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

]]>The need for a reliable quantum description of the motion of interacting electrons in nanostructures is of ever-growing significance. The usual approximations for the exchange-correlation (xc) potential of density functional theory can prove inadequate, especially in the time-dependent regime and when correlation is strong. Spatial steps (and related features) in the xc potential, in particular, are known to be frequently crucial for the accurate description of electron densities, but present challenges for the common xc approximations. Using a variety of model systems, the authors exhibit the nature of these steps, and describe the principles that determine their properties, to inform the development of improved functionals.

[Phys. Rev. B 93, 155146] Published Mon Apr 25, 2016

]]>The formation energies of semiconductor point defects are nowadays predicted much more accurately by density functional theory than a decade ago, thanks to the use of modern hybrid functionals. The drawback is the high computational effort compared to the traditional local density approximation (LDA) or generalized gradient approximation (GGA). In this work, the authors trace back the main variations between the functionals to differences in the position of the bulk valence-band maximum, as well as in the reference energies for the chemical potential obtained with each functional. For point defects relevant for $p$-type GaN, these differences are accounted for by corrections, reducing the maximum disagreement between the different functionals from more than 2 eV to below 0.2 eV. The correction scheme should be useful for performing high-throughput calculations in cases where full hybrid functional calculations are prohibitively expensive.

[Phys. Rev. B 93, 165206] Published Fri Apr 22, 2016

]]>The authors present a careful phase diagram determination of the hole-doped Sr${}_{1-x}$Na${}_{x}$Fe${}_{2}$As${}_{2}$ iron-based superconductors, with special attention paid to the region around $x$=0.36, where they recently found the existence of a magnetic $C$4 phase. This phase has already proven instrumental in settling longstanding questions in these materials. In contrast to the two previous hole-doped Ba-based materials exhibiting this phase, here they find the $C$4 phase to be more robust, existing over a larger range of compositions. This should prove useful for future studies.

[Phys. Rev. B 93, 134510] Published Wed Apr 20, 2016

]]>Topological insulators are promising materials for future spintronic applications due to the unique properties of their spin-polarized surface states. Using time-, spin-, and angle-resolved photoemission spectroscopy, the authors establish the utilization of photoexcited topological surface states as unique channels in which to drive fully controllable spin-polarized electrical currents on ultrafast time scales. Using circularly polarized femtosecond laser pulses of infrared light, the authors show that the dynamics of the generated currents exhibits two distinct timescales for the spin and charge degrees of freedom on the surface, and identify the underlying mechanisms giving rise to this behavior. Moreover, they find that the generated currents are completely reversible with the helicity of the circular-light polarization, paving the way for novel applications of topological insulators in spintronics at ultimate speeds.

[Phys. Rev. B 93, 155426] Published Wed Apr 20, 2016

]]>Weyl semimetals (WSMs) provide a platform to realize long-sought massless Weyl fermions, which can host a variety of exotic quantum phenomena such as chiral anomalies and anomalous Hall conductivity. The most intriguing nature of WSMs is the emergence of an unconventional surface state called a Fermi arc, which is distinct from the well-known Fermi surface of ordinary metals. The authors have used angle-resolved photoemission spectroscopy on NbP and succeeded in observing the electronic states for two differently terminated surfaces of this noncentrosymmetric crystal. Corroborated by first-principles calculations, it is unambiguously demonstrated that NbP is a Weyl semimetal with reduced spin-orbit coupling, exhibiting a drastic difference in the Fermi-surface topology between the P- and Nb-terminated surfaces. Through a direct comparison of Fermi-arc surface states between the two surfaces, the momentum location of Weyl nodes has been elucidated. The present results provides a pathway for exploring new quantum phenomena utilizing Fermi-arc properties of WSMs.

[Phys. Rev. B 93, 161112(R)] Published Wed Apr 20, 2016

]]>This paper provides a detailed analysis of the thermodynamic properties of the Ohmic two-state system as well as its transient dynamics following a quantum quench. The authors also quantify the accuracy of time-dependent NRG and compare it to a variety of techniques used to study time-dependent dynamics.

[Phys. Rev. B 93, 165130] Published Wed Apr 20, 2016

]]>Numerous experiments on solids in the vicinity of first-order phase transitions indicate that the two phases can coexist in equilibrium even away from coexistence curve, which seems to defy basic notions of thermodynamics. Examples include the magnetic quantum phase transition in the helical magnet MnSi, and the Mott transition in the rare-earth nickelates. Muon spin rotation experiments show that the volume fraction of the magnetic phase is less than 100% in a region away from the coexistence curve of the first-order phase transition. The authors provide an explanation for these puzzling observations: imperfections in the solid material can lead to small regions in space that locally favor one phase over the other. As a result, the material will not be homogeneous, but rather contain droplets of one phase within the other. These droplets are stable, and last indefinitely. The system thus never attains the equilibrium state one would naively expect, in which all of the volume would be taken up by one of the phases.

[Phys. Rev. B 93, 144203] Published Tue Apr 19, 2016

]]>How a disordered superconductor evolves into an insulator is still a matter of intense debate. Disorder and reduced screening of Coulomb repulsion may simply break the pairs or, alternatively, the Cooper pairs themselves may localize. Thin disordered NbN films fall between these two extremes and display intermediate scale inhomogeneity with anomalous properties, which are studied by the authors using Scanning Tunneling Spectroscopy and transport measurements. While STS exhibits inhomogeneity at distances much larger the typical size of the nanocrystals constituting the NbN films, the conductivity fluctuations near the superconductor-insulator transition have effectively a zero-dimensional character similar to the one previously found only in granular systems.

[Phys. Rev. B 93, 144509] Published Fri Apr 15, 2016

]]>Multiferroics present a dynamic crossover between magnetic and electronic order parameters. Using both the conventional pyroelectric current (PC) and the bias electric field (BE) methods, the authors investigate the electric polarization of multiferroic $M$Mn${}_{7}$O${}_{12}$ ($M$= Ca, Sr, Cd, Pb). While the pyrocurrent measurements show intense peaks below the magnetic ordering temperature, the peaks are not observed in the biased electric field method, which indicates the peaks are produced by thermal stimulation, not ferroelectricity. In CaMn${}_{7}$O${}_{12}$, the electric polarizations polarization produced is quite small, which suggests that the large polarizations reported in previously are not associated with intrinsic ferroelectricity.

[Phys. Rev. B 93, 155127] Published Fri Apr 15, 2016

]]>Resonant x-ray scattering is used to definitively show that a pure spin density wave state is the ground state in ultrathin LaNiO${}_{3}$ layers sandwiched between LAAlO${}_{3}$ layers. This result is also shown to be in agreement with first-principles calculations.

[Phys. Rev. B 93, 165121] Published Fri Apr 15, 2016

]]>Current control of magnetism, especially in multilayer thin films, hold great promise for applications in spintronics. Efficient control can be achieved using spin-orbit torques originating from either the spin Hall effect or the Rashba effect. In the context of spin waves, direct current pass through a heavy metal/ferromagnetic bilayer may be used to reduce/amplify damping or shift the resonant field. The effective field or damping introduced by the spin Hall or the Rashba effects depends on the current linearly, reversing the sign with a direct current in the opposite direction. The authors of this study report unexpected symmetric magnetic resonance shifts for spin waves passing through a CoFeB/Ta waveguide. This second-order effect arises from the dc modified magnetic anisotropy via anisotropic stress introduced between the substrate and the bilayer. Similar current-induced magnetoelastic effects may be relevant in many ferromagnetic heterostructures subject to large direct current.

[Phys. Rev. B 93, 140404(R)] Published Mon Apr 11, 2016

]]>The authors have used resonant ultrasound spectroscopy to characterize variations in the elastic and anelastic properties of GeTe associated with the ferroelectric/ferroelastic phase transition at ~625 K. These arise as a consequence of strong coupling between strain and driving order parameter, and their magnitudes are permissive of a significant order/disorder component to the transition. Remarkably, precursor softening due to dynamical disorder and Debye freezing of twin walls at low temperatures appear both to be controlled by the same thermally activated mechanism associated with small displacements of Ge within GeTe${}_{6}$ octahedra. This mechanism must also be important in constraining the stability and dynamics with respect to ferroelectric switching.

[Phys. Rev. B 93, 144109] Published Mon Apr 11, 2016

]]>The search for unconventional phases of matter, such as quantum spin liquids (QSL), is one of the fundamental and most debated issues in condensed matter physics. While gapped spin liquids have been widely studied and are now accepted to exist in nature, the existence and stability of gapless spin liquids is more controversial. The authors present their analysis of the frustrated antiferromagnetic spin model on a triangular lattice, which has been suggested very recently as a platform to host QSLs. They use a combination of several numerical techniques to show that a gapless spin liquid, which can be described via emergent interacting Dirac fermions, proves to be an excellent candidate ground state to describe the frustrated regime for the spin model.

[Phys. Rev. B 93, 144411] Published Mon Apr 11, 2016

]]>Current-induced spin-orbit torque (SOT) in magnetic heterostructures with perpendicular magnetic anisotropy is an efficient mechanism to control the magnetic moments therein. The authors present a Hall-voltage-based technique that allows them to simultaneously determine two key physical quantities describing SOT in magnetic heterostructures: the SOT efficiency and the magnitude of effective Dzyaloshinskii-Moriya interaction field. With the help of this new technique, the authors find that the spin-orbit torque, apart from the conventional spin-Hall-induced contribution, includes another geometrical component that depends on the shape of the ferromagnetic layer of the heterostructure. This opens a possibility for engineering SOT switching without any aid of external magnetic field, with possible applications for magnetic random access memory applications.

[Phys. Rev. B 93, 144409] Published Fri Apr 08, 2016

]]>The authors continue efforts to understand the origin of the peculiar phenomenon revealed in the electronic and magnetic properties of Na${}_{x}$CoO${}_{2}$. From ac and dc spin susceptibility measurements, they confirm the key role of the Co${}^{3+}$ intermediate spin state. This research should also have bearing on other layered transition metal oxides.

[Phys. Rev. B 93, 140402(R)] Published Tue Apr 05, 2016

]]>Perovskite and double perovskite oxides are perhaps the most studied ferroelectric and multiferroic materials. The authors present in this manuscript an extensive analysis of another promising family of ferroelectrics, the corundum derivatives $A\phantom{\rule{0}{0ex}}B\phantom{\rule{0}{0ex}}{O}_{3}$ and ${A}_{2}\phantom{\rule{0}{0ex}}B\phantom{\rule{0}{0ex}}{B}^{\prime}\phantom{\rule{0}{0ex}}{O}_{6}$. Despite a superficial similarity of the chemical formulas to single and double perovskites, the corundum derivatives have an entirely different structure and the mechanism for the polarization reversal. The authors perform a detailed first-principles study and arrive at empirical rules for evaluating the ferroelectric properties of materials in this class.

[Phys. Rev. B 93, 134303] Published Mon Apr 04, 2016

]]>Using a combination of first-principles calculation and high-resolution scanning TEM, the authors report novel structural characteristics of the ferreoelastic 90${}^{\circ}$ domain wall in PbTiO${}_{3}$. They highlight the discovery of a sharp discontinuity in the variation of the lattice parameters across the domain wall. The first-principles calculations show that oxygen vacancies prefer to cluster in the plane adjacent to the asymmetric domain walls. Understanding interaction of oxygen vacancies with domain walls is important because such defects are used to tailor properties of ferroelectric materials

[Phys. Rev. B 93, 144102] Published Mon Apr 04, 2016

]]>Iron-based superconductors, where the Cooper pairing is theorized to arise from magnetic interactions, have emerged as very good model systems for studies of the interplay between superconductivity and magnetism. Superconducting vortices, introduced by an applied magnetic field, can serve as a sensitive probe of the superconducting state in the host material. The authors have used small-angle neutron scattering to study the vortex lattice in KFe${}_{2}$As${}_{2}$, and perform a comprehensive characterization of the effects of multiband superconductivity and Pauli paramagnetism in this material. In particular, they have succeeded in separating orbital and paramagnetic contributions to the vortex lattice field modulation, and obtain a quantitative measure of the Pauli paramagnetic effects deep within the superconducting phase.

[Phys. Rev. B 93, 104527] Published Mon Mar 28, 2016

]]>GdTiO${}_{3}$ is representative of the rare-earth titanates, which are prototypical Mott insulators with a Ti 3${d}^{1}$ electron configuration. GdTiO${}_{3}$ has recently attracted a lot of attention due to the formation of a high-density two-dimensional electron gas when interfaced with SrTiO${}_{3}$, opening the way for new electronic devices. The authors present a comprehensive DFT study of native point defects and impurities in GdTiO${}_{3}$, finding that these introduce small hole polarons into the material and thus explaining the experimental observation of $p$-type hopping conductivity in a nominally undoped material. They also discuss how the defect-induced polarons impact optical properties and can act as traps in devices.

[Phys. Rev. B 93, 115316] Published Mon Mar 28, 2016

]]>Arranging $S$=$\frac{1}{2}$ spins into planes, chains, or other low-dimensional structures permits an experimental exploration of the effects of quantum fluctuations.In reality, such “arrangements” are often easier said than done. However, the use of molecular building blocks opens new possibilities for creating adjustable networks of spins. The authors of this paper have made and studied a series of two-dimensional square-lattice $S$=$\frac{1}{2}$ antiferromagnets, whose interlayer spacing is varied considerably by the substitution of increasingly bulky axial ligands. Using a variety of techniques, including low-temperature and high-magnetic-field magnetometry, muon-spin relaxation, and electron-spin resonance, they show that the increased layer spacing reduces the interlayer coupling, as desired. However, this also permits a small non-Heisenberg-like perturbation to emerge in the intralayer interaction. This stymies the order-suppressing effect of quantum fluctuations and promotes the development of a long-range ordered state at low-temperatures.

[Phys. Rev. B 93, 094430] Published Fri Mar 25, 2016

]]>The authors demonstrate the selective addressability of two nearly degenerate clock transitions in bismuth-doped ${}^{28}$Si without tuning the magnetic field or microwave frequency. Rather, they vary the microwave polarization between clockwise and counterclockwise circular polarizations, which can be done rapidly. To generate the circularly polarized microwaves, a superconducting coplanar waveguide microresonator was oriented inside a tunable dielectric resonator such that their microwave magnetic fields can be superimposed to give any arbitrary microwave polarization. This work not only opens up additional states near the Si:Bi clock transition, which can be used for different quantum computing schemes, it also demonstrates a clever method for generating arbitrarily polarized microwaves.

[Phys. Rev. B 93, 121306(R)] Published Fri Mar 25, 2016

]]>Mackinawite, the tetragonal form of FeS, is the newest iron-based superconductor that generates excitement in the field, as it represents a direction away from the purely arsenide- and selenide-based systems. Other sulfides have been shown recently to have superconducting properties, including BaFe${}_{2}$S${}_{3}$ under high pressures and H${}_{2}$S under extreme pressures, but tetragonal FeS is perhaps the simplest sulfide to exhibit superconductivity behavior at ambient pressure. The authors of this work present a novel method for preparing high-quality single crystals of FeS and investigate its superconducting properties. They find that the electronic and magnetic properties of FeS are highly anisotropic. In particular, the level of anisotropy characterized by the ratio of the upper critical fields in two orthogonal directions was found to be the largest of all the iron-based superconductors.

[Phys. Rev. B 93, 094522] Published Thu Mar 24, 2016

]]>The authors conduct a comprehensive study of thermodynamic and transport properties of $R$PtBi compounds ($R$=Gd, Dy, Tm, Lu). Temperature- and field-dependent resistivity measurements on high-quality single crystals reveal an unusually large nonsaturating magnetoresistance (MR) up to 300 K under a moderate magnetic field of 140 kOe. Analysis of the transport data in this series reveals that these rare-earth-based half-Heusler compounds provide opportunities to tune the MR effect through lanthanide contraction and to elucidate the mechanism of nontrivial MR.

[Phys. Rev. B 93, 115134] Published Mon Mar 21, 2016

]]>The optical properties of 2D semiconductors made of transition metal dichalcogenide monolayers, such as Mo${X}_{2}$ and W${X}_{2}$ ($X$=S, Se), are dominated by excitons. The light emission yield for optoelectronics applications depends on whether the electron-hole transitions are optically allowed (bright) or forbidden (dark). No direct evidence of the energetic ordering of these exciton states is currently available. The authors here solve the Bethe-Salpeter equation with $G\phantom{\rule{0}{0ex}}W$ wave functions in order to determine the sign and amplitude of the splitting between bright and dark exciton states in the entire Mo${X}_{2}$ and W${X}_{2}$ monolayer family. They also evaluate the influence of pin-orbit coupling on the optical spectra and clearly demonstrate the strong impact of the intravalley Coulomb exchange term on the dark-bright exciton splitting, an important ingredient for engineering optoelectronics and spintronics applications.

[Phys. Rev. B 93, 121107(R)] Published Wed Mar 16, 2016

]]>The authors report the first observation of dynamic nuclear spin polarization (DNP) assisted by $p$-shell carriers in a single self-assembled InAs/GaAs quantum dot (QD) without application of an external magnetic field. With the ideal case of complete nuclear spin polarization remaining an elusive goal, research to improve the degree and efficiency of DNP or to narrow the nuclear field fluctuation distribution has been getting significant attention. The work here has implications that could lead to breaking the current limit of nuclear spin polarization. Furthering our understanding of the mesoscopic nuclear spins in a self-assembled QD is imperative in the pursuit of a stable nuclear spin environment for prolonging spin qubit coherence time.

[Phys. Rev. B 93, 125306] Published Wed Mar 16, 2016

]]>There is growing interest in low-symmetry metal oxides because of their potential use in high-power electronics capable to sustain very high voltages. Very little is known about their fundamental physical properties, such as transverse and longitudinal optical phonon modes, dielectric constants, and how free charge carriers couple with lattice vibrations. This lack of information is partially due to the complexity by which these properties intertwine due to the low symmetry crystal systems. Here, the authors describe a general pathway to the analysis of long-wavelength experiments for monoclinic and triclinic crystal systems, and they report for the first time a complete set of phonon modes for the monoclinic phase of gallium oxide. These parameters may arrive just in time to support computational optimization of charge and heat transport for device designs. The concept for analysis of long wavelength properties in monoclinic and triclinic crystal systems can help access a widely uncharted field in condensed matter physics.

[Phys. Rev. B 93, 125209] Published Tue Mar 15, 2016

]]>Nearly half-filled Landau levels of an ultrapure two-dimensional electron gas host electronic nematic phases, commonly known as quantum Hall stripes. While original orientation of stripes is determined by the native symmetry-breaking potential of yet mysterious origin, an in-plane magnetic field can easily rotate stripes by 90 degrees. Experiments by the Minnesota/Purdue group in this paper reveal that, under a modest in-plane field, the stripe orientation can also be changed simply by making the topmost Landau level less or more than half-full. This finding reflects the strong filling factor dependence of the native symmetry-breaking potential and might provide a critical piece of information needed to finally identify its origin.

[Phys. Rev. B 93, 121404(R)] Published Mon Mar 14, 2016

]]>Recently, there has been interest in using the electronic states of the acceptor impurity boron in silicon as active laser media and as qubits for a semiconductor-based quantum information technology. Even though boron is the prototypical acceptor in silicon, the most thoroughly studied semiconductor, questions have recently been raised about the ordering of its excited states. Here, the authors use several experimental techniques, including Raman scattering spectroscopy, two-hole photoluminescence spectroscopy and temperature-dependent infrared absorption spectroscopy, to confirm the assignment of the deepest even-parity excited state and observe new spectral features associated with the even-parity states.

[Phys. Rev. B 93, 125207] Published Thu Mar 10, 2016

]]>The process of deformation twinning, as schematically indicated in the figure, plays an important role in establishing the ductility of many engineering materials such as titanium and magnesium and some refractory alloys. By fine tuning alloy compositions, the tendency for twin formation can be controlled, which can lead to guidelines for design of advanced alloys for applications spanning biomedical, energy conversion, and high-temperature structural applications. This work presents a theoretical framework to computationally study the effect of alloying on planar defect energies and is expected to be a useful tool for computationally guided alloy design.

[Phys. Rev. B 93, 094101] Published Wed Mar 09, 2016

]]>This work demonstrates optical vortices of visible light confined to tens of nanometers. Confinement to such dimensions is considered beyond standard optical capabilities. The significant down-scaling is achieved by engineering a short-wavelength platform, consisting of high-index dielectrics and metals that support hybrid photonic-plasmonic modes of very short wavelength propagating in two dimensions. Additional confinement is achieved using super-oscillations between two guided modes in carefully designed waveguides. The authors utilize phase-resolved near-field measurements to demonstrate optical vortices with topological charge as high as five that are still smaller than the size of a diffraction-limited spot of the illuminating light. The size of the vortices may allow for future orbital-angular-momentum based light-matter interactions with quantum systems.

[Phys. Rev. B 93, 121302(R)] Published Wed Mar 09, 2016

]]>Exciton spin lifetimes in semiconductor quantum wells are usually short and decrease when magnetic field is applied in the plane of the structure. The authors here have discovered that in coupled semiconductor quantum wells, the spin lifetime dependence on magnetic field is quite unusual. It strongly increases at low magnetic fields, then saturates and decreases at higher fields. This behavior is a consequence of mixing between direct and indirect exciton states, via the magnetic quantum confined Stark effect. This effect can be a convenient tool for exciton spin engineering, and may complement the traditional quantum confined Stark effect in structures where inhomogeneity is important.

[Phys. Rev. B 93, 115410] Published Tue Mar 08, 2016

]]>The authors measure the nonlinear optical response of Ca${}_{1-x}$La${}_{x}$FeAs${}_{2}$ to investigate the detailed structural symmetries of the recently discovered 112-type family of iron-based superconductors. They find a strong and anisotropic optical second-harmonic response, identifying ${C}_{2}$ and ${C}_{1}$ as the high- and low-temperature crystallographic point groups, respectively. This makes the 112-type materials the first known high-temperature superconductors to break structural inversion symmetry, allowing for the possible mixing of singlet and triplet Cooper pairs in the superconducting state. In addition, the intrinsically low crystallographic symmetry of this family of materials can potentially stabilize large single domains of the electronic nematic state ubiquitous in the iron-based superconductors without application of strain.

[Phys. Rev. B 93, 104506] Published Mon Mar 07, 2016

]]>The existence of electric dipole active magnetic excitations, or electromagnons, in rare-earth orthoferrites was theoretically predicted more than two decades ago but their experimental observation has not been confirmed until now. In this work, magnetic modes in DyFeO${}_{3}$ single crystals were studied at low temperatures and in external magnetic fields ($H$) by means of synchrotron radiation-based far-infrared spectroscopy. The authors report observation of two electromagnons, which appear in the spectra at 20 cm${}^{-1}$ and 50 cm${}^{-1}$ only below the magnetic ordering temperature ${T}_{N}$ of Dy${}^{3+}$ of 4.2 K. It is demonstrated that electromagnons provide a significant contribution to the static electric permittivity along the $c$ axis ${\epsilon}_{c}$(0). Both the frequency of the electromagnons and ${\epsilon}_{c}$(0,$H$) manifest a strong hysteresis upon cycling of the external magnetic field.

[Phys. Rev. B 93, 094403] Published Fri Mar 04, 2016

]]>SrRuO${}_{3}$, a conductive ferromagnetic oxide, is a simple, old, but still fascinating material. Previous angle-resolved photoemission (ARPES) studies found a kink structure in the electron bands, implying unusual electron dynamics. Here, the authors explore the origin of this kink in more detail via ARPES studies of high-quality films of varying thickness. The results indicate that the kink originates from the strong electron-phonon coupling rather than electron-magnon coupling.

[Phys. Rev. B 93, 121102(R)] Published Fri Mar 04, 2016

]]>Currently, there is great interest in the field of “active plasmonics”, where the properties of plasmonic excitations are tuned by an external stimulus. For practical applications, fast and efficient operation is required. Lattice vibrations in the plasmonic medium enable high-frequency modulation. So far, optical control by lattice vibrations was developed for the case of acoustic phonons only, which limits the operation frequencies to the gigaheretz range. Here, the authors go beyond this limit and demonstrate how optical phonons can be efficiently used for high-frequency terahertz modulation of light at the nanoscale. This is achieved by using novel hybrid plasmonic-semiconductor structures, wherein the modulation takes place in a nanometer-thin layer of elemental tellurium that forms under illumination by intense laser pulses.

[Phys. Rev. B 93, 125404] Published Thu Mar 03, 2016

]]>In quasi-one-dimensional (quasi-1D) systems, because of the finite interchain coupling and fluctuation, the charge density wave (CDW) transition temperature ${T}_{\mathrm{CDW}}$ is much lower than the mean-field-theory predicted ${T}_{\mathrm{MF}}$ and the CDW fluctuation region exists between ${T}_{\mathrm{CDW}}$ and ${T}_{\mathrm{MF}}$. Usually, CDW fluctuation has little effect on the electronic transport properties. Moreover, the CDW state is usually insensitive to magnetic field. The work done here on a PdTeI single crystal with quasi-1D PdTe chains shows that the above statements need to be revisited. It is found that the CDW fluctuation leads to a gradual decrease of carrier concentration before the static long-range CDW order occurs at ${T}_{1}$~110 K. On the other hand, the pinning and sliding of CDW state are observed below ${T}_{2}$~6 K. When such a low ${T}_{2}$ is combined with the existence of multiple quasi-1D bands, PdTeI exhibits exotic crossover behavior from negative to huge positive magnetoresistance under magnetic field and field-induced localization. It indicates that the CDW fluctuation could have a remarkable influence on transport properties of CDW materials. Moreover, there is an interaction between the CDW state and magnetic field.

[Phys. Rev. B 93, 121101(R)] Published Tue Mar 01, 2016

]]>One of the most peculiar properties of spin waves in magnetic thin films is nonreciprocal wave propagation, i.e., a dependence of the wave properties on the propagation direction. The authors study this effect in permalloy films of thickness varying from 6 to 40 nm and find that the difference in the frequencies can reach several tens of MHz. The authors show that this frequency nonreciprocity is mainly caused by asymmetric magnetic anisotropies at the two surfaces of the ferromagnetic film. This result is important to understand recent works on the interfacial Dzyaloshinskii-Moriya interaction, an unconventional chiral magnetic interaction that generates a similar frequency nonreciprocity. The authors demonstrate that while these contributions usually occur at the same time, the two can be distinguished via the difference in their dependence on the film thickness.

[Phys. Rev. B 93, 054430] Published Mon Feb 29, 2016

]]>A theoretical framework is developed to study a magnetically levitated nanomagnet in the quantum regime. The system is very rich due to the interaction between its degrees of freedom: position, orientation, and magnetization. In particular, the Einstein–de Haas effect, namely the coupling between the magnetization and the angular momentum of the object, plays a dramatic role. This work sheds new light on one of the few unsolved problem in quantum mechanics, the rigid rotor with a spin. At the same time, it opens up the possibility to model the dynamics of a single magnetic domain nanoparticle in a magnetic trap, an interesting new scenario for future experiments.

[Phys. Rev. B 93, 054427] Published Fri Feb 26, 2016

]]>Spin-orbit coupling gives rise to many interesting phenomena in solids and at surfaces as, for example, the Rashba splitting or the occurrence of the topological surface states in topological insulators. In addition, many technically important properties such as transverse charge and spin conductivities and magnetodichroic phenomena occur due to spin-orbit coupling. To provide a flexible and accurate basis to deal with these phenomena, the authors worked out a coherent scheme to construct the electronic Green function within multiple scattering theory for a general potential. This includes, in particular, the case of a nonlocal and complex potential to account for correlation effects to set up, for example, the framework of dynamical mean field theory. Various technical issues such as the need to distinguish left- and right-handed solutions and their relation to symmetry are discussed in detail.

[Phys. Rev. B 93, 075145] Published Tue Feb 23, 2016

]]>Insulators do not transport electrical currents. Recent experimental reports instead show magnetic insulators can conduct spin current in terms of magnons (excitations from magnetic ordering). The spin current could be converted to electric current by the inverse spin Hall effect. Thus, magnetic insulators may not be regarded as insulating. Recent experimental work has suggested such spin transport effects exist even above magnetic transition temperatures, challenging the conventional approach starting from magnetic ordering. In this paper, an alternative approach is taken to this problem using the Schwinger boson method. It is shown that spin pumping and spin current transport are possible at elevated temperatures, even higher than transition temperatures, suggesting that paramagnetic insulators could be used for spintronics applications.

[Phys. Rev. B 93, 064421] Published Mon Feb 22, 2016

]]>The authors measured the electrical and thermal Hall conductivities of YBa${}_{2}$Cu${}_{3}$O${}_{y}$ in high magnetic fields to investigate the nature of the nonsuperconducting ground state in underdoped cuprates. They found that the Wiedemann-Franz law is obeyed in the $T$=0 limit as soon as the vortex solid melts at high field. This shows that there is no vortex liquid at $T$=0 and it imposes a clear constraint on the nature of excitations in the enigmatic pseudogap phase of cuprate superconductors.

[Phys. Rev. B 93, 064513] Published Mon Feb 22, 2016

]]>Understanding the electronic and magnetic properties of 3$d$ transition-metal perovskites is one of the central issues in solid state theory. The physics at play here, involving electron localization and spin/orbital-ordering, is not easily captured by standard theories. By applying a combination of a variety of first-principles methods (DFT, HSE, GW), effective electronic Hamiltonian and Heisenberg spin Hamiltonian the authors study the manganite family $R$MnO${}_{3}$ ($R$=La, Pr, Nd, Sm, Eu, and Gd) characterized by a progressive enhancement of the orthorhombic distortion which causes an $A$-type to $E$-type magnetic transition. This study provides a comprehensive and accurate account of the dependence of the electronic structure and Néel temperature on the structural distortions. In particular, it is shown that the Coulomb repulsion and the Jahn-Teller coupling strength remain unchanged along the series, and are not responsible for the observed transition. Rather, the orthorhombic distortions induces a progressive reduction of the nearest-neighbor hopping term (kinetic) and a concomitant attenuation of the FM in-plane exchange interaction: this induces a a gradual destabilization of the $A$-type AFM ordering. Monte Carlo simulations predict Néel temperatures in very good agreement with experiment.

[Phys. Rev. B 93, 075139] Published Fri Feb 19, 2016

]]>Metallic surface states emerging at all-in-all-out type antiferromagnetic domain walls in bulk pyrochlore iridates have recently been observed. Such bulk crystals, however, inevitably contain huge amounts of the domains at random, and so it is quite challenging to investigate and utilize these novel surface conducting states. The results presented here are for a pyrochlore iridate heterointerface, which is realized by an advanced thin film growth technique. The authors successfully demonstrate the detection and control of a surface state linked to the antiferromagnetic domain wall, guiding further efforts for utilizing such states for surface transport in electronics and spintronics applications as well as for investigating the emergent topological transport at the interface.

[Phys. Rev. B 93, 064419] Published Wed Feb 17, 2016

]]>The authors present a time-dependent nonequilibrium Green’s function formalism for electrons interacting with bosonic excitation such as phonons, photons, or plasmons. In this scheme, the equations of motion for the Green’s functions – the Kadanoff-Baym equations – of electrons and bosons are treated on equal footing and therefore require substantial modifications as compared to the usual two-times propagation scheme. Motivated by recent experiments, the authors apply their formalism to time-resolved photoemission from bulk magnesium core states, a prominent system where the excitation of collective electronic motion in the conduction band leads to the characteristic plasmon satellites in the spectrum. The two corresponding pathways for this electron energy-loss effect – intrinsic or extrinsic losses – are usually hard to disentangle due to their quantum interference. The authors demonstrate how the loss channels manifest in a time-dependent picture and discuss how to discern them in the time domain.

[Phys. Rev. B 93, 054303] Published Tue Feb 16, 2016

]]>The possibility of observing a strongly interacting quantum critical fluid of electrons in a metal has intrigued physicists for a long time. The authors combine insight from string theory and condensed matter physics to develop a hydrodynamic theory of the thermal and electrical conductivity of electron fluids. They use this formalism to explain experimental data from a novel state of Dirac electron fluid in clean samples of graphene near the charge neutrality point, and observe substantially improved quantitative agreement over the existing hydrodynamic theories. This study marks the first quantitative connection between these exotic models of transport and experimentally realizable condensed matter systems.

[Phys. Rev. B 93, 075426] Published Tue Feb 16, 2016

]]>In this work, the ultrasound velocity is measured in the frustrated spin liquid material SrDy${}_{2}$O${}_{4}$ as a function of magnetic field and temperature. Experimental data reveals a complex phase diagram that depends heavily on the direction of the magnetic field. Whereas a dome for the three-dimensional long-range magnetic order is induced for magnetic field applied along the $b$ axis, no finite temperature magnetic order is observed for other field directions. When passing between the spin liquid and magnetically ordered phase, significant irreversibility is observed. “Solidification” of the spin liquid (through applied field) proceeds through three distinct steps while melting of the magnetic order into the spin-liquid phase is a more continuous process.

[Phys. Rev. B 93, 060404(R)] Published Fri Feb 12, 2016

]]>This paper reports on the experimental realization of ratchet effects in graphene, excited by alternating electric fields of terahertz frequency range. Such ratchet effects were previously seen in semiconductors. Two types of graphene lateral superlattices are studied: epitaxially grown and exfoliated graphene with an asymmetric lateral periodic potential. Laser radiation shining on the modulated devices results in the excitation of a direct electric current being sensitive to the radiation’s polarization state. This ratchet current consists of a few linearly independent contributions including the Seebeck thermoratchet effect as well as “linear” and “circular” ratchets. The results are analyzed with a theoretical model. Further development of the theory is required for a quantitative analysis.

[Phys. Rev. B 93, 075422] Published Fri Feb 12, 2016

]]>The study of the spin Hall effect in graphene is an active field of research, but its experimental realization is proven to be difficult due to the inherently weak intrinsic spin-orbit interaction. Thus, one needs to rely on extrinsic spin-orbit scattering to be able to probe this effect in graphene. The authors of this paper have reported some progress in this direction. They predict that adatoms on graphene can induce an anisotropic Rashba spin-orbit coupling, and the degree of anisotropy has a noticeable effect on the energy dependence of the spin Hall angle.

[Phys. Rev. B 93, 085418] Published Thu Feb 11, 2016

]]>The authors discuss the concept of the Brillouin zone, central to many fields of condensed matter physics. They challenge the universality of defining the Brillouin zone as a Wigner-Seitz cell of the reciprocal lattice. Experimental measurements of terahertz phonon-polariton dispersion in a photonic crystal fabricated in a thin slab of an anisotropic material (lithium niobate) demonstrate that the lowest band gap does not form at the boundary of the conventionally defined Brillouin zone, in contrast to a similar photonic crystal fabricated in a nearly isotropic lithium tantalate. The analysis motivated by this unexpected experimental result shows that in an anisotropic photonic crystal the Wigner-Seitz cell of the reciprocal lattice is no longer bounded by Bragg planes and, consequently, does not conform to the original definition of the Brillouin zone. The authors construct an alternative Brillouin zone bounded by the Bragg planes and show its utility in analyzing the dispersion bands of anisotropic photonic crystals.

[Phys. Rev. B 93, 054204] Published Tue Feb 09, 2016

]]>Noncentrosymmetric superconductors such as PbTaSe${}_{2}$ have been the focus of intensive study due to their unconventional properties and they have also recently been proposed as candidates for topological superconductivity and Majorana fermions. To test this, it is important to characterize the superconducting order parameter. The authors here report precise temperature-dependent measurements of the London penetration depth of PbTaSe${}_{2}$. The results indicate fully gapped $s$-wave BCS-type superconductivity, which is rather puzzling. This compound is known to have a topologically nontrivial band structure and a strong antisymmetric spin-orbit coupling, which, in 2D superconductors, is expected to produce an order parameter with mixed spin singlet and spin triplet components.

[Phys. Rev. B 93, 060506(R)] Published Fri Feb 05, 2016

]]>The half-filled Landau level can support a plethora of exotic strongly correlated states. The authors interconnect a recently proposed Dirac particle theory of the half-filled Landau level with the physics of spin liquids and the surface states of topological insulators. Additionally, they suggest that composite fermions can be viewed as dipolar bound states of charge/vortex composites and show that their transport properties violate the conventional Wiedemann-Franz law.

[Phys. Rev. B 93, 085110] Published Fri Feb 05, 2016

]]>The fate of magnetic ordering in frustrated systems with competing magnetic interactions is a long-standing question in the field of strong electronic correlation. Frustration can lead either to exotic phases that do not order down to zero temperature, the so-called spin liquids, or to spiral magnetism. In this paper, the authors consider a model for spins on the anisotropic triangular lattice, by using variational Monte Carlo, where they treat on the same ground spin liquids and magnetic states with spiral order. Moreover, they perform a systematic analysis of the spin-liquid states that can be constructed on the anisotropic triangular lattice by means of the fermionic projective symmetry group classification. The results allow for determining a complete phase diagram, as a function of the frustration ratio, encompassing different physical behaviors. They are relevant for the description of materials that are characterized by stacked triangular layers, such as charge-transfer salts and the compounds Cs${}_{2}$CuCl${}_{4}$ and Cs${}_{2}$CuBr${}_{4}$.

[Phys. Rev. B 93, 085111] Published Fri Feb 05, 2016

]]>The authors study the contribution of the chiral anomaly to various magnetotransport coefficients in Weyl semimetals. They do so using quasiclassical transport theory taking into account Berry curvature. In this framework, the authors compute the longitudinal conductivity, thermal conductivity, the spectrum of polaritons, and absorption coefficient for sound waves.

[Phys. Rev. B 93, 085107] Published Thu Feb 04, 2016

]]>It has recently been shown that disordered hyperuniform many-particle systems represent new distinguishable states of amorphous matter that are poised between a crystal and a liquid are are endowed with novel physical and thermodynamic properties. Such systems have shown to exist as ground states, i.e., at a temperature of absolute zero. Such “stealthy” and hyperuniform states are unique in that they are transparent to radiation for a range of wavelengths. In this paper, we ask whether Ising models of magnets, called spin chains in one dimension, can possess spin interactions that enable their ground states to be disordered, stealthy, and hyperuniform. Using inverse statistical-mechanical theoretical methods, we do demonstrate the existence of such states, which should be experimentally realizable.

[Phys. Rev. B 93, 064201] Published Wed Feb 03, 2016

]]>The authors report elastic and inelastic neutron scattering results on the quantum spin ice candidate Yb${}_{2}$Ti${}_{2}$O${}_{7}$. The experiments were performed on a well characterized stoichiometric powder, which displays a large and sharp heat capacity anomaly at 0.26 K. The authors show that, at low temperature, Yb${}_{2}$Ti${}_{2}$O${}_{7}$ exhibits long-range order with an ice-like ferromagnetic structure. However, the onset temperature is much higher than the temperature of the heat capacity anomaly. The spin excitations were found to be gapless on an energy scale < 0.09 meV and organized into a continuum of scattering with vestiges of highly overdamped ferromagnetic spin waves. The same spin dynamics is also observed for the single crystals, which indicates that those spin excitations are robust upon weak disorder.

[Phys. Rev. B 93, 064406] Published Wed Feb 03, 2016

]]>The authors show that static and Floquet topological insulators can be described (classified) by a unified-framework scattering theory, even when bulk Floquet bands fail to capture the topological nature of a phase. The paper introduces and characterizes for the first time invariants of weak topological Floquet insulators in two dimensions. This should help motivate research into Floquet weak topological insulators and Floquet topological crystalline insulators, fields which are largely unexplored.

[Phys. Rev. B 93, 075405] Published Tue Feb 02, 2016

]]>Inspired by the recently proposed Dirac composite fermion picture for half-filled Landau level the authors extend previously developed Hamiltonian formalism to incorporate considerations of particle-hole symmetry. They show that the magnetic translation algebra can be represented in the enlarged space of Dirac fermions, with particle-hole symmetry manifestly defined. A Hartree-Fock approximation then leads to a composite Fermi liquid of Dirac fermions.

[Phys. Rev. B 93, 085405] Published Tue Feb 02, 2016

]]>The multiorbital Hubbard model Hamiltonian is relevant to a host of strongly correlated materials, but there is no consistently accepted form for it. This manuscript derives the most general model Hamiltonian with $s$, $p$, and $d$-shell electrons and also identifies new terms describing pair hopping and quadrupole effects that were missed previously.

[Phys. Rev. B 93, 075101] Published Mon Feb 01, 2016

]]>Quantum kicked rotor (QKR) is a standard model of chaos that describes a particle moving on a ring under time-modulated kicking. This paper demonstrates that a large class of spin-$\frac{1}{2}$ quasiperiodic QKR models exhibits a dynamical analog of the integer quantum Hall effect, which is usually associated with two-dimensional electronic systems. Additionally, the authors reveal that the topological theta angle, a fundamental concept in quantum chromodynamics, can emerge in QKR models.

[Phys. Rev. B 93, 075403] Published Mon Feb 01, 2016

]]>In quantum magnetism, Kitaev’s honeycomb model is well known for its gapped and gapless spin liquid ground states in which the elementary spin degrees of freedom fractionalize into Majorana fermions and a ${\mathbb{Z}}_{2}$ gauge field. In this paper, the authors discuss the gapless spin liquids arising in three-dimensional Kitaev models and show that the gapless Majorana fermions form metallic states which, depending on the underlying lattice structure, exhibit Fermi surfaces, nodal lines, or Weyl points.

[Phys. Rev. B 93, 085101] Published Mon Feb 01, 2016

]]>The authors have conducted an experimental study of hexagonal boron nitride using steady-state and time-resolved photoluminescence. The assignment of previously observed lines of unknown origin is made convincingly to indirect excitons assisted by acoustic and optical phonons and to defect-assisted transitions. This is further evidence for $h$-BN being an indirect band-gap semiconductor.

[Phys. Rev. B 93, 035207] Published Thu Jan 28, 2016

]]>The authors discuss magnetoplasma and cyclotron resonances under microwave excitation in a high quality, GaAs/AlGaAs two-dimensional electron system. They observe that the cyclotron resonance originates as a pure resonance that does not hybridize with dimensional magnetoplasma excitations. The magnetoplasma resonances form a fine structure of the cyclotron resonance.

[Phys. Rev. B 93, 041110(R)] Published Thu Jan 28, 2016

]]>Reports on sulfur hydride attaining metallicity under pressure and exhibiting superconductivity at temperatures as high as 200 K have spurred an intense search for another room-temperature superconductor among hydrogen-rich compounds. Recently, compressed phosphorus hydride (phosphine) was reported to metallize at pressures above 45 GPa, reaching a superconducting transition temperature (${T}_{c}$) of 100 K at 200 GPa. However, neither the exact composition nor the crystal structure of the superconducting phase have been conclusively determined. This work reports an extensive study of the phase diagram of PH${}_{n}$ ($n$=1–6) by means of \textit{ab initio} crystal structure predictions using the minima hopping method. The results do not support the existence of thermodynamically stable PH${}_{n}$ compounds, which exhibit a tendency for elemental decomposition at high pressure even when vibrational contributions to the free energies are taken into account. Although the lowest energy phases of PH${}_{1,2,3}$ display ${T}_{c}$’s comparable to experiments, it remains uncertain if the measured values of ${T}_{c}$ can be fully attributed to a phase-pure compound of PH${}_{n}$.

[Phys. Rev. B 93, 020508(R)] Published Tue Jan 26, 2016

]]>The authors report x-ray absorption spectroscopy of samples in large electrical fields. The observed effect is linear in electric field and only observable in polar materials. It can be thought of as the x-ray analogue of the linear Stark effect. It may open up a new area of research by studying electrical polarization with element specificity for multiconstituent functional materials

[Phys. Rev. B 93, 035136] Published Tue Jan 26, 2016

]]>Phonons in iron pnictide materials were predicted to show large effects from magnetic ordering that had not been observed, and, indeed, the overall phonon dispersion has been in poor agreement with \textit{ab initio} calculations. By carefully detwinning a sample of SrFe${}_{2}$As${}_{2}$, clear phonon splitting was observed below ${T}_{N}$, allowing the authors to suggest an improved model for the phonon response for both the antiferromagnetically ordered phase below ${T}_{N}$ and the paramagnetic phase above ${T}_{N}$.

[Phys. Rev. B 93, 020301(R)] Published Mon Jan 25, 2016

]]>The authors report a first-principles study of the structural and electronic properties of the transition-metal-based compounds TiO${}_{2}$, ZnS, and NiO. Total energies (and resulting quantities) are calculated within the random-phase approximation (RPA) total-energy functional, which is being discussed actively in the DFT community. The effect of Hubbard $U$ corrections are shown. These calculations should stimulate more study and use of the RPA functional.

[Phys. Rev. B 93, 035133] Published Mon Jan 25, 2016

]]>The $g$ tensor, which describes the interaction energy of the spin with an external magnetic field, plays an essential role in controlling and manipulating single spins in semiconductor quantum dots. Spin-correlated orbital currents can strongly affect the $g$ tensor. This paper investigates theoretically and experimentally how these currents depend on the shape of quantum dots and how they affect the anisotropy of the electron $g$ tensor. It is found that the spin-correlated orbital currents form a simple current loop perpendicular to the magnetic moment’s orientation, and are therefore directly sensitive to the shape of the nanostructure. This simple and intuitive picture is validated by a systematic experimental magnetoluminescence study of the size dependence of the separate electron and hole $g$ tensors of InAs/InP quantum dots.

[Phys. Rev. B 93, 035311] Published Mon Jan 25, 2016

]]>Phenomenon of many-body localization violates one of the basic rules of statistical mechanics: It states that certain ‘localized’ macroscopic systems cannot act as a bath for themselves and hence do not relax to equilibrium. The authors of this paper find that, whenever a system is not localized at all thermodynamic parameters (particle density, energy density), then local fluctuations into the non-localized phase, dubbed bubbles, can slowly destroy localization globally. This result provides a rather strong restriction on the existence of many-body localized phases and runs contrary to the idea that there could be genuine localization transitions as a function of temperature.

[Phys. Rev. B 93, 014203] Published Fri Jan 22, 2016

]]>A voltage biased Josephson junction coupled to two microwave cavities which are in turn coupled to two “hot” and “cold” thermal environments, constitutes the proposal for the first thermoelectric heat engine where the heat current is completely separated from the electronic degrees of freedom.

[Phys. Rev. B 93, 041418(R)] Published Fri Jan 22, 2016

]]>Diffusive transport of magnons in YIG has recently attracted a lot of research attention, particularly in the context of the spin Seebeck effect and the new field of magnon spintronics. Here, the authors perform a systematic study of the influence of an external magnetic field on the magnon spin diffusion length, making use of a nonlocal measurement scheme that allows for direct extraction of this length scale. The results demonstrate a new mechanism for the manipulation of diffusive magnonic spin currents.

[Phys. Rev. B 93, 020403(R)] Published Tue Jan 19, 2016

]]>This paper provides a warning to the acoustic metamaterials community to be careful not to mistake the sign or value of the effective parameters, i.e. the density and modulus, of acoustic metamaterials for want of an overall picture of the physics involved. The authors here provide a new picture of the functioning of acoustic metamaterials based on hidden forces and hidden sources of volume. The new ansatz is tested on some established acoustic metamaterials with elements based on membranes, Helmholtz resonators, springs, and masses. It should provide the basis for a clearer vision of acoustic metamaterials and a faster route to real-world applications.

[Phys. Rev. B 93, 024302] Published Thu Jan 14, 2016

]]>Massless Dirac fermions with a pseudospin of $\frac{1}{2}$ are tied to many intriguing properties of graphene. Previous studies have discussed the possibility of constructing a higher pseudospin system in artificial lattices of ultracold atoms, which can also support equally interesting physical properties. The authors of this paper demonstrate that photon transport in certain photonic crystals corresponds to a pseudospin-1 system. This is a significant development because, contrary to ultracold systems, photonic crystals offer a better opportunity to study novel pseudospin-1 physics in experimentally realizable materials and at room temperature.

[Phys. Rev. B 93, 035422] Published Thu Jan 14, 2016

]]>Spin-orbitronics, which exploits the coupling between the spin and the orbital momentum of electrons, relies on the possibility to electrically create and detect pure spin currents without need of ferromagnetic elements. An efficient way to achieve this spin-to-charge conversion (and vice versa) is expected by exploiting the Rashba-Edelstein effect. This phenomenon is related to the well-known spin Hall effect, but in the former, the spin-to-charge current conversion occurs at the interface of materials with a strong spin splitting of the surface states, instead of the bulk. This paper reports an observation of the inverse Rashba-Edelstein effect, i.e., conversion of a spin current into a charge current, at a bismuth/copper interface, by using spin absorption with lateral spin valves. The induced charge current changes sign with temperature, a phenomenon that the authors can explain theoretically owing to the complex spin structure and dispersion of the surface states at the Fermi energy.

[Phys. Rev. B 93, 014420] Published Wed Jan 13, 2016

]]>In quantum magnetism, peculiar features of quantum spin system sometimes forbid the existence of gapped “featureless” paramagnets which are fully symmetric and unfractionalized. The celebrated Lieb-Schultz-Mattis theorem is an example of such a constraint, but it is not known what the most general restriction might be. For given lattice structure and symmetries, it is an interesting question whether a featureless paramagnetic phase exists. In this paper, the authors focus on the existence of such featureless paramagnets on the square and honeycomb lattices and, on the basis of analytical and numerical arguments, propose the corresponding wave functions for several such models. The existence of these nontrivial states not only offers a proof of principle, but also suggests an exotic field theory description when these states are brought to the vicinity of a second-order phase transition.

[Phys. Rev. B 93, 035114] Published Wed Jan 13, 2016

]]>Using a perturbation expansion to the fourth order in a junction critical current, the authors study the generation of nonclassical microwave radiation by inelastic Cooper pair tunneling in a small voltage-biased Josephson junction connected to a superconducting transmission line. The second order in the tunneling current only allows consideration of independent events. Pushing the calculation to the fourth order allows capture of the temporal correlations in the charge transport and their connection to the ones in the emitted electromagnetic radiation.

[Phys. Rev. B 93, 014506] Published Mon Jan 11, 2016

]]>Hydrogen sulfides have recently received a great deal of interest due to the record high-${T}_{c}$ of up to 203 K observed on strong compression of H${}_{2}$S . In this paper, a joint theoretical and experimental study is presented to characterize the dissociation products of compressed H${}_{2}$S , which is essential to understand its complex superconducting states. Based on the results here, the authors found H${}_{2}$S partially decomposes into S + H${}_{3}$S + H${}_{4}$S${}_{3}$ above 27 GPa, and H${}_{4}$S${}_{3}$ emerges as the major component at around 66 GPa. Interestingly, x-ray diffraction (XRD) experiments observed a small fraction of H${}_{3}$S and residual H${}_{2}$S at least up to 140 GPa, which are believed to be responsible for the two superconducting states observed in experiments. This paper provides the first XRD evidence on the existence of H${}_{3}$S at high pressure from the decomposition of H${}_{2}$S.

[Phys. Rev. B 93, 020103(R)] Published Mon Jan 11, 2016

]]>The prevailing theme of “form follows function” requires structural coherence over sufficient distances to allow subtle magnetic coupling to be revealed in high magnetic field studies at low temperatures. Single-crystal studies of the $S$=2 quasi-one dimensional chain material MnCl${}_{3}$(2,2’-bipridine) allows the long-range order to be identified, while also revealing the presence of an energy gap at the spin-flop transition field, whose nature remains intriguing.

[Phys. Rev. B 93, 014407] Published Fri Jan 08, 2016

]]>In modern quantum many-body physics, the Kadanoff-Baym equations have become a crucial component in the treatment of strongly and weakly correlated systems far from equilibrium. From the one-particle Green function $G$($t$,${t}^{\prime}$), they allow for the calculation of time-dependent expectation values of all one-particle observables and the total energy. In this work, for isolated Coulomb systems, the numerical behavior of the Kadanoff-Baym equations is investigated. The electron density dynamics are damped to an unphysical homogeneous density distribution, across both the linear and nonlinear response regimes. Unphysical features are shown to exist for $\mathrm{\Phi}$-derivable self-energy approximations, such as Hartree-Fock, second-Born, or $G\phantom{\rule{0}{0ex}}W$, in Hubbard and Coulomb systems, irrespective of interaction strength. With this degree of universality, these findings are pertinent to all two-time formalisms, and suggest the need for a different approach to the dynamics of quantum systems.

[Phys. Rev. B 93, 041103(R)] Published Fri Jan 08, 2016

]]>Doping a semiconductor induces a change in its lattice parameter. This change is caused not only by the different sizes of the impurity and host atoms, but also by the strain developed when the occupation of electronic energy bands is modified. While these ideas are conceptually simple, experimentalists and theorists alike have struggled for decades to separate the size and electronic effects. Following up on the most significant earlier developments, which focused on doped Si, the authors present data for $n$-type Ge doped with novel precursors and suggest that trends as a function of the donor species in both Ge and Si hold the key to resolving the doping dependence of the lattice parameter into its two fundamental physical components.

[Phys. Rev. B 93, 041201(R)] Published Fri Jan 08, 2016

]]>Through a combination of modeling and density functional theory calculations, the authors solve the structure of the only known quasicrystalline oxide, Ba-Ti-O, and introduce a new crystal-chemistry motif that explains all previously unsolved ultrathin Ba-Ti-O structures.

[Phys. Rev. B 93, 020101(R)] Published Thu Jan 07, 2016

]]>This paper reports on a photophysics study of gate-doped single-wall carbon nanotubes suspended over trenches. The authors experimentally demonstrate trion emissions from electrostatically doped nanotubes. They observe that the trion binding energies can be manipulated by varying the nanotube diameter.

[Phys. Rev. B 93, 041402(R)] Published Tue Jan 05, 2016

]]>Previously, closed optical transitions were only possible in crystals where the rare-earth ion site had perfect axial symmetry. The possibility of creating closed hyperfine transitions for all site symmetries by applying an appropriate oriented magnetic field is explored. The new technique should allow studies that are currently impossible for rare-earth-ion crystals both in the single ion and ensemble regimes.

[Phys. Rev. B 93, 014401] Published Mon Jan 04, 2016

]]>Dirac cones are at the heart of all the quasirelativistic phenomena oberved or predicted to occur in graphene. For a long time, it was believed that their existence is tightly bound to the honeycomb geometry of the graphene lattice. However, recent studies show that Dirac cones can be found in a variety of two-dimensional materials, with square or more complicated lattices. Here, the authors present a complete classification of these different systems, and show how Dirac cones emerge as a consequence of the symmetries of the underlying crystal. These results also provide guidelines to engineer novel materials that go beyond graphene.

[Phys. Rev. B 93, 035401] Published Mon Jan 04, 2016

]]>Based on experiments and simulations, the authors use waveguides of nonuniform cross-sectional width to tailor the spatial distribution of the energy density of transmission channels and to realize conversion between evanescent and propagating channels. For particular geometries, perfect reflection channels are created, and their large penetration depth into the medium as well as the complete return of probe light to the input end should greatly benefit sensing and imaging applications.

[Phys. Rev. B 92, 214206] Published Thu Dec 31, 2015

]]>Enhancing light transmission through scattering media is of much interest in many fields. Here, numerical simulations are performed to study the impact of gain on the value of reflection of the minimum reflection channel in a random medium. Surprisingly, in some random configurations, the reflection falls when gain is added even through the intensity inside the medium presumably increases once gain is introduced. This unexpected behavior is explained based on a decomposition of quasinormal modes.

[Phys. Rev. B 92, 224202] Published Tue Dec 29, 2015

]]>The first observation of two pressure-induced quantum phase transitions in a YbB${}_{6}$ single crystal is reported. This material is currently of much interest as there is conflicting experimental support for it belonging to a new class of topological insulators. Based on the results here the authors conclude that while at ambient-pressure YbB${}_{6}$ is topologically trivial, a newly found high-pressure gapped phase could be topologically nontrivial, although a partially gapped semimetal state has also been suggested. This interesting phase deserves further investigation.

[Phys. Rev. B 92, 241118(R)] Published Tue Dec 29, 2015

]]>BiFeO${}_{3}$ is one of the few known materials in which ferroelectricity and magnetism coexist at room temperature. Apart from possible practical applications, this multiferroic material has been attracting significant interest because of the interactions between its complex magnetic structure and different structural modes. The authors of this state-of-the-art first-principles study explore the electrical phase diagram of bulk BiFeO${}_{3}$ and reveal important aspects of the competition between ferroelectric and antiferroelectric phases.

[Phys. Rev. B 92, 235148] Published Mon Dec 28, 2015

]]>Room-temperature electronic properties of semiconductors, especially in the case of higher charge densities, are commonly discussed in terms of single-particle excitations – free electrons and holes. Many-particle effects, such as the formation of excitons (Coulomb-bound electron-hole pairs), are usually seen as low-temperature and low-density phenomena. In this paper, using ultrafast terahertz and photoluminescence measurements, the authors find that under certain conditions typical for wide-band-gap semiconductors, the radiative excitons can be efficiently formed at high charge density and at room temperature. This effect is believed to contribute to the extraordinarily high quantum efficiency of group III nitride light emitters.

[Phys. Rev. B 92, 241305(R)] Published Mon Dec 28, 2015

]]>Exactly solvable models often provide valuable insights in theoretical studies of topological phases. Here the authors introduce exactly solvable models of interacting Majorana fermions each with extensive topological ground-state degeneracy and a hierarchy of pointlike, topological excitations that are only free to move within submanifolds of the full lattice. These very different models make up a new kind of topological quantum order.

[Phys. Rev. B 92, 235136] Published Mon Dec 21, 2015

]]>For the first time, it is experimentally shown that a cloud of cesium (Cs) atoms can be used as a spectrally selective and tunable delay line for single photons emitted by semiconductor quantum dots. This delay line – significant slowing down of photons separated in frequency by only a few gigahertz – may represent the missing ingredient for the demonstration of a time-reordering scheme for entangled photon generation, and it could play a key role in future quantum networks and quantum communication.

[Phys. Rev. B 92, 235306] Published Mon Dec 21, 2015

]]>Understanding the conditions determining the appearance of stable chiral spin topological excitations is a central problem in condensed matter physics and related fields. This work provides the first unambiguous demonstration of discretization due to the macroscopic phase coherence and metastability for chiral topological excitations, and evidence of the mechanisms underlying its appearance. This was achieved through the use of an advanced Lorentz electron microscope with the highest possible resolution imaging of spin textures. The results provide new insights into dynamics in the phase transition of the chiral system, and will raise new questions concerning the mechanisms governing loss of chiral ordering.

[Phys. Rev. B 92, 220412(R)] Published Thu Dec 17, 2015

]]>van der Waals heterostructures, created by stacking two-dimensional materials, represent a novel and largely unexplored class of materials with very interesting optoelectronic properties. Excitons, strongly bound electron-hole pairs, play a crucial role in determining these properties, especially in 2D materials where the electron-hole binding is strong. However, a complete understanding of excitonic effects in 2D layered materials, i.e., when the electronic system transitions from a 2D geometry to a 3D one, is still missing. Here, the authors present a first-principles based multiscale method that attempts to fill this gap. With the help of their framework, one can predict the optoelectronics properties of van der Waals heterostructures and make a closer connection between the available theoretical models and experimental measurements in these materials.

[Phys. Rev. B 92, 245123] Published Thu Dec 17, 2015

]]>Building on recent advances by several groups to solve the fermion-sign problem in continuous-time quantum Monte Carlo calculations, the authors present two new algorithms and simulate the magnetic properties and phase transitions of the mass-imbalanced Hubbard model for square and honeycomb lattices. These theoretical calculations provide unbiased predictions that are relevant for experiments on ultracold atoms.

[Phys. Rev. B 92, 235129] Published Wed Dec 16, 2015

]]>A recent experimental identification of mono-pnictides (TaAs, TaP, NbP, and others) as Weyl semimetals has triggered an enormous effort to understand the unconventional physical phenomena in these materials. The chiral magnetic effect (CME), i.e., the occurrence of electrical current along the direction of an applied magnetic field, has been a particularly controversial subject, with a number of claims and counter-claims published in the literature. In this work Jing Ma and D. A. Pesin from the University of Utah provide a systematic theory of the CME and the related natural optical activity in these materials. They show that the natural optical activity of metals can be distinguished from well known phenomena in insulators by its singular frequency dependence in the terahertz range.

[Phys. Rev. B 92, 235205] Published Wed Dec 16, 2015

]]>The current through a driven two-level system, here realized in a double quantum dot, is determined by the phases acquired between avoided crossings. As a function of the detuning and the driving amplitude, it exhibits a characteristic interference pattern both in real space and in Fourier space. The authors demonstrate theoretically and experimentally the case of bichromatic driving creating fundamental relations between commensurability and symmetry properties.

[Phys. Rev. B 92, 245422] Published Tue Dec 15, 2015

]]>In an experimental and theoretical study, the authors find that the ac stress by a diamond mechanical resonator leads to a dressed spin basis whereby an NV center spin qubit is protected against magnetic fluctuations but remains sensitive to other environmental perturbations such as strain and temperature. By analyzing how the qubit protection scales with mechanical resonator amplitude, they demonstrate this technique’s value for precision thermometry, strain sensing, and quantum information.

[Phys. Rev. B 92, 224419] Published Mon Dec 14, 2015

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