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

]]>Here, the authors investigate the possibility that particle-hole symmetry in the lowest Landau level at half filling arises as an emergent low-energy symmetry. Specifically, they start with the traditional approach to the half-filled Landau level: an effective field theory known as the composite Fermi liquid pioneered by Halperin, Lee, and Read. In this theory, composite fermions arise when flux is attached to electrons in a half-filled Landau level. However, an equivalent starting point is one where one starts with a filled Landau level, depletes electrons (or equivalently, adds holes) to the point of half filling. One then attaches flux to the holes, and the resulting composite hole description is another viable description of the half-filled Landau level. The authors construct a particle-hole symmetric theory by studying a system consisting of alternating quasi-one-dimensional strips of composite Fermi liquid (CFL) and composite hole liquid (CHL), both of which break particle-hole symmetry. Gauge invariance requires the existence of gapless edge modes at the interface between the CFL and CHL. Out of these modes, they construct a neutral Dirac fermion coupled to an emergent U(1) gauge field. Thus, at distances long compared to the strip width, they demonstrate that the system is described by a Dirac fermion coupled to an emergent gauge field, with an anti-unitary particle-hole symmetry, precisely the form conjectured recently by Son to be the low-energy description of a particle-hole symmetric half-filled Landau level.

[Phys. Rev. B 94, 075101] Published Mon Aug 01, 2016

]]>The authors perform near-field nanoimaging of various waveguide modes inside thin flakes of WSe${}_{2}$ by using state-of-the-art near-field scanning optical microscopy. This compound is a prototypical transition metal dichalcogenide with strongly bound excitons. The nanoimaging data provides direct evidence of strong coupling between waveguide photon modes and excitons. Such coupling generates hybrid polaritonic modes that are imaged for the first time as propagative modes in real space. This work uncovers in real space the physics of nanoscale light-matter interactions in van der Waals semiconductors and thus provides guidelines for future applications of this class of materials in nanophotonics and optoelectronics.

[Phys. Rev. B 94, 081402(R)] Published Mon Aug 01, 2016

]]>The authors here explore the properties of Weyl semimetals and axion insulators coupled to geometry. The novel interplay between axion strings and lattice dislocation defects are studied through both effective field theory and microscopic arguments. Various new phenomena are proposed for a Weyl semimetal with dislocations. This work should be of great interest both in the context of topological semimetal/insulators and for the problem of topological defects in strongly correlated systems.

[Phys. Rev. B 94, 085102] Published Mon Aug 01, 2016

]]>Strain engineering in graphene is of high current interest because of the possibility to induce new physical phenomena by means of mechanical strain. The authors of this work study the effect of strain on the electron-phonon interaction that plays an important role in the vibrational, thermal, and transport properties of graphene. It has been commonly assumed that Kohn anomalies and electron-phonon coupling in strained graphene occur only for the longitudinal and transverse optical phonons, as in pristine graphene. In contrast, this study shows that uniaxial strain in graphene induces a Kohn anomaly and enhancement of the electron-phonon coupling in the longitudinal acoustic phonon branch. This feature is a major difference between pristine and uniaxially strained graphene, due to the presence of a new intervalley phonon-scattering channel for electronic states close to the Dirac point.

[Phys. Rev. B 94, 085401] Published Mon Aug 01, 2016

]]>The phenomenon of many-body localization generalizes Anderson localization to interacting systems. Understanding this conceptually novel phenomenon requires a study of many-body eigenstates at finite energy densities. This is a very challenging task since the most efficient numerical methods such as, e.g., the density matrix renormalization group method, can only access the ground state and low lying excitations. In this work, the authors introduce a unitary tensor network based variational method that approximately finds all many-body eigenstates of fully localized Hamiltonians and scales polynomially with system size. The usefulness of their approach is demonstrated by considering the Heisenberg chain in a strongly disordered magnetic field.

[Phys. Rev. B 94, 041116(R)] Published Thu Jul 28, 2016

]]>Two-dimensional potentials are excellent test beds for the physics of interacting excitonic gases, but engineering highly homogeneous potentials can be challenging. In this work, the authors investigate excitons bound to a single 10-μm scale stacking fault, finding excitonic luminescence with unprecedented homogeneity. The authors further show that stacking-fault-bound excitons are mobile and have a giant permanent dipole moment by means of the magneto-Stark effect. These results indicate that stacking faults may be a promising platform to probe the many-body physics of interacting dipoles in an atomically smooth potential.

[Phys. Rev. B 94, 041201(R)] Published Mon Jul 25, 2016

]]>The creation of electron-hole pairs by photoexcitation in quasi-two-dimensional semiconductor heterostructures has long provided one motivation for the study of a positive impurity immersed in an electron gas. Improved understanding of such systems is vital to realize the full potential of solar cells and optoelectronic transistors and memory devices. Collective, many-body phenomena dominate the behavior of the hole-in-electron-gas system at high carrier density, while excitonic species such as the exciton (a bound state of an electron and the hole) and the trion (a bound state formed from two electrons and the hole) emerge at low density. Confinement enhances the binding energy in quantum wells compared to bulk samples but, despite much experimental and theoretical work, the properties of excitons and trions are still uncertain. The authors perform diffusion Monte Carlo calculations of a single positive impurity immersed in a two-dimensional, paramagnetic electron gas at zero temperature using the Coulomb ($1/r$) potential. A gradual crossover from the collective, many-body behavior to the trion state is studied over a range of carrier densities and electron-hole mass ratios consistent with recent experiments in GaAs and CdTe quantum wells. Relaxation energies, electron-hole pair correlation functions, and electron-hole center-of-mass momentum densities are reported for these systems and may be used, for example, in density functional theory studies of optoelectronic devices.

[Phys. Rev. B 94, 041410(R)] Published Fri Jul 22, 2016

]]>In a wide range of experiments where electrical currents are used to inject angular momentum, or spin currents, from a metallic ferromagnet into a nonmagnetic metal, the interface plays a critical role. Whether in the giant magnetoresistance effect or in nonlocal spin valves, a loss of spin polarization of an electrical current crossing the interface is seen to reduce the spin current and lead to smaller overall response in sensors. The authors here compare electrical spin injection to the more recently discovered thermal spin injection in a nonlocal spin valve. They show that, despite a strong reduction of electrical spin injection that they tie to the loss of interfacial spin polarization, thermal spin injection remains a large effect. This highlights that thermal spin injection (also called the spin-dependent Seebeck effect) can proceed by new physical mechanisms not possible in the electrically driven case, potentially involving incoherent spin pumping and collective behavior of magnetization in the oxidized interface layer. Better understanding of these interface effects could lead to new ways to increase sensitivity of next-generation magnetic sensors and efficiency of sources of spin current in metallic systems.

[Phys. Rev. B 94, 024426] Published Thu Jul 21, 2016

]]>Atomic force microscopy is one of the techniques that allows us to visualize objects that we cannot see directly. The question of what we actually see when we look at an atomic force microscopy image is, however, a subtle one. This paper discusses a scheme that permits the calculation of atomic force microscopy images with relatively little effort. Using electronic density functional theory, the force on the cantilever is calculated as the gradient of the effective potential that governs the electronic structure in density functional theory. In this way, nonclassical contributions to the force can efficiently be taken into account and are translated into an image. Comparison to experimental images shows that the nonclassical contribution to the force can make a visible difference.

[Phys. Rev. B 94, 035426] Published Mon Jul 18, 2016

]]>The Kitaev model is a toy model for $S$=$\frac{1}{2}$ moments on a honeycomb lattice, which interact via strongly bond-dependent Ising-like interactions. In real material candidates like Na${}_{2}$IrO${}_{3}$, small Heisenberg interactions are also present in addition to dominant Kitaev interactions. The doped Kitaev-Heisenberg model has been predicted to show unconventional superconductivity and spin or charge density wave, and bond-order instabilities. In this paper, the authors have succeeded in surface-doping Na${}_{2}$IrO${}_{3}$ crystals by argon plasma etching and in changing the conductivity by several orders of magnitude. The doped samples show several unusual transport behaviors, including first order spin or charge density wave-like transitions.

[Phys. Rev. B 94, 041109(R)] Published Mon Jul 18, 2016

]]>URu${}_{2}$Si${}_{2}$ undergoes a second-order phase transition at 17.5 K that has defied attempts to identify its order parameter despite a vast literature extending over the last 30 years. A wide variety of theories have been posed to explain this so-called “hidden order”, some with exotic ground states. An important dividing line between these theories is the character of the 5$f$ orbital: on one side sit theories that require a localized ${f}^{2}$ configuration ($f$-manifold crystalline electric field effects, hastatic order, etc.), while on the other sit those that start from an itinerant, partially occupied ${f}^{3}$ orbital (band structure + hybridization). Unfortunately, the experimental measures remain muddy on this issue, with indications of both localized ${f}^{2}$ and itinerant ${f}^{3}$ behavior depending on the experiment. Here, the authors report U ${L}_{I\phantom{\rule{0}{0ex}}I\phantom{\rule{0}{0ex}}I}$-edge absorption/${L}_{\alpha 1}$ emission resonant x-ray emission spectroscopy measurements comparing data from URu${}_{2}$Si${}_{2}$ to UCd${}_{11}$, UF${}_{4}$, and UO${}_{2}$ data to unravel the various roles of $f$-orbital occupation, delocalization, and ligand-field splitting of the $d$ manifold. The data indicate a dominant delocalized ${f}^{3}$ configuration that is likely partially occupied, with no measurable change in occupancy to temperatures as low as 10 K. While these measurements do not rule out a minority localized ${f}^{2}$ configuration, any theory relying on such a configuration must account for its relatively small contribution to the Fermi surface.

[Phys. Rev. B 94, 045121] Published Fri Jul 15, 2016

]]>The ability to explain and predict the behavior in magnetic field is essential for any possible application of multiferroic materials. Motivated by the complex phase diagrams of MnWO${}_{4}$ and other spiral multiferroics, the authors formulate and investigate the anisotropic next-nearest-neighbor Heisenberg (ANNNH) model, which is a generalization of the celebrated ANNNI model to three component quantum spins. Using real-space mean-field simulations allowed for an unbiased construction of the magnetic phase diagram with multiple commensurate and incommensurate states. In particular, the biaxial anisotropy can explain the puzzling nonferroic reentrant phase of MnWO${}_{4}$ at high magnetic fields. The employed numerical technique is capable of predicting the temperature and field dependence of the ordering wave vector produced by the competition between helicity and anisotropy. An important issue raised in the present study is the effect of quantum fluctuations on the devil’s staircase known to exist for the ANNNI model, but which is replaced by a “floating” incommensurate phase for the ANNNH model.

[Phys. Rev. B 94, 020406(R)] Published Thu Jul 14, 2016

]]>An infinite projected entangled-pair state (iPEPS) is a powerful variational tensor network ansatz for two-dimensional ground states in the thermodynamic limit, and can be seen as a natural generalization of a matrix-product state to two dimensions. One of the main challenges in iPEPS simulations is the optimization of the tensors, i.e., finding the optimal variational parameters, in order to have the best representation of the ground state of a given Hamiltonian. The author presents a variational optimization scheme, in which the energy is minimized in an iterative way by sweeping over all the tensors in the ansatz, in a similar spirit as done in the density-matrix renormalization group method. Benchmark results for challenging problems are presented that show that the variational scheme yields considerably more accurate results than the previously best imaginary-time evolution algorithm, with a similar computational cost and with a faster convergence towards the ground state.

[Phys. Rev. B 94, 035133] Published Thu Jul 14, 2016

]]>Many earlier high-resolution x-ray studies of criticality seemingly invalidated the single-length scaling hypothesis that is predicted by the three-dimensional Ising model, a cornerstone in the theory of critical phenomena. Speculations about the reasons for these discrepancies have concentrated on the role of impurities and on surface-induced strain in the crystalline samples. The general validity of the Ising model for real systems has also been questioned. Here, the authors present x-ray scattering data on the temperature-induced order-disorder transition in $\beta $-brass, nature’s archetypical realization of the Ising model. These investigations unambiguously show that the Ising model does precisely describe the critical behavior observed and, in particular, that the short-range order parameter indeed exhibits single-length scaling. The surface sensitivity has been varied in the experiments without any substantial differences observed.

[Phys. Rev. B 94, 014111] Published Wed Jul 13, 2016

]]>This paper focuses on developing a new algorithm (En-DMRG) for obtaining highly excited states of disordered and strongly interacting spin systems. This line of research is stimulated by the growing interest in understanding the many-body localized (MBL) state, which emerges in disordered interacting systems at high energy densities through a disorder-driven dynamic phase transition. By studying the one-dimensional Heisenberg spin chain (with up to 72 spins) in a random field, the authors demonstrate the accuracy of the method in obtaining energy eigenstates and the corresponding statistical results of the quantum system in the MBL phase. The method presented in this work has the potential to be applicable in a variety of quantum statistical physics problems, possibly including the MBL phase in higher dimensions.

[Phys. Rev. B 94, 045111] Published Wed Jul 13, 2016

]]>Ultrathin films of the heavy elements Pd and Pt are widely used in spintronics as converters between charge and spin currents and vice versa. One open question has been whether the strongly paramagnetic nature of these elements influences the interconversion process. The authors show, using a combination of ferromagnetic resonance and x-ray magnetic circular dichroism measurements, that a related spin current absorption process (spin pumping damping) responds to the presence of ferromagnetic order in the polarizable Pd and Pt layers. They observe a change in the thickness dependence of spin current absorption when the Pd and Pt layers are in direct contact with a ferromagnet, compared with when they are in indirect contact. This change correlates with the presence of induced magnetic moments in Pd and Pt as revealed by element-specific x-ray magnetic circular dichroism magnetometry. The results show that the decoherence process for spin current in such layers is not independent of geometry, but rather is influenced by the presence of induced ferromagnetic order.

[Phys. Rev. B 94, 014414] Published Tue Jul 12, 2016

]]>Arrays of subwavelength slit cavities have been of interest to the acoustic metamaterial community for some years now, giving rise to such phenomena as “enhanced acoustic transmission”, where sound of certain wavelengths can transmit through the slit array structure as if it were not there. This behaviour is widely accepted as a coupling effect between diffraction evanescent waves arising from structure factor, and Fabry-Perot like resonances supported along the length of each slit cavity. Here, the authors explore the effect of altering the spacing, or width, of some of the slits that form the array, thereby forming a “compound” grating, where each periodic unit cell is comprised of multiple slit cavities. The transmission of such structures is experimentally recorded, where it is found that as one increases the number of slit cavities per unit cell, broad and deep minima can appear in the peaks of the primary enhanced acoustic transmission resonance that may have a strong angular dependence. These features owe their existence to the “phase resonance”, where new degrees of freedom available to the near field allow adjacent slit cavities to be out-of-phase with one another and thus destructively interfere with the re-radiated pressure wave. Such behaviour has been studied extensively in the electromagnetic case, where a surface wave band folding picture could be used to predict the dispersive nature of these phase resonances. A similar explanation of the underlying surface wave physics is extended here to the acoustic case.

[Phys. Rev. B 94, 024304] Published Tue Jul 12, 2016

]]>The authors have studied SmB${}_{6}$ and Ce${}_{3}$Bi${}_{4}$Pt${}_{3}$. Both of these Kondo insulators show a saturation of the increasing resistivity at low temperature that suggests an additional conduction channel and is consistent with the theoretical prediction that these systems should host robust surface states due to their nontrivial topology. While previous work has shown that the resistance saturation in SmB${}_{6}$ is due to conducting surface states, this work demonstrates that the origin of the resistance saturation in Ce${}_{3}$Bi${}_{4}$Pt${}_{3}$ is through the bulk of the material and not the surface. The contrasting behavior of the two materials when the surface is disordered with ion-irradiation supports this conclusion, and also demonstrates the sensitivity of the resistance of the surface state in SmB${}_{6}$ to small levels of disorder. Finally, the low-temperature specific heat of SmB${}_{6}$ shows a residual linear term, and the origin of this term has been highly debated recently. Through measurements of single crystals and powders, it is shown that this term dominantly arises from the bulk and not the surface state.

[Phys. Rev. B 94, 035127] Published Tue Jul 12, 2016

]]>The properties of an electron beam can be manipulated by electromagnetic fields in vacuum via the ponderomotive force. Such an interaction is also at the core of the Kapitza-Dirac effect, which describes the diffraction of electrons by an optical standing wave. Here, the authors predict a new type of interaction between electrons and the electromagnetic field, opening up new possibilities for the manipulation of electron beams. If surface plasmon polaritons are tailored to interfere forming a periodic field pattern, it becomes possible to diffract electrons from such near field. With the proper manipulation of the plasmonic fields, orbital angular momentum can be imparted to the electrons, and even the phase of their wave functions can be manipulated. An additional degree of freedom is provided by the possibility to tailor the spatial properties of the light and the materials supporting the surface plasmons. This arbitrary control can be extended to different substrates such as graphene or layered systems and may open up a viable route to create tunable phase plates for electron microscopes.

[Phys. Rev. B 94, 041404(R)] Published Mon Jul 11, 2016

]]>“Strange metal phases”, displaying strong deviations from Fermi liquid theory, are discussed in the context of cuprates, itinerant magnets, or heavy-fermion metals. Several Yb-based materials, e.g., $\beta $-YbAlB${}_{4}$ or the Au-Al-Yb quasicrystal, display $T$/$B$ scaling of magnetic and thermodynamic properties. This has been taken as evidence for zero-field quantum criticality without requirement to fine-tune composition and pressure. However, it appears unlikely that materials are accidentally located at such a special point in multidimensional phase space. In this paper, the authors demonstrate by a thermodynamic study on the new Kondo lattice compound YbCo${}_{2}$Ge${}_{4}$ that $T$/$B$ scaling and a divergence of the magnetic Grüneisen parameter can arise without a quasiparticle mass divergence and in the absence of zero-field quantum criticality. They discuss alternative scenarios for such strange metal behavior.

[Phys. Rev. B 94, 041106(R)] Published Fri Jul 08, 2016

]]>Engineering the gauge field of a physical system is a crucial step in the effort to control the behavior of waves. In the past few years, scientists have demonstrated the ability to design and implement artificial gauge fields in a variety of physical systems, which have played a major role in demonstrating various fascinating phenomena in both photonic and cold-atoms settings. In this Rapid Communication, the authors explore, theoretically and experimentally, a method to create artificial gauge fields. They implement this method for demonstrating the Rashba effect – the photonic equivalent of the spin-orbit coupling known from electronic systems.

[Phys. Rev. B 94, 020301(R)] Published Thu Jul 07, 2016

]]>The control of octahedral rotations in perovskite heterostructures is an emerging strategy for inducing new functionality as evidenced by recent predictions of improper ferroelectricity, polar metals, and multiferroics. Many of these predictions are predicated on the presence of a specific rotation pattern (${a}^{-}\phantom{\rule{0}{0ex}}{a}^{-}\phantom{\rule{0}{0ex}}{c}^{+}$) in superlattices that exhibit the orthorhombic ($P\phantom{\rule{0}{0ex}}b\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}m$) perovskite structural variant. The authors use synchrotron diffraction to measure the octahedral rotation patterns in strained ferrite, manganite, and gallate perovskite films finding that compressive strain strongly favors ${a}^{+}\phantom{\rule{0}{0ex}}{a}^{-}\phantom{\rule{0}{0ex}}{c}^{-}$ rotation patterns and tensile strain weakly favors ${a}^{-}\phantom{\rule{0}{0ex}}{a}^{-}\phantom{\rule{0}{0ex}}{c}^{+}$ structures. In contrast, films grown on orthorhombic substrates exhibit the same rotation pattern orientation as the substrate, even for epitaxial conditions where strain would favor the opposite structural orientation. The results indicate that substrate imprinting is a more robust method than strain for controlling the rotation pattern in $P\phantom{\rule{0}{0ex}}b\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}m$-type perovskite films, a finding that should enable more efficient experimental pursuits of rotation-driven ferroic states in oxide heterostructures.

[Phys. Rev. B 94, 024105] Published Thu Jul 07, 2016

]]>Ferromagnetism and superconductivity are two generally incompatible states of matter. Their coexistence has been observed only under very unusual circumstances and so far only in metals. In contrast, most semiconductors are not naturally magnetic or superconducting, but introducing magnetism or superconductivity into semiconductors is seen as an important step towards radical improvement of our electronics capabilities and therefore a hotly pursued goal. Here, the authors report the discovery of superconductivity coexisting with ferromagnetism in the semiconducting material samarium nitride (SmN). The large intrinsic exchange splitting of the conduction band in SmN requires the superconducting order to be of unconventional (likely $p$-wave) type. Superconductivity is observed to be even further enhanced in superlattices where layers of SmN alternate with layers made of the strongly ferromagnetic but non-superconducting material gadolinium nitride. These features render SmN an interesting laboratory for understanding more about the fundamentals of ferromagnetism and superconductivity in semiconductors and exploring opportunities for integrating superconducting spintronics into the design of semiconductor-based electronic devices.

[Phys. Rev. B 94, 024106] Published Thu Jul 07, 2016

]]>A ${\mathbb{Z}}_{2}$ fractionalized Fermi liquid (FL*) is a novel state of strongly correlated quantum matter. Although it is metallic, it violates Luttinger’s theorem on the volume enclosed by the Fermi surface obeyed by conventional metals; this is possible due to the presence of emergent gauge excitations. The authors study here the superconducting states that can arise out of instabilities of such a topological metal. In particular, they focus on a FL* topological metal that has favorable energetics on the square lattice with nematic order, relevant to the cuprate superconductors. They find that a Higgs transition out of this FL* results in confinement of the anyonic degrees of freedom, and the resulting state is a superconductor with broken translation symmetry or time-reversal invariance. In the process, they also establish a complete mapping between bosonic and fermionic descriptions of time-reversal invariant gapped insulating ${\mathbb{Z}}_{2}$ spin liquid states on the rectangular lattice, which, on doping with charge carriers, can give rise to FL* phases. They also note a possible connection to the recent observation of pair density waves in the superconducting state of the underdoped cuprates.

[Phys. Rev. B 94, 024502] Published Tue Jul 05, 2016

]]>Magnetic vortex is one of the simplest nontrivial topological structure that can exist in magnetic nanodots. Due to nonzero topological charge, vortices exhibit complex and interesting dynamic under external excitation, which includes excitation of translational (gyrotropic) and spin-wave modes. Here, the authors study the vortex dynamic in thick magnetic nanodots, when the vortex becomes essentially three dimensional. The third dimension is shown to lead to the appearance of additional spin-wave modes with characteristic curled structure (“curled” modes), which becomes the lowest ones in frequency among spin-wave modes for a sufficiently thick nanodot. Besides an uncommon profile, the curled modes also show uncommon hybridization with gyrotropic modes: modes propagating in opposite directions (clockwise and counterclockwise) that can be hybridized with different order gyrotropic modes.

[Phys. Rev. B 93, 214437] Published Tue Jun 28, 2016

]]>Magnetic tunnel junctions are under active consideration owing to their applications including the potential next generations of spin transfer torque memories. In view of the high symmetry of the magnetic properties and of the high frequency of the system eigenexcitations, out-of-plane magnetized systems are thought to enable a faster and simpler spin-torque-induced switching process than the formerly used in-plane magnetized systems. Unfortunately, the lack of high frequency and low current switching experiments have precluded, so far, the assessment of the speed potential of perpendicular anisotropy systems at low junction dimensions. By time resolving the switching and evidencing its stochastic aspects at the nanosecond-scale, the authors here demonstrate that a complex dynamics happens and persists down to small sized elements with always a very strong asymmetry between the two switching directions. The reversal is explained by the complex interplay between the spatial profile of the stray field emanating from the fixed system of the tunnel junction and the constraint that the switching is preferably initiated from the device edge.

[Phys. Rev. B 93, 224432] Published Tue Jun 28, 2016

]]>Much interest has emerged in the realization of materials with quantum spin liquid ground states, which would provide fertile ground for realizing exotic excitations. Three “Kitaev” spin liquid candidate materials have been proposed – Na${}_{2}$IrO${}_{3}$, $\alpha $-RuCl${}_{3}$, and $\alpha $-Li${}_{2}$IrO${}_{3}$ – and there has been many diverging proposals for the most relevant interactions. Here, the authors re-evaluate carefully the magnetic interactions in these materials, using nonperturbative exact diagonalization methods. The results clarify misconceptions in the literature and help estimate the potential for realizing the spin liquid state in real materials.

[Phys. Rev. B 93, 214431] Published Mon Jun 27, 2016

]]>The role of spin-orbit coupling (SOC) in 4$d$ and 5$d$ transition metal oxides is relatively poorly understood outside of the LS or JJ coupling limits. In this work, the importance of the intermediate regime is demonstrated through the identification of SOC as an essential feature of the collective properties in nominally orbitally quenched 5${d}^{3}$ systems. The authors probe the magnetic excitations in model system Sr${}_{2}$ScOsO${}_{6}$ via inelastic neutron scattering and model the results including the effects of SOC-induced anisotropy. Experimentally determining the strengths of nearest and next-nearest neighbor interactions leads to the important discovery that the magnetic ground state is controlled by anisotropy, which would not be possible in the LS coupling limit. This indicates that SOC plays a key role in relieving frustration and promoting heightened transition temperatures in 5${d}^{3}$ double perovskites.

[Phys. Rev. B 93, 220408(R)] Published Mon Jun 27, 2016

]]>Motivated by the particle-hole symmetry at the half-filled lowest Landau level, many articles have taken the point of view, following D. T. Son, that composite fermions are Dirac particles and their Fermi sea is akin to that occurring at the surface of a three dimensional topological insulator. This presents a paradox, because the microscopic theory of composite fermions, which treats composite fermions as non-relativistic particles, has been confirmed in great detail. The authors discuss what features of the Dirac-composite fermion view can be reconciled with the microscopic theory. They also show how particle-hole symmetry can emerge in the Chern-Simons formulation of composite fermions.

[Phys. Rev. B 93, 235152] Published Mon Jun 27, 2016

]]>Phases of matter are traditionally seen as families of static systems exhibiting the same long-distance and low-energy correlations. In this work, the authors propose and classify a new family of phases of matter. They are novel insofar as they are intrinsically driven and out of equilibrium — they can only be realized in systems with time-dependent Hamiltonians. The phases we consider arise in 1D periodically driven systems, in the presence of strong disorder and interactions, and are similar to but qualitatively distinct from the symmetry-protected topological phases now well known in the equilibrium setting. In a companion paper, the authors examine similar intrinsically driven families of states with long-range order, and an order parameter that oscillates at a frequency that is robustly an integer multiple of the underlying drive frequency.

[Phys. Rev. B 93, 245145] Published Mon Jun 27, 2016

]]>Duality is a powerful theoretical tool whereby the strongly interacting and intractable regime of a theory can be accessed in terms of weakly interacting “dual” variables. In condensed matter, this is familiar from the Kramers-Wannier duality of the Ising model, and boson-vortex duality. Here, a duality for another fundamental problem – Dirac fermions in two spatial dimensions – is obtained, which provides a new window to the physics of strongly interacting electrons. This leads to unexpected connections between topological insulators in three dimensions and the physics of the half-filled Landau level, including composite fermions and Pfaffian quantum Hall states.

[Phys. Rev. B 93, 245151] Published Mon Jun 27, 2016

]]>This study is motivated by a challenge to the third law of the thermodynamics: how does the degenerated ground state in a textbook example behave in a real compound? At very low temperatures, nature tries to preserve the basic law using small perturbations and a novel quantum state should appear. The authors here study the classic frustrated magnet Ba${}_{3}$Yb${}_{2}$Zn${}_{5}$O${}_{11}$, a spin tetrahedron having two-fold degeneracy realized in a breathing pyrochlore lattice. They measure by inelastic neutron scattering the low-energy excitations in order to identify the effective spin Hamiltonian and macroscopic properties at very low temperatures. They demonstrate how the degeneracy of the ground state is lifted and a unique quantum state is selected.

[Phys. Rev. B 93, 220407(R)] Published Fri Jun 24, 2016

]]>Unraveling the peculiar properties of topological materials often requires tuning of the bulk chemical potential, which has been addressed by compensation of acceptors and donors. Compensation necessarily creates Coulomb disorder which, however, impedes chemical potential tuning and gives rise to self-organized charge puddles. In the topological insulator BiSbTeSe${}_{2}$, optical data and Monte Carlo simulations presented here reveal the existence of puddles at low temperatures as well as their surprising “evaporation” on a temperature scale of about 40 K. The highly nonlinear screening effect of thermal excitations is also important for Dirac matter or Weyl semimetals. Understanding and controlling the puddles will be a key to realizing novel phenomena in 3D topological materials, as was the case with 2D materials such as graphene.

[Phys. Rev. B 93, 245149] Published Fri Jun 24, 2016

]]>Cubic chiral magnets crystallizing in space group $P$2${}_{1}$3 give rise to a spin structure of nontrivial topology, the so-called skyrmion lattice. The strongly doped semiconductor Fe${}_{1-x}$Co${}_{x}$Si is a fascinating member of this material class of compounds as it exhibits helimagnetism over a wide compositional range. The authors study the magnetic properties of this itinerant magnet for different values of $x$, combine their results with previous reports, and infer the magnetic phase diagrams. The latter are rather isotropic but depend on the field and temperature history. In particular, when cooled in an applied magnetic field, the skyrmion lattice may persist down to lowest temperatures. This history dependence, in combination with the variation of temperature, field, and length scale of the magnetic order by means of compositional tuning, makes Fe${}_{1-x}$Co${}_{x}$Si an excellent material for future experiments studying skyrmions.

[Phys. Rev. B 93, 235144] Published Tue Jun 21, 2016

]]>Quantum dots have long been considered systems that could deliver a significant advantage for practical quantum information applications. Their atomic dipolelike transitions and their compatibility to be integrated into a variety of photonic structures has highlighted this fact. Here, the authors measure an unexpectedly high single-photon level phase shift (~6${}^{\circ}$), induced upon reflection from a singly negatively charged quantum dot incorporated in a low-$Q$-factor cavity ($Q$~290). This measurement provides the first evidence for a system with unit fidelity and a high efficiency in its interactions. The result opens a new road to realistic spin-photon entanglement devices, in a simple and easy to fabricate photonic structure.

[Phys. Rev. B 93, 241409(R)] Published Tue Jun 21, 2016

]]>A special type of anisotropic spin exchange called the “Kitaev interaction” provides rare exactly-soluble models of gapless quantum spin liquids in 2d and 3d. The main goals of this work were to identify other lattice geometries that support the elusive Kitaev interactions and understand the magnetic properties of such materials. By performing inelastic neutron scattering on La${}_{2}$MgIrO${}_{6}$ and La${}_{2}$ZnIrO${}_{6}$, the authors find that these magnets, which possess long-range antiferromagnetic order, yield clear signatures of gapped and nearly-dispersionless magnon excitations. They also show that spin-wave models with a dominant Kitaev interaction can naturally account for the magnetic ordering pattern as well as the inelastic neutron scattering data.

[Phys. Rev. B 93, 214426] Published Mon Jun 20, 2016

]]>Coupled-cluster theory is a “gold standard” of molecular quantum chemistry, but has only recently been extended to condensed-phase systems. In this work, excited-state coupled-cluster techniques are brought to bear on the spectral function of the uniform electron gas. The diagrammatic content of coupled-cluster theory goes beyond that of the $G\phantom{\rule{0}{0ex}}W$ approximation, which has become a canonical approach in first-principles condensed-matter physics. Benchmark calculations demonstrate that the coupled-cluster results are more accurate than those of $G\phantom{\rule{0}{0ex}}W$-based approaches, insensitive to the mean-field reference, and systematically improvable towards the exact result. Calculations on large systems approaching the thermodynamic limit reveal an accurate quasiparticle bandwidth as well as plasmonic satellite structure in the core-hole spectral function.

[Phys. Rev. B 93, 235139] Published Mon Jun 20, 2016

]]>The search for Majorana bound states, motivated by the prospect of discovering non-Abelian exchange statistics and developing topological quantum computing, is an ongoing challenge in condensed matter physics. Crafting a wire hosting Majorana states is not a trivial task, requiring great care in the choice of materials and in the device design. Experimentalists rely on the electron transport measurements to characterize the devices. It is therefore important to have a detailed theory for interpreting the experimental data and reliably detecting the nontrivial (topological) superconductivity needed for the appearance of Majorana states. Here, the authors have developed such a theory for short small-capacitance wires. In such wires, the apperance of Majorana states coexists with the effects of single-electron charging. The theory here provides quantitative results for the electric conduction of wires in these conditions and provides guidance for those in the field seeking understanding of important recent experiments, as well as those designing the future ones.

[Phys. Rev. B 93, 235431] Published Mon Jun 20, 2016

]]>The existence of the surface Fermi arc states is a signature property of the Weyl semimetals. In view of their one-dimensional chiral nature, one might naively expect that the corresponding surface transport is nondissipative. In a drastic contrast to such an expectation, the authors show here that the dynamics of the chiral surface states is dissipative in semimetals. In the presence of impurities, the origin of the dissipation is the scattering of surface Fermi arc states into the bulk of the semimetal. The gapless spectrum of the bulk states in semimetals is the key feature that allows such processes to happen. By taking into account all scattering processes, the authors perform a model calculation of the electric conductivity of the surface Fermi arc states.

[Phys. Rev. B 93, 235127] Published Wed Jun 15, 2016

]]>Two important scenarios have been put forth to explain high-temperature superconductivity. One scenario emphasizes, following Philip W. Anderson, that for many high-${T}_{c}$ superconductors their undoped parent compounds are Mott insulators, with local physics playing a major role. Another scenario stresses instead the importance of spin fluctuations near a quantum critical point. In this work, the authors reconcile both viewpoints by constructing a functional of the Green’s function of the electron ($G$), of the fluctuations ($W$) and of the vertex that couples the two ($\mathrm{\Lambda}$). In the approach presented by the authors, Mott physics as well as the charge and spin fluctuations are taken into account in the description of correlations. This is done at a computational cost comparable to that of the dynamical mean field theory, making the method suitable for multiorbital extensions.

[Phys. Rev. B 93, 235124] Published Tue Jun 14, 2016

]]>Because of the unconventional nature of high-temperature superconductors, it has been proposed that magnons, rather than phonons, play an important role in Cooper pair formation. It is therefore of great importance to understand how magnetic excitations behave and evolve between superconducting and nonsuperconducting compounds. In this work, the authors present a comprehensive study of the magnetic excitations in electron-doped NaFe${}_{1-x}$Co${}_{x}$As in several samples across the phase diagram, including both superconducting and nonsuperconducting compounds. They find a strong suppression of low-energy magnetic scattering connected with the onset of superconductivity but no effect on either high-energy scattering intensity or bandwidth. Additionally, through comparison with the related but structurally distinct compound BaFe${}_{2-x}$Ni${}_{x}$As${}_{2}$, they find a comparable total moment despite a smaller ordered moment, narrower scattering bandwidth, and T${}_{c}$. These features all point to magnetism as an agent in the formation of superconductivity.

[Phys. Rev. B 93, 214506] Published Mon Jun 13, 2016

]]>Magnetic phases and phase transitions in artificial spin ice, constructed from tailor-made arrays of nanomagnets, are currently subjects of great interest. The authors present measurements of the diffuse soft x-ray resonant magnetic scattering in artificial spin ice, where the moments of the magnets, arranged on the sites of the kagome lattice, are highly dynamic. Comparing experimental scattering patterns with the patterns calculated from Monte Carlo simulations based on a needle-dipole model, they show the emergence of quasi-pinch-points in the kagome ice I phase, and explain their relation to the pinch-point singularities in spin ice pyrochlores. As in the bulk pyrochlore spin ices, measurement of diffuse scattering from artificial kagome spin ice provides unique information on the magnetic correlations and can be applied to a number of other problems concerning nanomagnetic systems.

[Phys. Rev. B 93, 224413] Published Mon Jun 13, 2016

]]>Measuring spin coherence time is essential in general characterization of a qubit. Here, a novel method to determine the coherence time of a spin qubit is implemented, based on the coherence properties of inelastic Raman scattering obtained with an unbalanced Mach-Zehnder interferometer. The authors demonstrate this method on both a single electron spin and a hole spin confined in a self-assembled quantum dot. The visibility of the first-order coherence of Raman scattered photon in the weak excitation regime exhibits a Gaussian decay as a function of the time delay, allowing a direct extraction of ${T}_{2}^{*}$, which is immune to the limitations that influence standard Ramsey interference with self-assembled quantum dots.

[Phys. Rev. B 93, 241302(R)] Published Mon Jun 13, 2016

]]>Rendering arbitrary objects invisible is currently of great interest in photonics. While there are some optical tricks to hide macroscopic objects, such as camouflaging and mimetics, true invisibility remains out of reach. However, for sufficiently small objects one can use advanced techniques to prevent their detection at least at discrete wavelengths. The scattered light is suppressed, as envisioned using the recently developed scattering cancellation technique. At optical frequencies, metallic nanoparticles that mantle a small object do the desired job. However, tuning the operational frequency of such a mantle cloak remained a challenge. The authors here solve this problem by using silver ellipsoids instead of spheres. Depending on the axis ratio, the cloaking frequency is tunable throughout the entire visible spectrum. The cloak performance has been rigorously analyzed using the T-matrix method. It is envisioned that the optimized structure opens the door to many applications where the scattering signal will be suppressed at a predefined wavelength, e.g., to suppress the cross-talk between adjacent optical nanoantennas or the spurious signal from a tip in a near-field optical microscope.

[Phys. Rev. B 93, 245127] Published Mon Jun 13, 2016

]]>The tricritical Ising universality class emerges at the end point of a line of continuous Ising phase transitions, above which the transition is first order. In 1+1 dimensions, it is described by the second minimal model of conformal field theory (CFT) with central charge $c=7/10$. Recently, there has been a revival of interest in the tricritical Ising model due to its emergent space-time supersymmetry. Combining numerical (infinite density-matrix renormalization group) and field theoretical (bosonization and CFT) techniques, there authors demonstrate here that Ising tricriticality occurs in a paradigmatic extended one dimensional Hubbard model with explicit bond dimerization. The low-energy sector can be described by a triple sine-Gordon model, which, in the transition region, reduces to the tricritical Ising CFT. The theoretically predicted decay of various two-point correlation functions is found to be in excellent agreement with the density-matrix renormalization group data.

[Phys. Rev. B 93, 235118] Published Fri Jun 10, 2016

]]>Ferroelectrics exhibit switchable spontaneous polarization, a functionality that can be harnessed in nonvolatile memory devices. The main characteristics of these devices are governed by the electronic band alignment at the interfaces, particularly at the metallic contacts. Here, the authors study such an interface using state-of-the-art high-energy x-ray photoelectron spectroscopy, with in situ polarization reversal achieved by applying poling voltage. They confirm a result central to the physics of ferroelectric devices: an electrode-dependent potential drop at the interface that is linear in the spontaneous polarization. They also reveal a novel effect: electric-field-dependent increase of the energy offset due to the field-induced strain in the ferroelectric material.

[Phys. Rev. B 93, 235415] Published Fri Jun 10, 2016

]]>Angle-resolved photoemission spectroscopy (ARPES) provides direct access to the electronic band structure of materials. Very recently, the spatial resolution of ARPES setups has dramatically increased, with a spot size at the sample shrinking from 50$\times $50 $\mu $m${}^{2}$, typical of most synchrotron-based setups, down to 100$\times $100 nm${}^{2}$ or less. In this work, the authors demonstrate the impact of increased spatial resolution by evidencing faint spatial inhomogeneity in the ARPES signal of a graphene sample that would go undetected in ARPES setups possessing lower spatial resolution.

[Phys. Rev. B 93, 241101(R)] Published Thu Jun 09, 2016

]]>Superconductivity (SC) and local-moment ferromagnetism (FM) are mutually antagonistic collective phenomena, and their genuine coexistence is rarely seen in nature. Although recent years have witnessed the possible coexistence of SC and Eu-spin FM in some doped EuFe${}_{2}$As${}_{2}$ systems, evidence of simultaneous bulk SC and obvious FM is still absent. Here, the authors report a robust coexistence of SC and FM in the iron pnictide RbEuFe${}_{4}$As${}_{4}$. The newly synthesized material is virtually an intergrowth of RbFe${}_{2}$As${}_{2}$ and EuFe${}_{2}$As${}_{2}$, making it hole doped by itself. With decreasing temperature, SC emerges at 36.5 K, followed by a Eu-spin ferromagnetic ordering at 15 K. Bulk SC is unambiguously confirmed by a large specific heat jump $\mathrm{\Delta}\phantom{\rule{0}{0ex}}C\phantom{\rule{1.000em}{0ex}}$=7.5 J mol${}^{-1}$ K${}^{-1}$ at 36 K, while the Eu-spin FM is demonstrated by a coersive field of 360 Oe as well as a saturation magnetization of 6.5 ${\mu}_{B}$/Eu at 2 K. Additional interesting issues include a rarely observed third-order ferromagnetic transition and a possible Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state below 5 K. The findings not only provide an excellent playground to investigate the long-standing issue of how SC coexists with long-range local-moment FM, but also give hints for the mechanism of iron-based SC by answering why SC and FM are compatible in this particular system.

[Phys. Rev. B 93, 214503] Published Wed Jun 08, 2016

]]>An additional degree of freedom in cobalt oxides that makes them fascinating is the spin state of the Co${}^{3+}$ ions, which possess six $d$-shall electrons and can form configurations with spins $S$=0, 1, or 2. In the prototypical member of the family, LaCoO${}_{3}$, the ground state is believed to be insulating with the spin state $S$=0. However, there has been a great deal of controversy over the last decades about the nature of the excited states and the phases observed at higher temperatures. The authors here approach this long-standing problem by applying ultrahigh magnetic fields reaching 133 Tesla at temperatures ranging from 2 to 120 K. Surprisingly, at magnetic fields above 100 T, they find two novel magnetic phases that are identified as spin-state crystalline states, possibly with some orbital ordering.

[Phys. Rev. B 93, 220401(R)] Published Mon Jun 06, 2016

]]>The remarkable quantization of the Hall conductivity is routinely observed in two-dimensional electron gases subject to strong magnetic fields. Such quantum transport experiments are performed by measuring the current that flows in 2D systems in response to an applied electric field, and result in a quantized conductance proportional to a topological index, known as the Chern number. Recently, a similar effect was proposed and observed in gases of ultracold atoms, when these are loaded into Bloch bands with a nonzero Chern number. In this context, the center of mass of the atomic gas performs a measurable differential drift, in direct analogy with the electronic Hall effect, from which the corresponding Chern number can be obtained. Such detection relies on the assumption that the center-of-mass drift of the atomic cloud is directly related to the current density, which itself depends on the Chern number of the band. Here, the authors carefully revisit and analyze the center-of-mass detection scheme. Interestingly, we find that the center-of-mass motion also depends on the particle density, which in the presence of magnetic perturbations, can be explicitly related to the topology of the band. Whereas this additional dependence offers a correction to existing experimental results, it opens up a rich variety of new possible quantized responses and detection schemes in 2D and 4D engineered systems, such as cold atoms and photonic lattices.

[Phys. Rev. B 93, 245113] Published Mon Jun 06, 2016

]]>Flexoelectricity has become a hot topic, and recent substantial advances in theoretical modeling have been key to this success. Establishing a strong synergy between continuum and first-principles approaches appears crucial for future progress. This manuscript presents a powerful innovative approach to achieve this goal. Moreover, it unravels unsuspected and deep connections between flexoelectricity and strain-gradient elasticity. The latter is an important field of its own, where a formal $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ theory was still missing. This manuscript unifies two fields, provides a fundamental advance in both, pushing them into unexplored directions.

[Phys. Rev. B 93, 245107] Published Fri Jun 03, 2016

]]>The understanding of the microscopic origin of electron pairing in strongly correlated electron systems remains an ultimate goal in the field of unconventional superconductivity. Detailed tunneling spectroscopy of vortex core states can provide important insight to the momentum structure of the superconducting order parameter, which is closely related to the nature of the electron pairing. The authors performed a fully self-consistent real-space BdG study of vortex core states in five-orbital models relevant to Fe-based superconductors. The superconducting order was stabilized by spin fluctuation-derived pairing vertices generating an ${s}_{\pm}$-wave gap structure. By application to LiFeAs, they find striking agreement with STS measurements by Hanaguri $e\phantom{\rule{0}{0ex}}t$ $a\phantom{\rule{0}{0ex}}l$. on this compound. In particular, the details of the energy dependence and the spatial structure seems in almost quantitative agreement without any tuning parameters. From this fact they conclude that their model provides a reasonable description of LiFeAs, without the necessity to invoke more exotic paring states of this material.

[Phys. Rev. B 93, 224503] Published Thu Jun 02, 2016

]]>The finding of new multiferroic materials, where electric and magnetic orders coexist, is a challenging task currently. The double perovskite Y${}_{2}$CoMnO${}_{6}$ shows spontaneous magnetization and electrical polarization at low temperature. Previous investigations of this compound did not reach agreement about the type of magnetic structure present. This study demonstrates that this compound exhibits a collinear ferromagnetic ordering of Co${}^{2+}$ and Mn${}^{4+}$ moments in the $a\phantom{\rule{0}{0ex}}c$ plane with a small antiferromagnetic canting along the $b$ axis. A thorough characterization of the dielectric properties reveals the absence of any related anomaly in the dielectric permittivity and the lack of spontaneous electrical polarization ($P$) in the $P$($E$, electric field) loops. The pyroelectric current is strongly dependent on the number of antisite defects in the Co/Mn arrangement, the heating rate, and the poling field. Thus, the observed electric polarization is due to thermally stimulated depolarization currents ascribed to defect dipoles mainly placed at the antiphase boundaries. No ferroelectric transition occurs in this material, disproving the existence of intrinsic magnetoelectric multiferroicity.

[Phys. Rev. B 93, 214401] Published Wed Jun 01, 2016

]]>For polariton condensates, nonresonant excitation creates carriers that act both as a repulsive potential and a gain medium for the condensate. The repulsive potential allows for direct imprinting of functional optical elements and simple circuits based on flowing polaritons just by shaping the excitation beam. However, extended geometries will also require amplification of the polariton beam. Another nonresonant pump beam may act as an amplifier, but will necessarily also result in an undesired deflection of the traveling polariton beam. The authors show that a deflectionless amplifier can be realized by embedding the amplification beam in a photonic trap that cancels the repulsive potential created by the amplification beam. This work emphasizes the potential of polariton condensates as a platform for reconfigurable optical circuits.

[Phys. Rev. B 93, 235301] Published Wed Jun 01, 2016

]]>Photoemission spectroscopy is one of the most direct routes to investigate electronic systems. However, if electron correlations are strong, the identification of the predominant scattering processes through the analysis of the photoelectron spectra becomes a formidable challenge. The authors introduce here a decomposition of the electronic self-energy (see image), based on the so-called parquet equations, which, as suggested by their name, take into account all scattering channels and their interconnections. The application of this new parquet decomposition to self-energies computed in the case of local Coulomb interaction in two-dimensions (2D Hubbard model) yields several new results. Future investigations based on parquet decompositions and/or the fluctuation diagnostics of the self-energy could shed further light on the microscopic processes controlling the spectral functions in correlated electron models.

[Phys. Rev. B 93, 245102] Published Wed Jun 01, 2016

]]>Quantum thermalization of isolated systems undergoing unitary time evolution is a fundamental problem in quantum statistical mechanics. Its study has been revived recently in the context of many-body Anderson localization. Previous works have focused on localization of many-body systems with all the single-particle states being localized. As a significant step forward, this work studies localization aspects of noninteracting many-particle systems in the presence of a single-particle mobility edge. By systemically investigating entanglement entropy scaling and nonthermal fluctuations in various lattice models, the authors establish a nonergodic extended phase as a generic intermediate phase (between purely ergodic extended and nonergodic localized phases) for the many-body localization transition of noninteracting fermions. This work also sheds light on the interacting transition scenario as well.

[Phys. Rev. B 93, 184204] Published Tue May 31, 2016

]]>Topological band structures are ubiquitous in both fermionic and bosonic problems, but conventional wisdom holds that the topological features in a noninteracting bosonic system are only visible to finite-frequency probes. Utilizing a recently proposed boson-to-fermion mapping, the authors demonstrate that topological gapless phonon modes can in fact be brought all the way down to zero frequency. In particular, they construct spring-mass models that are analogs of two-dimensional Dirac semimetals, and hence feature topological zero-frequency phonon modes. These zero modes are physically distinct from the familiar Goldstone phonons, and are in fact present even in pinned disordered systems due to topological protection.

[Phys. Rev. B 93, 205158] Published Tue May 31, 2016

]]>In conventional superconductors, the energy gap lies in the frequency range of few tenths of terahertz (THz), making THz spectroscopy the best tool to access its fundamental excitations. Recently it has been shown by R. Matsunaga $e\phantom{\rule{0}{0ex}}t$ $a\phantom{\rule{0}{0ex}}l$. that the use of intense, coherent multicycle THz pulses allows one to measure, in a NbN film, a component of the transmitted pulse oscillating three times faster than the incident light. It is found that this effect, named third-harmonic generation, has its maximum intensity at the temperature below ${T}_{c}$, where the light frequency $\omega $ matches the superconducting gap value $\mathrm{\Delta}$(T), pointing to a resonant process involving excitations specific of the superconducting state. What is the nature of this resonance? While previous work attributed this resonance to the Higgs mode, i.e., to amplitude fluctuations of the superconducting order parameter, the present paper comes to a different conclusion. By providing a detailed microscopic derivation of the nonlinear optical response, the authors show that the 3$\omega $ current response is controlled by the lattice-modulated density fluctuations. As a consequence, the third-harmonic generation turns out to be largely dominated by Cooper-pair excitations, which pile up at 2$\mathrm{\Delta}$. At the same time, in analogy with the standard Raman response, the Higgs signal is suppressed by the extremely small coupling to the probing field. The authors discuss also the polarization dependence of the nonlinear 3$\omega $ response, which opens the route to a Raman-like symmetry-selective probe of the superconducting excitations. This result offers challenging perspectives for THz spectroscopy of several systems, including cuprate superconductors.

[Phys. Rev. B 93, 180507(R)] Published Wed May 25, 2016

]]>Defect-assisted nonradiative Shockley-Read-Hall recombination is an important process in wide-band-gap semiconductors. However, nonradiative capture rates decrease exponentially with the energy of the transition, and therefore the mechanisms by which such recombination can take place are unclear. The authors show here that excited states of defects play a key role in turning specific defects into efficient nonradiative centers. The wider the band gap of the material, the greater a role these excited states are likely to play. Specifically, the authors can explain why certain gallium vacancy complexes are efficient nonradiative centers in group-III nitrides, the key materials for solid-state lighting.

[Phys. Rev. B 93, 201304(R)] Published Wed May 25, 2016

]]>Effective mass calculations are currently based on finite-difference estimations of electronic band curvature (or Hessians). This requires an extra convergence study that can easily reach problematic levels if not carefully monitored. While these issues could be considered hindrances in traditional first-principles calculations, they become more fundamental limitations within the quickly expanding field of high-throughput material design. To eliminate these issues, a new method is developed based on density functional perturbation theory (DFPT) that directly computes the Hessian of DFT bands. The new method includes a generalized k.p formalism within which it is possible to define effective masses at degenerate band extrema. The result is a simulation method of effective masses that is general, robust, and simple to use, which should open new routes in high-throughput material design.

[Phys. Rev. B 93, 205147] Published Wed May 25, 2016

]]>Some of the plasmonic phenomena can be imitated by exploiting the structural dispersion of parallel-plate waveguides filled with positive dielectrics. This synthetic platform, as a test bed for exploring plasmonic features, is more suitable for longer-wavelength regimes and may exhibit lower loss, since positive dielectric materials are utilized.

[Phys. Rev. B 93, 195152] Published Tue May 24, 2016

]]>Identifying topological phases in strongly interacting many-body systems is a very challenging task, especially beyond fine-tuned exactly solvable models. The construction of topological phases from coupled quantum wires provides a controllable and systematic theoretical handle to those interactions. The authors show an explicit connection between the coupled wire construction and a lattice Hamiltonian of interacting bosons, which host a $U$(1) symmetry-protected topological phase (also referred to as the bosonic integer quantum Hall state).

[Phys. Rev. B 93, 195143] Published Fri May 20, 2016

]]>Multiferroic materials that simultaneously show electricity and magnetism provide an interesting playground for novel optical phenomena. One example is nonreciprocity, a change in optical properties with the reversal of the propagation direction of light. In this paper, the authors report that the luminescence intensity in multiferroic CuB${}_{2}$O${}_{4}$ changes by 70% between the opposite directions of the emitted light. The observed asymmetry is about 100 times larger than the previously reported value. They demonstrate that the gigantic directional asymmetry of luminescence provides a tool to visualize the antiferromagnetic domain structure, which is impossible using the conventional linear optical Faraday or Kerr effects.

[Phys. Rev. B 93, 201109(R)] Published Fri May 20, 2016

]]>Disordered superconductors show remarkable physics governed by the interplay of superconductivity and Anderson localization. The competition between these phenomena leads to a quantum phase transition: the superconductor-insulator transition (SIT). In this paper, the authors develop a theory of local density of states (LDOS) – including its average and mesoscopic fluctuations – as measured in tunneling experiments near the SIT. They use the nonlinear sigma-model renormalization-group framework (“fermionic approach”) and treat systems with short-range and Coulomb interactions on equal footing. The average LDOS obtained shows a pronounced depletion around the Fermi energy, both in the metallic phase (i.e., above the superconducting critical temperature) and in the insulating phase near the SIT. The LDOS fluctuations are found to be particularly strong for the case of short-range interactions. The findings compare well with experimental observations of depletion of LDOS and of its large point-to-point fluctuations in the metallic and insulating phases near the SIT in TiN, InO, and NbN films. The observed effects are thus intrinsic properties of a macroscopically homogeneous system and do not require any additional assumptions, such as granularity.

[Phys. Rev. B 93, 205432] Published Fri May 20, 2016

]]>Electrons in semiconductor nanowires can be radially confined to form a thin tubular conductive channel a few nanometers below the surface. The peculiar potential landscape, which can be either a result of Fermi level pinning due to surface reconstruction or band mismatch at the heterointerface in core/shell nanowires, gives rise to both Rashba and Dresselhaus spin-orbit coupling (SOC). By utilizing k$\cdot $p theory, the authors have developed models for the SOC in nanowires with zinc-blende crystal structure for various nanowire growth directions. Taking advantage of these models, the weak (anti)localization correction $\mathrm{\Delta}\phantom{\rule{0}{0ex}}\sigma $ to the static Drude conductivity is derived. Finally, the theory is fitted to experimental data of an undoped $<$111$>$ InAs nanowire device, which exhibits a gate-controlled crossover from positive to negative magnetoconductivity. Thereby, the authors extract transport parameters and quantify the distinct types of SOC individually. The close agreement of theory and experiment suggests that the model provides reliable information and underlines the relevance of Dresselhaus SOC, which was previously often considered absent.

[Phys. Rev. B 93, 205306] Published Thu May 19, 2016

]]>The statistical physics of well isolated nonequilibrium quantum systems has been drawing intense interest recently, providing new paradigms for understanding equilibration and ergodicity (and failure thereof). Tools from the field of quantum information, such as quantum entanglement, have been instrumental in this development. Past investigations of the entanglement structure in localized and thermalizing phases have focused on diagnostics, such as entanglement entropy, which provide a useful but only partial characterization of the pattern of entanglement. In this work, inspired by random matrix theory, the authors examine localization and thermalization through the lens of the entanglement spectrum. This approach provides a complete characterization of the pattern of entanglement and yields new insights into many-body localization, thermalization, and nonequilibrium statistical mechanics.

[Phys. Rev. B 93, 174202] Published Wed May 18, 2016

]]>Protected chiral edge modes are a well-known signature of topologically ordered phases, such as the fractional quantum Hall states (FQHS). Using the framework of projected entangled pair states (PEPS) on the square lattice, the authors construct a family of chiral spin-$\frac{1}{2}$ quantum spin liquids with ${\mathbb{Z}}_{2}$ gauge symmetry and analyze in full detail the properties of the edge modes. Surprisingly, the results show that the latter can be well described by a chiral conformal field theory of free bosons (SU(2)${}_{1}$), as is the case for the $\nu $=$\frac{1}{2}$ (bosonic) gapped Laughlin state, despite the fact that the numerical data here suggest a critical bulk. The authors thus propose that our family of PEPS physically describes a boundary between a chiral topological phase and a trivial phase and might be closely connected to an (unknown) analogous FQHS.

[Phys. Rev. B 93, 174414] Published Wed May 18, 2016

]]>Lu${}_{2}$MnCoO${}_{6}$ is a rare example of a double-perovskite material exhibiting multiferroic behavior – coupling of magnetic and electric order – and a rare example of hysteretic coupling in a bulk material. Here, the authors lay to rest questions about whether it occurs in single crystals and verify that it is an intrinsic effect. The Mn and Co spins are arranged in a frustrated up-up-down-down ordering along the $c$ axis. Data from single crystals show that exchange striction produces electric polarization, albeit with a twist. Unlike other ‘up up down down’ multiferroics, the ferroelectricity emerges orthogonal to the magnetic ordering easy axis. This may be explained by a model taking into account the oxygen bond distortion in Lu${}_{2}$MnCoO${}_{6}$. Consequently, the system has a rich phase diagram: a dielectric response emerges below the magnetic ordering temperature, whereas a hysteretic polarization emerges in the region of magnetic hysteresis, as shown in this paper.

[Phys. Rev. B 93, 180405(R)] Published Tue May 17, 2016

]]>Topological nodal line semimetals exhibit protected one-dimensional Fermi lines, which arise due to an intricate interplay between the symmetry and topology of the electronic wave functions. In this paper, the authors derive the $\mathbb{Z}$ invariants that guarantee the stability of the line nodes in the bulk under reflection symmetry and show that a quantized Berry phase (i.e, a ${\mathbb{Z}}_{2}$ invariant) leads to the appearance of protected surfaces states, which take the shape of a drumhead. Most importantly, a relation between the $\mathbb{Z}$ invariant, which characterizes the bulk, and the quantized Berry phase is derived. This relation is generally applicable to any topological nodal line semimetal with or without spin-orbit coupling. Moreover, it is shown that the Berry phase invariant can be simply obtained by computing the reflection parity eigenvalues. As a representative example of a topological nodal line semimetal, the authors examine Ca${}_{3}$P${}_{2}$, which has been identified as an ideal system with the line nodes at the Fermi energy. Using numerical calculations, they show that the drumhead surface state of Ca${}_{3}$P${}_{2}$ has a rather weak dispersion, which implies that correlation effects are enhanced at the surface.

[Phys. Rev. B 93, 205132] Published Tue May 17, 2016

]]>The study of optical properties in semiconductors after excitation with an ultrashort laser pulse represents a unique method to investigate many-body physics out of equilibrium. Indeed, the presence of a nonequilibrium carrier population alters the quasiparticle band structure and the excitonic binding energy. These peculiar effects can be captured only by computing exchange and correlation effects out of equilibrium. This paper provides an accurate and quantitative description of the nonequilibrium response of silicon by solving the Green’s function theory equation within a first-principles scheme. The method is shown to reproduce well the experimental transient reflectivity spectrum of bulk silicon.

[Phys. Rev. B 93, 195205] Published Fri May 13, 2016

]]>In this paper, a family of non-Abelian topologically ordered states are built from arrays of quantum wires with fine-tuned electron-electron interactions. With the help of non-Abelian bosonization, the decoupled wires are described by conformal field theories. Electron-electron interactions are chosen to gap all conformal field theories except the one that describes the edge state. Hence, the construction makes it easy to engineer different topological ordered states. The authors prescribe, in particular, how to arrive at all the edge states described by the unitary conformal field theory minimal models using the spin degrees of freedom of the quantum wires alone. The construction provides a bridge between the field theory descriptions and the microscopic models of topological ordered states.

[Phys. Rev. B 93, 205123] Published Fri May 13, 2016

]]>The challenge of understanding the optical response and plasmon behavior in nanostructured materials, such as graphene and its nanoribbons, lies in describing screening in the presence of several concurrent electron scattering mechanisms. In this paper, the authors present a technique for calculating the dielectric function of nanostructures with an arbitrary band dispersion and Bloch wave functions. To calculate the linear response of a dissipative electronic system to an external electromagnetic field, the authors derive a quantum master equation within a self-consistent-field approach and solve it along with full-wave electromagnetic equations. This technique accurately captures interband electron-hole pair generation, as well as both interband and intraband electron scattering with phonons and impurities. The technique is employed to calculate the dielectric function, complex conductivity, as well as plasmon dispersion and propagation length for supported graphene. Plasmon propagation lengths are comparable on polar and nonpolar substrates and are on the order of tens of nanometers, considerably shorter than previously reported. They improve with fewer impurities, at lower temperatures, and at higher carrier densities.

[Phys. Rev. B 93, 205421] Published Thu May 12, 2016

]]>The search for new topological phases of matter is a major new direction in condensed matter physics. Recent experimental realizations of Dirac and Weyl semimetal phases pave the way to look for other exotic phases of matter in real materials. In this paper, the authors present a systematic angle-resolved photoemission spectroscopy study of ZrSiS, a potential topological nodal semimetal candidate. Their systematic measurements establish the spinless nodal fermion semimetal phase in ZrSiS, which is supported by their first-principles calculations. This work puts forward the ZrSiS-type material family as a new platform to explore exotic states of quantum matter.

[Phys. Rev. B 93, 201104(R)] Published Wed May 11, 2016

]]>Subwavelength focusing (and hence imaging) of waves is a topic of major interest for many applications. Several approaches have been proposed so far, mostly relying on metamaterials, but they suffer from drawbacks of being inherently narrowband and from dissipation into the material. The authors here suggest an innovative approach to overcome these issues by using an extended fractal resonator alongside time reversal. By coupling the fractal resonator with a reverberating cavity that transforms a single source of waves into multiple ones, they demonstrate experimentally that very wide bandwidth subwavelength focusing of microwaves is possible from the far field anywhere on the fractal.

[Phys. Rev. B 93, 180201(R)] Published Thu May 05, 2016

]]>Due to its noncentrosymmetric lattice structure, a monolayer of transition metal dichalcogenides (TMD) possesses a very special type of spin-orbit coupling (SOC) called Ising SOC. Unlike Rashba SOC, Ising SOC pins electron spins to the out-of-plane rather than the in-plane direction. In TMD materials, the Ising SOC is quite strong and substantially enhances the in-plane upper critical field of their superconducting state. Despite this, the authors show here that the Ising SOC can in fact lead to spin-triplet Cooper pairing with electrons spins pointing in the in-plane direction. Such pairing induces a topological superconducting state in spin-polarized proximity coupled wires and generates Majorana end states. So-formed Majorana states can be more accessible experimentally due to the strong Ising SOC and a wider topologically nontrivial regime.

[Phys. Rev. B 93, 180501(R)] Published Wed May 04, 2016

]]>This paper discusses what happens when Weyl fermions acquire a Majorana mass through coupling to a charged condensate. Such a mass makes the system become a topological superconductor. Chiral vortices in this superconductor bind one-dimensional Majorana-Weyl modes in their core. The Majorana-Weyl modes are electrically neutral, and are also chiral in that particles occupying them propagate in only one direction. Previous work had suggested that these modes would possess their own axial anomaly, where charge appears on the vortex as a result of a radial inflow from outside. This charge accumulation seems paradoxical as the core modes are neutral.Stimulated by this paradox, the authors reexamine the derivation of the low-energy effective action functional that describes the system’s response to external fields. They find that both the original and another paradox that arise in the process are resolved by an interplay between the universal topological terms in the action functional that are determined entirely by anomaly effects and the nonuniversal nontopological terms that depend on the details of the band structure.

[Phys. Rev. B 93, 174501] Published Tue May 03, 2016

]]>The authors systematically characterize the three-orbital Hubbard model using state-of-the-art determinant quantum Monte Carlo (DQMC) simulations with parameters relevant to the cuprate high-temperature superconductors. The DQMC results agree well with those from cuprate experiments, such as photoemission, and enable the identification of orbital content in the bands. A comparison of DQMC results to those from exact diagonalization and cluster perturbation theory elucidates how these different numerical techniques complement one another to produce a more complete understanding of the model and the cuprates.

[Phys. Rev. B 93, 155166] Published Fri Apr 29, 2016

]]>Unconventional superconductors, with order parameters that are predicted to have short range spatial modulations, have held long standing interest in the field. The Josephson effect, which directly probes the strength of the pairing potential is an ideal technique to study these materials, in contrast to a majority of probes which rely on deductions made from quasiparticle measurements. The authors combine the Josephson effect with the high spatial resolution afforded by scanning tunneling microscopy to study atomic scale variations of the order parameter in a model system consisting of magnetic adatoms on a BCS superconductor. The atomic resolution achieved establishes scanning Josephson spectroscopy as a promising tool for the study of novel superconducting materials.

[Phys. Rev. B 93, 161115(R)] Published Thu Apr 28, 2016

]]>The electron-phonon interaction alters substantially the conventional picture of the band structure. It also changes the properties of excitonic states, which are very pronounced in many 2D materials. Using many-body perturbation theory, the authors describe how the inclusion of temperature modifies the electronic bands of single-layer MoS${}_{2}$. Different bands and different regions in the Brillouin zone are affected in different ways by electron-phonon coupling. Using the temperature-broadened bands as input for the Bethe-Salpeter equation, the authors explain why, for the bound $A$ and $B$ excitons, the electron-phonon coupling changes mainly the position, and for the $C$ exciton, only the width is affected by temperature, while the energy is rather constant.

[Phys. Rev. B 93, 155435] Published Tue Apr 26, 2016

]]>The family of iron-based superconductors has recently acquired a new member material, FeS. Theoretically, this compound has been shown to have electronic structure similar to that of the superconducting FeSe. However, contradictory ground states have been predicted for FeS. In this work, a collaboration of authors from Switzerland and Germany use muon spin rotation and relaxation to show that weak-moment magnetism microscopically coexists with bulk superconductivity. Additionally, in contrast with some earlier studies, the results suggest a fully gapped superconducting state of FeS.

[Phys. Rev. B 93, 140506(R)] Published Mon Apr 25, 2016

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