations, the effective trapping rate depends on temperature, which is a consequence of the strong energy dependence of the quasiparticle density of states in the superconductor. At low temperatures, the relaxation process in the normal metal is the bottleneck limiting the effectiveness of traps. The authors also show that the trapping rate saturates for larger traps. At saturation, the rate is limited by the inverse of the time it takes for quasiparticles to diffuse across the device.

[Phys. Rev. B 94, 104516] Published Tue Sep 20, 2016

]]>Spin transport at interfaces between nonmagnets and ferromagnets plays an important role in spintronic devices. Lately, there is an increasing suspicion that spin-orbit coupling, which couples the spin and momentum of carriers, might contribute significantly to this process. Unfortunately, the existing description of spin transport at such interfaces, magnetoelectronic circuit theory, is not valid when spin-orbit coupling is present at the interface. This paper presents a generalization of magnetoelectronic circuit theory to interfaces with spin-orbit coupling. Like the original theory, this generalization describes spin transport in terms of drops in spin and charge accumulations across the interface, but also includes responses to in-plane electric fields and offsets in spin accumulations. The most important result is a description of the way in-plane electric fields generate spin accumulations, spin currents, and torques at the interface. The effects described by this generalized circuit theory impact the interpretation of experiments involving spin-orbit torques, spin pumping, spin memory loss, the Rashba-Edelstein effect, and spin Hall magnetoresistance.

[Phys. Rev. B 94, 104419] Published Fri Sep 16, 2016

]]>Most spintronic devices share two features: they utilize spin-orbit coupling and they contain interfaces. While bulk spin-orbit effects are thought to be well described by phenomenological theories, interfacial spin-orbit effects are not. A theory that could describe interfacial spin-orbit effects would be useful in analyzing experiments on heavy-metal ferromagnet bilayers, which are a key feature of potential energy-efficient implementations of MRAM. To develop such a theory, the authors present the boundary conditions needed for drift-diffusion models to treat interfaces with spin-orbit coupling. Together with the drift-diffusion equations, these boundary conditions give an analytical model of spin-orbit torques caused by both the spin Hall and Rashba-Edelstein effects. A key feature of these boundary conditions is that they capture spin currents generated by interfacial spin-orbit scattering. The authors validate this phenomenological approach by comparing the results with those obtained by solving the spin-dependent Boltzmann equation. They discuss the interpretation of current experiments, and describe in particular how interfacial effects give rise to torques on a nearby ferromagnetic layer even through a nonmagnetic spacer layer.

[Phys. Rev. B 94, 104420] Published Fri Sep 16, 2016

]]>Landau theory of Fermi liquids is the foundational concept behind our understanding of conventional metals. Its basic postulate is that the low-energy physics can be described by weakly interacting quasiparticles — a dressed but effectively free version of electrons formally represented by poles of the single-particle Green’s function. However, this concept notoriously fails for strongly interacting systems such as high-${T}_{c}$ cuprate superconductors, and our understanding of properties of these “non-Fermi” quantum liquids is far from complete. One well-known candidate theory for non-Fermi liquids is Hertz-Millis theory: a finite-density fermion system coupled to a massless critical boson. In (2+1) dimensions, this theory is strongly interacting at low energies, but as of yet there is no controlled insight in this regime; conventional perturbation theory fails. In this article, the authors show that in the so-called quenched approximation the fermionic spectrum can be computed exactly to all orders in the coupling constant. They show extensive detailed analysis that, in this quenched limit, the nonperturbative low-energy dynamics are indeed that of a non-Fermi liquid. Instead of coherent quasiparticles one finds the emergence of critical fermionic excitations described by branch cuts in the single particle Green’s function.

[Phys. Rev. B 94, 115134] Published Wed Sep 14, 2016

]]>The authors present here several advances towards a comprehensive understanding of WTe${}_{2}$. Using microfocus laser ARPES, they resolve for the first time the distinct electronic structure of both inequivalent top and bottom (001) surfaces of WTe${}_{2}$. Moreover, they demonstrate for the first time the presence of large surface state Fermi arcs on both surfaces. Using surface electronic structure calculations, they further demonstrate that these Fermi arcs are topologically trivial and that their existence is independent of the presence of type-II Weyl points in the bulk band structure. Contrary to common belief, the observation of surface state Fermi arcs is thus not suitable to robustly identify a type-II Weyl semimetal. Finally, the authors use the observation of Fermi arcs and distinct top and bottom surfaces to clarify the controversial bulk electronic structure of WTe${}_{2}$. They show that the bulk Fermi surface is formed by three-dimensional electron and hole pockets with areas that are found to be in good agreement with transport experiments, with the exception of small hole pockets that have not been observed in quantum oscillation experiments.

[Phys. Rev. B 94, 121112(R)] Published Wed Sep 14, 2016

]]>In recently predicted type-II Weyl semimetals, the Weyl states connect hole and electron bands that would otherwise be separated by an indirect gap. The set of points in the momentum space at which both bands touch are called Weyl points and are connected by Fermi arcs at the surface of the sample. Experimental evidence confirmed existence of these exotic fermions in MoTe${}_{2}$ and Mo${}_{x}$W${}_{1-x}$Te${}_{2}$. Here, the authors show that such states also exist in pure WTe${}_{2}$ and are very sensitive to strain. While flat undistorted portions of the sample show well defined Fermi arcs – a clear signature of Weyl fermions – strain due to bulking of the surface in other locations suppresses this state and destroys the Weyl state. Such high sensitivity to strain provides a simple way to tune properties of Weyl Fermions and explore the physics of these unusual objects.

[Phys. Rev. B 94, 121113(R)] Published Wed Sep 14, 2016

]]>Slicing the 3D Brillouin zone into 2D as a dimensional reduction makes the topological structure of the 3D Weyl point clear as a topological critical point. The Chern number of the sliced 2D system (section Chern number) changes discontinuously at the gap-closing momentum that guarantees topological stability of the Weyl point. This section Chern number is just a mathematical tool and cannot be directly measured experimentally. However, propagating edge modes of the 3D system with boundaries are necessarily momentum selective, associated with the section Chern number. This is a direct consequence of the bulk-edge correspondence. Using a localized basis with/without boundaries, the authors have demonstrated this here in a 3D helical photonic crystal without inversion symmetry

[Phys. Rev. B 94, 125125] Published Wed Sep 14, 2016

]]>Ferroelectrics that allow a coupling between the polarization and another order parameter are of great interest because they could make the electric field control of nonpolar order parameters possible. In recent years, “hybrid improper’” ferroelectrics - materials where the polarization couples to two different structural distortions - have emerged as a possible way to realize this goal. Theoretical predictions of hybrid improper ferroelectricity in layered perovskite materials were followed by its first experimental realization in Ca${}_{3}$Ti${}_{2}$O${}_{7}$ in 2015. However, the precise pathway by which the polarization reverses in this material during ferroelectric switching remains an open question. The authors address this question and lay the groundwork for understanding the unexpectedly complex domain structure of Ca${}_{3}$Ti${}_{2}$O${}_{7}$, consisting of a network of multiple types of domain walls and topological defects.

[Phys. Rev. B 94, 104105] Published Thu Sep 08, 2016

]]>Entanglement entropy is a powerful tool for characterizing spin liquids and other systems with long-range entanglement. For gapped spin liquids, this problem has been well-studied, finding a universal subleading constant term in the entanglement entropy, known as the topological entanglement entropy. In this paper, the idea of topological entanglement entropy is generalized to the gapless $U$(1) spin liquid. First, it is shown that the entanglement entropy separates into two pieces, one due to emergent charges of the theory (assumed to be gapped) and one due to the emergent gapless gauge mode. Both give universal subheading contributions to the entanglement entropy, which are logarithmic in the size of the region considered. The photon contribution is due to the local physics of the gapless mode. The particle contribution, on the other hand, is a topological term, generalizing the notion of topological entanglement entropy. A construction is presented which can isolate this topological piece by eliminating the photon contribution and other local terms, such as the area law. This topological term in the entanglement entropy can serve as a powerful diagnostic tool for $U$(1) spin liquids.

[Phys. Rev. B 94, 125112] Published Thu Sep 08, 2016

]]>In a strong magnetic field, two-dimensional electron systems are dominated by Coulomb interactions between the electrons. While the most familiar manifestation of these interactions is the fractional quantum Hall effect, this is actually a relatively subtle variation in the strong electronic correlations that exist throughout the high-field regime. This is clearly revealed in tunneling experiments in which an uncorrelated electron is rapidly thrust into a strongly correlated electron gas. A robust Coulomb pseudogap is observed, with tunneling strongly suppressed until the energy of the incoming electron is large enough. The Coulomb gap is insensitive to the precise magnetic field and is typically much larger than the energy gap associated with the fractional quantum Hall effect. In the present paper, the authors demonstrate that the size of the Coulomb gap observed in tunneling between two parallel two-dimensional electron systems depends sensitively on their spin configuration, with larger gaps associated with a larger net spin polarization. In effect, it is easier for electrons ‘to get out of the way’ of an incoming tunneling electron when the spin polarization of the electron gas is less than complete.

[Phys. Rev. B 94, 125409] Published Thu Sep 08, 2016

]]>There is much interest from the physics community in materials with a gapped nonmagnetic ground state, in which quantum critical points can be reached by application of a magnetic field. Here, the authors report magnetization data as a function of temperature, field, and hydrostatic pressure for the gapped quantum magnet CsFeCl${}_{3}$, a quite intriguing system in which the large easy-plane single-ion anisotropy competes with the exchange interactions. They establish the phase boundary, and identify a quantum phase transition at a critical value of the pressure. It is likely that experiments by other techniques will follow this discovery of both magnetic-field and pressure-controlled transitions in this interesting compound.

[Phys. Rev. B 94, 104409] Published Wed Sep 07, 2016

]]>Spintronics holds the promise of novel functionality and higher device performance, but a massive challenge is to find semiconducting materials with good electronic properties that are also ferromagnetic. Dilute magnetic semiconductors (DMS) such as Mn-doped GaAs have great potential but are notoriously difficult to make, only stabilizing in thin films in most cases. Recently, a promising new class of DMS materials based on the chemistry and structure of iron-based superconductors has been discovered, including the compound (Ba,K)(Zn,Mn)${}_{2}$As${}_{2}$. These materials can be synthesized in bulk form, providing an unprecedented opportunity to study the mechanism of ferromagnetism in semiconductors. In this work, the authors present detailed temperature-dependent characterization of the atomic and magnetic structure of (Ba,K)(Zn,Mn)${}_{2}$As${}_{2}$ using x-ray and neutron pair distribution function techniques, establishing the existence of an unexpected structural distortion on a short length scale and surprisingly robust short-range ferromagnetic correlations that persist even at room temperature. These results fill in important gaps in the experimental understanding of DMS materials and provide valuable guidance to their theoretical description.

[Phys. Rev. B 94, 094102] Published Tue Sep 06, 2016

]]>Scaling, universality, and renormalization are three pillars of modern critical phenomena. According to the hypothesis of universality, continuous phase transitions fall into classes mainly determined by spatial dimensionality and symmetry of the order parameter. The latter is usually reflected by the degeneracy of the ground state of the Hamiltonian. However, for certain systems at criticality, a higher symmetry emerges in the order parameter, and the associated critical behavior may become very rich. Such emergent symmetry has been found in spin ice systems, deconfined quantum critical points, high-${T}_{c}$ superconductors, and so forth, and are often accompanied by very interesting critical phenomena. New results presented here are based on Monte Carlo simulations and finite-size scaling of a series of $q$-state Potts models on the simple cubic lattice with ferromagnetic interactions in one lattice direction and antiferromagnetic interactions in the other two directions. The staggered magnetization appears to display an emergent continuous O($n$) symmetry with $n$=$q$-1, as illustrated by two-dimensional intersections of the distribution functions. Also the estimated critical exponents are consistent with the O($n$=$q$-1) universality classes.

[Phys. Rev. B 94, 104402] Published Thu Sep 01, 2016

]]>It has been corroborated in the past that Pauli blockade mechanisms of weakly spin-coupled charge carrier pairs undergoing transitions into doubly occupied singlet states are among the most dominant spin-selection rules influencing room-temperature magnetoresistance and luminescence of organic semiconductors. It has remained unclear though whether these pairs consist of equal or opposite charges, and thus, whether the spin-dependent transitions are recombination or charge transport processes. Here, the authors report on simultaneous transient measurements of electric current and optical emission changes in organic light emitting diodes due to pulsed magnetic resonance induced charge carrier spin manipulation. When a room-temperature steady-state current is changed magnetic resonantly, the electroluminescence changes show identical dynamic and magnetic resonant signatures, indicating that both observables are controlled by the same spin-dependent electronic process. However, a quantitative analysis of the optical emission change reveals that its magnitude significantly exceeds what is expected from the magnitude of the electrical current change, implying that the optical emission change occurs not only because of the device current change but also due to a change of the radiative recombination rate, thereby showing that the observed spin-dependent process is due to recombination, not transport.

[Phys. Rev. B 94, 075209] Published Wed Aug 31, 2016

]]>The discovery of new spin-to-charge current conversion effects such as the spin Hall effect (SHE) is expanding the potential of spintronic applications. In contrast to the anomalous Hall effect (AHE), a systematic experimental study of the different mechanisms contributing to the SHE is lacking. Finding routes to maximize the SHE is not possible as long as it remains unclear which is the dominant mechanism in a material. The authors systematically study Pt, the prototypical SHE material, using the spin absorption method and find a single intrinsic spin Hall conductivity in a wide range of Pt conductivities, in good agreement with theory. By tuning the conductivity, they observe for the first time the crossover between the moderately dirty and the superclean scaling regimes of the SHE, equivalent to that obtained for the AHE. These results explain the dispersion of values in the literature and show a clear path to enhance this important effect.

[Phys. Rev. B 94, 060412(R)] Published Tue Aug 30, 2016

]]>Driven-dissipative quantum many-body systems, i.e., many-particle systems where quantum coherent dynamics and dissipative effects occur on the same footing, can constitute a promising platform to initiate a systematic classification of quantum dynamical criticality. In this work, the authors show that both nonequilibrium and quantum features can be simultaneously present and persist in the low-frequency properties of a critical one-dimensional driven open Bose gas, and no effective equilibrium description is applicable for this novel type of nonequilibrium quantum criticality.

[Phys. Rev. B 94, 085150] Published Tue Aug 30, 2016

]]>Nanowire/superconductor hybrid devices are important systems that are currently used in many experimental programs. In these systems, deliberate or accidental quantum dots in the nanowire create Yu-Shiba-Rusinov subgap states when they couple to the superconductor. In this work, an array of bottom gates under an InAs nanowire is used to tune the coupling (${\mathrm{\Gamma}}_{S}$) of individual quantum states to a superconductor as the system is probed with a weakly coupled normal lead. The paper includes measurements of a coupling-induced quantum phase transition as well as the Zeeman splitting of a subgap state for different values of ${\mathrm{\Gamma}}_{S}$. The authors analyze their data using an extension of the Anderson model and the numerical renormalization group procedure and give quantitative values for key parameters of the system.

[Phys. Rev. B 94, 064520] Published Mon Aug 29, 2016

]]>Using optical absorption and scanning tunneling spectroscopy techniques, the authors investigate band-gap properties of single layers of transition metal dichalcogenide (TMDC) alloyed crystals (Mo${}_{x}$W${}_{1-x}$S${}_{2}$). They observe a significant modulation of the excitonic transitions, quasiparticle band gaps, and exciton binding energies. These findings may hold promise for fundamental studies of intralayer phenomena and for potential applications of TMDC alloys in electronic and optoelectronic devices.

[Phys. Rev. B 94, 075440] Published Mon Aug 29, 2016

]]>Individual dopants in semiconductors are attracting considerable attention due to the prospect of using them in a wide range of applications. In particular, phosphorus donors in silicon have attracted significant interest owing to their weak interaction with the host crystal. However, harnessing their attributes toward the construction of scalable circuitry will require low resistive interconnects at a comparable scale as the dopant atoms. In this Rapid Communication, the authors investigate the transition from quantum coherent to the semiclassical diffusive transport in a 4.6-nm quasi-one-dimensional Si:P metal wire. Analyzing the temperature dependence of universal conductance fluctuations (UCF) the authors show that electron transport evolves from a quantum coherent to a semiclassical regime at temperatures as low as ~4 K and confirm that concepts of UCF and weak localization remain valid in metallic conductors at the atomic-scale.

[Phys. Rev. B 94, 081412(R)] Published Mon Aug 29, 2016

]]>In the usual theory of electrical insulators, a small conductivity is permitted at finite temperature by the process of phonon-assisted quantum tunneling, or “hopping”, of electrons across a disordered energy landscape. The long-standing assumption has been that during this process each electron hops between essentially random locations in space. Here the authors show that the paths followed by electrons through a compensated semiconductor are in fact far from random. Instead, these electron pathways spontaneously form a fractal shape with effective dimension 2. The existence of these fractal pathways allows an unexpectedly high electrical current to move through the material at low temperature. The authors’ results can be used to explain recent surprising observations about electron transport through compensated topological insulators and nanocrystal arrays.

[Phys. Rev. B 94, 085146] Published Mon Aug 29, 2016

]]>What is the long-time behavior of a quantum system after it is brought out of equilibrium? In a generic case, it has been proposed that an effective thermalization occurs, and the local properties of the system can then be described by a thermal Gibbs ensemble. However, in integrable systems, with infinitely many conservation laws, the answer is far more complicated. It was shown in several special cases that the long-time steady state can be exactly described by the so called complete generalized Gibbs ensemble. The latter can be obtained by maximizing the entropy with constraints imposed by the conservation of energy, but also by additional carefully chosen subset of the integrals of motion. In this work, the authors analyze more general initial states, focusing on integrable XXZ Heisenberg spin chains. They find that the complete generalized Gibbs ensemble indeed correctly describes the system at long times after the quench.

[Phys. Rev. B 94, 054313] Published Tue Aug 23, 2016

]]>Quantum Monte Carlo simulations provide a powerful tool to compute static properties of quantum many-particles models and realistic systems. A vast array of ground-state energies and properties defined by equal-time correlation functions can be computed with high accuracy. On the other hand, the extensions needed to study time-displaced correlation functions, which provide access to excited states, have been challenging, especially for fermionic strongly correlated systems. The authors present a methodology that allows a high-accuracy calculation of the dynamical Green functions of many-fermions systems in the framework of the auxiliary-field quantum Monte Carlo method. They extract the the charge gap of the repulsive two-dimensional Hubbard model at half-filling as a function of the interaction strength and propose strategies for applying their methodology to more realistic systems.

[Phys. Rev. B 94, 085140] Published Tue Aug 23, 2016

]]>The Dzyaloshinski-Moriya interaction (DMI) originates from the relativistic spin-orbit coupling, and in low-symmetry crystal structures without inversion centers it gives rise to antisymmetric magnetic exchange. DMI plays an important role in the formation of inhomogeneous spin structures such as long-wavelength spirals, vortex states, and skyrmion textures. Even in high-symmetry lattices, where the antisymmetric DM term would normally vanish, DMI is present in the vicinity of any lattice defect and can influence the magnetic microstructure of polycrystalline materials with a large defect density. In this work the authors have studied the impact of the DMI on the magnetic small-angle neutron scattering (SANS) cross section, which is able to probe long-wavelength magnetization fluctuations in the bulk and on the nanometer length scale. The analysis of the authors provides a promising way to characterize the DMI in defect-rich materials.

[Phys. Rev. B 94, 054424] Published Mon Aug 22, 2016

]]>Members of the Mo${}_{x}$W${}_{1-x}$Te${}_{2}$ series are predicted to be Weyl semimetals, hosting type-II Weyl fermions, which have yet to be experimentally realized and which are unusual because they strongly violate Lorentz invariance. Crucially, the Weyl points in this system are predicted to sit above the Fermi level. Here, the authors show that for a type-II Weyl cone, although not for a type-I Weyl cone, if the Weyl point is above the Fermi level, then it’s necessary to see the band structure above the Fermi level to observe a topological Fermi arc. The authors also discover that pump-probe angle-resolved photoemission beautifully displays the unoccupied band structure in Mo${}_{x}$W${}_{1-x}$Te${}_{2}$. Their work sets the stage for demonstrating that this system is the first type-II Weyl semimetal, as well as the first tunable Weyl semimetal.

[Phys. Rev. B 94, 085127] Published Mon Aug 15, 2016

]]>Mid-infrared plasmonics has the potential to revolutionize molecular sensing technology, if integrated into optoelectronic chips. Recently,several groups working on plasmonics have substituted metals with heavily doped semiconductors for the sake of integration, also opening up the possibility of tuning the device response via the doping level. In this work, the authors analyze the relevant case of heavily doped Ge films by combining transport measurements with infrared spectroscopy. They demonstrate a broad tunability of the screened plasma frequency up to the mid-infrared range. The main loss channels are identified through comparison of the experimental scattering rates with quantum calculations and pump-probe measurements. Heavily doped Ge is highlighted as a viable route for the integration of mid-infrared plasmonics into silicon optoelectronic platforms.

[Phys. Rev. B 94, 085202] Published Mon Aug 15, 2016

]]>Here, the authors demonstrate a new first-principles computational method for performing predictive calculations of the optical absorption spectra of solids at finite temperature, including both zero-point fluctuations and phonon-assisted optical transitions. The comparison between calculations and measurements for the case of silicon, diamond, and gallium arsenide shows that this new method is not only efficient, but also very accurate, and is capable of yielding absorption coefficients in agreement with experiment over several orders of magnitude.

[Phys. Rev. B 94, 075125] Published Fri Aug 12, 2016

]]>Spin-resolved and circular dichroism angle-resolved photoemission spectroscopies (ARPES) are powerful techniques to detect the spin information of electrons in materials. In this work, the authors distinguish the following three types of spin in the ARPES experiment: the real spin, the pseudospin, and the spin polarization of photoemitted electrons. The authors present an interesting theoretical study on these three spin textures for topological surface states of Bi${}_{2}$Se${}_{3}$ and SmB${}_{6}$. Based on a model-type surface state Hamiltonian, they discuss the connections and differences of these spin textures as a function of the symmetry of the investigated system and the polarization of the incident detection light. This work provides a detailed and quantitative analytical study of the spin polarization and circular dichroism spectrum of the photoelectrons detected in an ARPES experiment.

[Phys. Rev. B 94, 085123] Published Fri Aug 12, 2016

]]>One of the key mechanisms for transverse spin-current generation in metals is the electron skew scattering off defects and impurities, where the spin-orbit interaction leads to an effective deflection of the electrons away from the direction of an applied electric field. Recently, there has been an intensive interest in the skew-scattering part of the Hall effects in an important spintronics material, $L{1}_{0}$-ordered FePt alloy, where conflicting data with regard to the skew-scattering magnitude have been reported in previous studies. To understand the reason for this controversy, the authors considered various sources of scattering in FePt and determined the skew scattering Hall angles from microscopic density functional theory calculations and Boltzmann transport theory. Based on their results, the authors are able to explain the extreme spread of available data for FePt with drastic dependence of the skew-scattering properties on the details of disorder.

[Phys. Rev. B 94, 060406(R)] Published Thu Aug 11, 2016

]]>The Dzyaloshinskii-Moriya Interaction (DMI) has recently attracted considerable interest owing to its fundamental role in the stabilization of chiral spin textures in ultrathin ferromagnets, which are interesting candidates for novel spintronic technologies. Here, the authors present an experimental study of the DMI strength that is induced in nanometer-thick CoFeB ferromagnetic thin films in contact with different nonmagnetic metal underlayers. They use a novel technique for noninvasive high-sensitivity sensing of magnetic field: a scanning nanomagnetometer based on the magnetic response of a single nitrogen-vacancy defect in diamond. The magnetic domain walls are mapped, the spin structure and type of domain wall determined, and the DMI strength extracted. Importantly, the authors find local variation of the DMI constant, which clearly suggests that local field mapping techniques are extremely important to study this physics.

[Phys. Rev. B 94, 064413] Published Thu Aug 11, 2016

]]>The fractional Josephson effect is of the most striking phenomena heralding topological superconductivity and Majorana bound states. In contrast to the conventional Josephson current that is 2$\pi $-periodic in the applied phase difference, the periodicity is 4$\pi $ in the junctions connecting topological superconductors. Unfortunately, this effect can be easily masked in experiments due to the quasiparticle poisoning processes resulting from inelastic transitions between the localized subgap states of the junction and delocalized states in the quasiparticle continuum. The authors propose a simple and feasible solution to this problem by embedding the topological Josephson junction in a SQUID setup. They show that signatures of the topological junction can be found in the switching probability of the supercurrent through the SQUID, even in the presence of quasiparticle poisoning processes.

[Phys. Rev. B 94, 085409] Published Thu Aug 11, 2016

]]>Magnetoresistance studies are one of the simplest yet powerful tools to investigate the electronic properties of solids. The authors here report on high-field angle-dependent magnetotransport measurements on epitaxial thin films of Bi${}_{2}$Se${}_{3}$, a three-dimensional topological insulator. At low temperature, they observe quantum oscillations that demonstrate the simultaneous presence of bulk and surface carriers. Their key observation is a strong anisotropy in the magnetoresistance in Bi${}_{2}$Se${}_{3}$ that depends on the orientation of the applied current (electric field) with respect to the applied magnetic field. When the magnetic field is applied parallel to the electric field, they find a strong negative longitudinal magnetoresistance (NLMR) that, quite strikingly, persists even up to room temperature. With this finding, the authors demonstrate that the observation of a NLMR is not unique to topological semimetals, such as Weyl semimetals, and therefore cannot by itself be taken as a diagnostic tool and as conclusive evidence for the existence of Weyl fermions. These results could pave the way towards a general understanding of the emergence of the axial anomaly that is suggested to be a universal phenomenon for generic three-dimensional metals in the presence of parallel electric and magnetic fields.

[Phys. Rev. B 94, 081302(R)] Published Wed Aug 10, 2016

]]>Magnetism due to vacancies in graphene has been a controversial topic, with conflicting results in the literature. This work aims to shine some light on the discussion by calculating the charging energy $U$, a key measure that encodes the local electron-electron Coulomb interaction in the vacancy-induced state. By combining analytical and numerical tools, the authors obtain values of $U$ reaching about 1 eV for typical micron-sized samples, which is significantly higher than the previous estimates. These results provide a strong theoretical support to the emergence of vacancy-mediated magnetism in bulk graphene with a dilute concentration of vacancies and are in agreement with recent experiments in the literature.

[Phys. Rev. B 94, 075114] Published Mon Aug 08, 2016

]]>The transport properties of zirconium pentatelluride ZrTe${}_{5}$, where the sign of carriers changes with temperature, have so far eluded a clear understanding. In this work, the authors investigate by angle-resolved photoemission the temperature dependence of the low-energy electronic bands, and find that the transport behavior can be explained by the presence of a van Hove singularity in the density of states. By surface doping with alkali atoms, the authors also access the unoccupied states in the conduction band and define this material as being a weak topological insulator.

[Phys. Rev. B 94, 081101(R)] Published Thu Aug 04, 2016

]]>Unique spin configurations in thin films, including skyrmions and chiral domain walls, present fascinating physical questions while also promising future, low-energy computing systems. The emergent interfacial Dzyaloshinskii-Moriya interaction (DMI), which stabilizes these features, greatly changes the energetic and symmetry-based arguments by which magnetic domains have previously been described. The authors here analyze the behavior of magnetic bubbles in thin films influenced by DMI using classical interface thermodynamics via the Wulff construction, originally conceived to describe crystal facets more than 100 years ago. The striking resemblance of experimentally observed domain shapes to those predicted theoretically suggests that a significant energetic driving force for domain growth exists due to an anisotropic wall energy term from the interfacial DMI. This description explains the observed reversal in growth symmetry found here in Co/Ni multilayer films while presenting a new paradigm for describing the broader behavior of chiral domain walls.

[Phys. Rev. B 94, 060401(R)] Published Mon Aug 01, 2016

]]>Superconductivity at high-pressures has attracted considerable interest after the report of a record critical temperature (${T}_{c}$) of 203 K in sulfur hydride (H${}_{3}$S) at 200 GPa. In this work, the authors predict several novel lithium-sulfur compounds at high pressure using evolutionary crystal structure prediction techniques and investigate their superconducting properties with density functional linear response calculations. The calculations reveal an intrinsic correlation between superconductivity and real-space electron localization. We find that high-${T}_{c}$ superconductivity in the Li-S system occurs at pressures much higher than in H${}_{3}$S, i.e., only when the electrode-like interstitial charge localization typical of alkali-metal compounds is suppressed. This is illustrated in the image, which shows a comparison of the electron-phonon spectral function ${\alpha}^{2}\phantom{\rule{0}{0ex}}F(\omega )$ of Li${}_{3}$S in the low- pressure (panel $a$, ${T}_{c}=0$ K) and high-pressure phases (panel $c$, ${T}_{c}=80$ K), calculated at 500 GPa. The right panels of the image show that the increase in ${T}_{c}$ is accompanied by a shift of the electronic charge from $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l$ regions to Li-S bonds.

[Phys. Rev. B 94, 060502(R)] Published Mon Aug 01, 2016

]]>Since 2008, iron-based superconductors have been the focus of intense research. Unfortunately, the higher transition temperatures and most promising high-magnetic field properties have been associated with compounds that have to be chemically disordered, making it hard to fully understand their intrinsic properties. A fully ordered compound with near optimal properties would be ideal and CaKFe${}_{4}$As${}_{4}$ appears to be that compound. In this work, the authors present detailed anisotropic, structural, thermodynamic, and transport measurements that, in addition to revealing high upper critical fields, also provide the basis for further understanding of this fully ordered, high-${T}_{c}$, iron-based superconductor. It is shown that CaKFe${}_{4}$As${}_{4}$ has exceptionally large upper critical fields (in excess of 650 kOe and possibly as high as 900 kOe) as well as high ${T}_{c}$ values (35 K), which can be obtained and studied without any disorder-related effects obscuring or complicating the results.

[Phys. Rev. B 94, 064501] Published Mon Aug 01, 2016

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