Rare events can have major effects on quantum matter. Extremely unlikely events cause certain physical properties to diverge to infinity near the quantum phase transition of the disordered Ising antiferromagnet in a transverse field, but destroy criticality of the clean system completely when a longitudinal component of the field is present. Using a tree tensor network renormalization group method combined with a novel matrix product operator representation, the authors detect signatures of rare events and determine the zero-temperature phase diagram of the disordered antiferromagnetic Ising chain in the presence of both longitudinal and transverse magnetic fields. The numerical technique used in this paper is generalizable to more complicated many-body systems and higher dimensions.

[Phys. Rev. B 96, 064427] Published Mon Aug 21, 2017

]]>Novel solid-state qubits have a lot to offer for quantum information processing because of the potential simplicity of engineering quantum computational hardware similar to modern silicon-based electronics. This work presents optical and electron-spin properties of a novel germanium-vacancy defect in diamond. The defect center combines high brightness and exceptional spectral stability with microwave and optical access to electron spin. The combination of a spin-½ qubit and an optical interface makes possible of the germanium-vacancy defect in scalable quantum networks.

[Phys. Rev. B 96, 081201(R)] Published Fri Aug 18, 2017

]]>Confinement – the process by which particles with fractional quantum numbers bind to form quasiparticles with integer quantum numbers – is as important to condensed matter as to particle physics. By a combination of inelastic neutron scattering experiments and theoretical calculations, the authors establish the compound SrCo${}_{2}$V${}_{2}$O${}_{8}$ as a beautiful paradigm for spinon confinement in a quasi-one-dimensional quantum magnet. In particular, using state-of-the-art theoretical techniques, the authors provide a quantitative and macroscopic explanation for the existence of free spinons above the ordering temperature. They also explain how those spinons get confined into pairs with transverse and longitudinal polarizations by interchain interactions as well as how these bound pairs evolve with temperature.

[Phys. Rev. B 96, 054423] Published Thu Aug 17, 2017

]]>In most semiconductors and insulators the presence of a small density of charged impurities cannot be avoided, but their effect can be reduced by introducing defects of opposite charge. Screening in such a system leads to the formation of electron-hole puddles, which dominate bulk transport, as first recognized by Efros and Shklovskii. This paper investigates the typical length scale, which determines the distance between puddles and the suppression of puddle formation close to metallic surface states. The authors find that this length scale is much smaller than previously anticipated, with profound consequences for the sample quality of thin films of topological insulators.

[Phys. Rev. B 96, 075204] Published Thu Aug 17, 2017

]]>Periodically driven system may provide a route for realizing novel phases of matter. Particularly, Floquet topological insulators could be induced by resonantly driving a transition in a spin-orbit coupled quantum well. In this work, disorder, dynamics, and topology are all considered on the same footing within the realm of such phases. First, it is found that Floquet topological phases are robust to disorder. Next, it is found that disorder actually induces a transition into a Floquet topological phase, when the pure driven system is trivial. The resulting driven, disordered, topological phase is a realization of the Floquet topological Anderson insulator.

[Phys. Rev. B 96, 054207] Published Wed Aug 16, 2017

]]>Using the recently conjectured fermion particle-vortex duality, the authors demonstrate that the theory of a single (2+1)-dimensional Dirac fermion coupled to an Abelian gauge field in a (3+1)-dimensional bulk exhibits a strong-weak duality. The duality is a manifestation of the electromagnetic S-duality in the bulk. At the self-dual point, they find several exact relations between transport coefficients, such as conductivity, Hall conductivity, and thermoelectric coefficients. Consequently, the duality provides new constraints on the physics of a strongly interacting system.

[Phys. Rev. B 96, 075127] Published Mon Aug 14, 2017

]]>The authors succeeded in growing high-quality crystals of binary FeAs, a spin density wave antiferromagnet with similarities to two families of unconventional superconductors. Quantum oscillations were measured in magnetic torque at high fields, making it possible to compare experimental electronic structure to prior density functional theory calculations. The two agree on the multicarrier nature of FeAs in the magnetic state, but diverge in some aspects of the Fermi-surface geometry. The theoretical results have so far been unable to describe the magnetic structure, and the presented experimental data now indicate that that could be due to an incorrect Fermi surface picture. Oscillations analysis also shows enhanced effective masses compared to theory, indicating that electron correlations may have previously been underappreciated in this material.

[Phys. Rev. B 96, 075120] Published Thu Aug 10, 2017

]]>The microscopic mechanism of the subpicosecond magnetization dynamics observed in transition metals irradiated by intense optical laser pulses is not yet completely understood. Here, the authors analyze this effect in the case of the frustrated antiferromagnet FeMn within the framework of time-dependent spin-density-functional theory. Detailed spatial and temporal analysis of the first-principles spin dynamics allows an effective fully dynamical nonlocal in space exchange interaction to be introduced and evaluated. The study shows that it is meaningful to distinguish between two types of excitations governing the early dynamics of the antiferromagnetic system. The first type can be described as Stoner-like excitations, while the second type is of a spin-wave nature and is directly related to the ultrafast modification of the effective exchange interaction.

[Phys. Rev. B 96, 054411] Published Wed Aug 09, 2017

]]>Quantum and frustrated magnets are a great platform to probe collective phenomena of fundamental and technological importance. Antiferromagnets on a diamond lattice have not been as extensively studied as other model magnetic materials due to the absence of obvious geometrical frustration. This is changing with the prediction — and in some cases observation — of several unique phenomena in magnetic A-site spinels like, e.g., degenerate spin spirals, spin-orbital entanglement and topological paramagnetism. In this work, the authors add two model materials to the growing family of diamond lattice antiferromagnets: CuRh${}_{2}$O${}_{4}$ and CoRh${}_{2}$O${}_{4}$. The authors deploy an arsenal of experimental and theoretical techniques to develop a detailed understanding of their results. Their work evidences that CoRh${}_{2}$O${}_{4}$ is a canonical diamond-lattice antiferromagnet while CuRh${}_{2}$O${}_{4}$ displays an unexpected spin-helix ground-state with a very strong influence from quantum fluctuations.

[Phys. Rev. B 96, 064413] Published Wed Aug 09, 2017

]]>The gyrotropic resonance frequency of a magnetic vortex, widely studied for applications in data storage and radio-frequency signal processing, increases with the vortex core’s stiffness. In traditional disk-shaped elements, this stiffness increases if the core is shifted towards the element’s edges. Here, the authors show that introducing a flat edge into a disk locally de-stiffens the core, resulting in the resonant frequency strongly decreasing when the core approaches the element’s flat edge. By controllably displacing the core within the disk one can thus both increase or decrease the core’s resonant frequency with respect to its value when unshifted. This has the effect of more than doubling the accessible range of resonant frequencies. However, more fundamentally, the above properties lead to a chirality-mediated bistability: for a given finite static in-plane magnetic field, one of two values of gyrotropic frequencies can be observed depending on the vortex chirality.

[Phys. Rev. B 96, 060405(R)] Published Mon Aug 07, 2017

]]>Feynman diagrams represent one of the most powerful techniques of modern many-particle theory. Applying such diagrammatic techniques to many-body processes involving angular momentum is, however, substantially challenging. Here, the authors introduce a diagrammatic approach to the angulon quasiparticle, which consists of an orbital quantum impurity – such as a molecule or electron – exchanging angular momentum with a bath of bosons (superfluid, lattice phonons). The new approach fuses the Feynman diagrams of solid-state physics with the angular momentum diagrams previously used in atomic and nuclear structure calculations.

[Phys. Rev. B 96, 085410] Published Mon Aug 07, 2017

]]>Microwave spectroscopy of a Josephson junction is a useful tool to directly measure the properties of Andreev bound states. Recently, this method has been successfully applied to junctions defined in proximitized InAs/Al nanowires whose realization is aimed at the study and use of Majorana zero modes. In these nanowires, Andreev bound states are sensitive to the interplay of spin-orbit coupling and applied magnetic field. Our theory investigates the evolution of the Andreev levels and of the associated microwave spectrum upon application and increase of magnetic field. The authors find that a drastic change in the spectrum occurs once the magnetic field is strong enough to affect the ground-state parity, which can happen before the topological transition. Besides explaining existing experimental data, the presented theory provides guidance for future experiments, involving the spectroscopic observation of the effects associated with Majorana zero modes.

[Phys. Rev. B 96, 075404] Published Fri Aug 04, 2017

]]>Solid-state simulations at the quantum mechanics level are mostly done by means of density functional theory (DFT). With the development of a new type of functionals about a decade ago, the accuracy of DFT for systems with relevant van der Waals interactions increased. These functionals, the so-called van der Waals functionals, have been used extensively by the pseudopotential solid-state community, but, for technical reasons, not by the community that uses all-electron methods. Here, the authors present a very simple way for the implementation of van der Waals functionals within the framework of all-electron methods. At that, the advantages of the algorithm, used in the pseudopotential methods, are preserved. The development should lead to a much more widespread use of the van der Waals functionals in the all-electron community.

[Phys. Rev. B 96, 054103] Published Wed Aug 02, 2017

]]>Chiral magnetic thin films have been a recent source of interest due to their ability to stabilize topological magnetic states, known as skyrmions. However, there exists disagreement in the literature over their existence in epitaxial MnSi films, grown on Si(111) substrates. The authors have combined polarized neutron reflectometry and small-angle neutron reflectometry to uncover direct evidence for a unique skyrmion geometry in MnSi/Si(111) films where the core magnetization of the vortex-like magnetic textures points in the plane of the film.

[Phys. Rev. B 96, 054402] Published Tue Aug 01, 2017

]]>Using a combination of experiment and simulation, the authors have investigated comprehensively how the spin-wave transmission across a curved magnonic waveguide is modified through rotation of an externally delivered uniform magnetic field. They reveal that, while the spin-wave transmission is reasonable when the bend is magnetized along an axis of symmetry, the transmission can be substantially improved (by a factor of 2) through just rotating the magnetic field by 15° in a particular direction. To explain their results, the authors study the landscape of the internal magnetic field spanning the structure, and discuss how it can be tuned in order to optimally distribute the flow of anisotropic spin waves across networks of magnonic waveguides.

[Phys. Rev. B 96, 060401(R)] Published Tue Aug 01, 2017

]]>Growing thin films of ferroelectric oxides in direct contact with semiconductors is a key research goal due to its technological promise. This work presents a comprehensive theoretical study of ultrathin films of BaTiO${}_{3}$ on Ge. For the experimentally stable interface stoichiometry, there are two distinct interfacial configurations with differing out-of-plane electrical polarizations. The two configurations can be made energetically degenerate by choosing an appropriate top electrode, thus enabling, in principle, an epitaxial ferroelectric thin film oxide. Surprisingly, in the intrinsic limit for Ge, switching the oxide polarization state modulates the band alignments to such a degree that the dominant charge carrier switches between electrons and holes.

[Phys. Rev. B 96, 075301] Published Tue Aug 01, 2017

]]>This work advances a physical picture of the Dzyaloshinskii-Moriya interaction (DMI), capable of addressing the apparently contradicting conclusions of recent studies about the location of the DMI energy in the real and reciprocal space as well as about the relation between the values of atomic moments and DMI strength. It is demonstrated on the example of a Co/Pt bilayer how DMI emerges from the chirality-dependent hybridization of the Co and Pt magnetic-spiral states under the influence of spin-orbit coupling and broken spatial inversion.

[Phys. Rev. B 96, 024450] Published Mon Jul 31, 2017

]]>A key challenge in realizing a Majorana-based topological quantum computer is to identify a scalable platform granting easy control over networks of Majorana bound states. A promising candidate is a two-dimensional electron gas with strong spin-orbit coupling and a superconducting layer on top. Here, the authors propose two different ways of defining Majorana networks in these structures and theoretically model the basic network elements. They show that Majoranas can be exchanged (braided) by controlling their coupling with electric gates.

[Phys. Rev. B 96, 035444] Published Mon Jul 31, 2017

]]>Spin states of a three-electron system can encode one quantum bit in such a way that universal qubit control by voltage pulses becomes possible. In this work, a triple-dot qubit is operated in a highly symmetric charge configuration, using resonant radio-frequency voltage pulses applied to one gate electrode. The authors show that careful symmetrization reduces the susceptibility of the qubit frequency to voltage fluctuations by more than an order of magnitude, compared to conventional triple-dot spin qubits. Combining voltage control with symmetric operating regimes may lead to multiqubit processors based on semiconducting spin qubits that at the same time alleviate decoherence due to electrical noise.

[Phys. Rev. B 96, 045443] Published Mon Jul 31, 2017

]]>The fractional quantum Hall (FQH) nematic state is a conjectured state of matter in which FQH topological order coexists with spontaneously broken rotation symmetry. Recent experiments on GaAs quantum wells suggest this state, as well as the concomitant isotropic-nematic phase transition, may be realized in a two-dimensional electron gas at certain rational filling factors. Although effective field theories of the FQH nematic state have been constructed, there has been so far no explicit realization of this state as the ground state of a microscopic model of interacting electrons. The authors show via a variational approach and numerical exact diagonalization that tuning Haldane pseudopotentials in a model of interacting electrons in the lowest Landau level induces a quantum phase transition out of the isotropic Laughlin FQH liquid and into the FQH nematic state.

[Phys. Rev. B 96, 035150] Published Thu Jul 27, 2017

]]>Conduction electrons in Weyl semimetals move as massless relativistic particles with a definite chirality, meaning that the spin points parallel or antiparallel to the momentum. Such a spin-momentum locking allows for a mechanism to control Cooper-pair transfer at the interface between a magnetic Weyl semimetal and a superconductor. The authors demonstrate that Andreev reflection is fully suppressed when the internal magnetization lies in the plane of the interface. External magnetic field can activate Andreev reflection by relaxing the requirement of zero-spin transfer into the superconductor. The $c\phantom{\rule{0}{0ex}}h\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}y\phantom{\rule{0.333em}{0ex}}b\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}k\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}d\phantom{\rule{0}{0ex}}e$ that the team has discovered provides a phase-insensitive mechanism to control supercurrent. It is also the unique signature of a pseudoscalar superconducting pair potential.

[Phys. Rev. B 96, 035437] Published Wed Jul 26, 2017

]]>The authors examine the phenomenon of “Cheshire charge” in topological phases of matter in three spatial dimensions. A looplike excitation is said to carry Cheshire charge if the charge is not locally detectable, that is, it can only be observed by a nonlocal process such as shrinking the loop to a point. The authors show that “Cheshire charge” is a generic feature of three-dimensional topological phases. They relate it to other features of these phases, such as three-loop braiding, as well as to higher-category theory that is hypothesized to be the general mathematical framework describing three-dimensional topological phases.

[Phys. Rev. B 96, 045136] Published Tue Jul 25, 2017

]]>The authors study the finite-size energy spectrum of the O($N$) symmetric Wilson-Fisher conformal field theory (CFT) on the spatial torus. They first perform an analytic calculation of the spectrum using the $\u03f5$ expansion with a careful treatment of the zero-momentum modes. The authors also perform exact diagonalization on several lattice Hamiltonians whose critical points are known to be described by the Wilson-Fisher CFT. Comparison of the analytic and numerical data demonstrates that the energy spectrum is a useful diagnostic for determining the universality class of a critical point. These results have applications in determining the critical behavior of more complicated many-body systems.

[Phys. Rev. B 96, 035142] Published Mon Jul 24, 2017

]]>The high-field Hall number directly measures the carrier density in metals with closed Fermi surfaces. However, no general theorem is known to govern the behavior of the Hall number in systems with open Fermi surfaces or the critical behavior across a Lifshitz transition from a closed to open topology. Using Boltzmann transport theory, the authors derive analytic expressions for the behavior of the Hall number through such a transition, and recent experiments on hole doped cuprate superconductors that suggest a sudden drop of the Hall number below optimal doping are shown to be consistent with a nematicity-induced Lifshitz transition.

[Phys. Rev. B 96, 045132] Published Mon Jul 24, 2017

]]>Many-body localization (MBL) enables new nonergodic dynamical phases of matter that avoid the fate of thermalization. Do other ergodicity-breaking phases beyond the MBL paradigm exist? Here, the authors introduce a method for constructing and probing the stability of candidate nonergodic phases. As an example, they consider disordered spin systems, in which a non-Abelian SU(2) symmetry precludes the conventional MBL phase that is characterized by quasilocal integrals of motion and area-law entanglement entropy of eigenstates. Instead, the authors construct a candidate nonergodic phase characterized by eigenstates having logarithmic scaling of entanglement entropy. They find that the phase is only marginally unstable and, therefore, nonthermal yet non-MBL dynamics can be observed in such disordered systems.

[Phys. Rev. B 96, 041122(R)] Published Fri Jul 21, 2017

]]>The authors have investigated the dispersion of magnetic excitations in thin films of antiferromagnetic NiO using resonant inelastic x-ray scattering (RIXS) at the ${L}_{3}$ edge of nickel. They demonstrate here that, thanks to the recent improvements in energy resolution and scattering geometry, soft x-ray RIXS is a valid complement to inelastic neutron scattering in this field because it allows a clear identification of the spin-wave peaks not only in bulk crystals but also in thin films and heterostructures too small for neutron studies. Moreover, the spin-wave dispersion measured along three high-symmetry directions of the 3D fcc Brillouin zone allows an accurate determination of the main super-exchange integral ${J}^{\prime}$ that establishes the antiferromagnetic structure of NiO. This report demonstrates definitively that RIXS is not confined to 2D materials such as cuprates and can be a useful tool for the study of magnetic interaction in a wide class of 3$d$ transition metal systems.

[Phys. Rev. B 96, 020409(R)] Published Thu Jul 20, 2017

]]>In a periodically driven nonlinear lattice, small lattice deformations are parametrically amplified and the dynamic phonon response function is broadly enhanced. As a consequence, the phonon-mediated electron-electron attraction becomes stronger, allowing Cooper pairs to be formed at higher temperatures.

[Phys. Rev. B 96, 014512] Published Wed Jul 19, 2017

]]>The optical manipulation of phonon degrees of freedom can change the effective interactions in solids and potentially enhance superconductivity. However, heating and nonthermal energy distributions need to be considered to understand the ultimate effects of such a driving on ordered states. Using nonequilibrium dynamical mean field theory in the Kadanoff-Baym and Floquet implementation, the authors systematically discuss the effects of parametric phonon driving on conventional superconductivity. Both the transient dynamics of isolated systems and the nonequilibrium steady states in driven-dissipative systems are considered. While the phonon-mediated attractive interaction can be enhanced in certain driving regimes because of Floquet phonon sidebands, the heating and nonthermal distribution effects dominate and generically result in a suppression of superconductivity.

[Phys. Rev. B 96, 045125] Published Wed Jul 19, 2017

]]>Topological semimetals harbor relativistic fermions featuring light effective mass, high mobility, and a nontrivial Berry phase. Quantum oscillations are an effective means to probe the properties of relativistic fermions. In the Dirac nodal-line semimetal ZrSiS, the extremely high Dirac fermion density gives rise to very strong de Haas–van Alphen quantum oscillations even at low magnetic fields, by which nearly massless Dirac fermions with surprisingly strong Zeeman effect are revealed.

[Phys. Rev. B 96, 045127] Published Wed Jul 19, 2017

]]>In the field of ultrafast spin dynamics, magnetic inertia (spin nutation) is considered to hold great promise for achieving ultrafast magnetization switching. However, the fundamental origin of magnetic inertial dynamics is unknown. Here, the authors derive a complete theory of spin dynamics, including magnetic inertia, from fundamental Dirac theory. They show that magnetic inertia exists for any single Dirac particle and that it is a higher-order relativistic spin-orbit coupling effect compared to Gilbert damping. Magnetic inertia is therefore expected to play a role only on an ultrashort subpicosecond timescale.

[Phys. Rev. B 96, 024425] Published Tue Jul 18, 2017

]]>The authors have studied the multiferroic properties of a triangular lattice magnet with ferromagnetic nearest-neighbor (NN) interactions, using single-crystal $\alpha $-NaFeO${}_{2}$ grown by hydrothermal synthesis. Despite the ferromagnetic NN interactions, this system exhibits very rich magnetoelectric phase diagrams induced by application of magnetic field. The relationship between complex magnetic ordering and ferroelectric polarization can be explained by the inverse Dzyaloshinskii-Moriya effect. Competition between antiferromagnetic second NN interactions in the triangular lattice plane and weak inter-plane interactions leads to a rich phase diagram for the crystal under study.

[Phys. Rev. B 96, 035128] Published Mon Jul 17, 2017

]]>Multiferroic phases of materials like YM${}_{2}$O${}_{5}$ feature an optical magnetoelectric effect on the resonance of the electromagnon. The effect manifests itself in nonreciprocal directional dichroism. Here, the authors examine the role of the commensurability of noncollinear spin order for the optical magnetoelectric effect for two types of electromagnon. For the electromagnon driven by the exchange striction, the crucial role of the commensurability is indicated by the suppression of the directional dichroism by the magnetic phase transition. On the other hand, the gapped electromagnon due to the spin-current mechanism is identified by the directional dichroism, regardless of (in)commensurability. These results provide new insights into the light-matter interaction driven by dynamical magnetoelectric coupling.

[Phys. Rev. B 96, 041117(R)] Published Mon Jul 17, 2017

]]>The interplay of quantum correlations and thermal decoherence is key to understanding the fundamental properties of many-body systems. Coherence is usually easily destroyed by interaction with the environment and local fluctuations inherent to thermal ensembles. Hence experimental demonstrations of many-body quantum correlations rely on cooling the system close to its ground state or pumping it into a well-defined quantum state. Here, the authors instead show how quantum correlations can arise in the opposite limit of an infinite temperature. Motivated by recent experiments with ultracold atoms in optical lattices, they study the dynamics of a hole created in a fermionic or bosonic Mott state in the atomic limit. They demonstrate that the propagating hole entangles the surrounding spins leading to sizable and lasting dynamical spin correlations. In the absence of interactions, these correlations arise solely due to quantum interference. Furthermore, they are both ferromagnetic and antiferromagnetic, in contrast to the equilibrium Nagaoka effect.

[Phys. Rev. B 96, 014303] Published Fri Jul 14, 2017

]]>Topological phases of multiple bands near the Fermi energy are investigated for strained Hg${}_{x}$Cd${}_{1-x}$Te, both from a bulk topological invariants and a surface states perspective. The main result of the paper is that nontrivial topology is induced solely by the spin-orbit coupling and not by the $s\phantom{\rule{0}{0ex}}p$ band inversion. It is obtained through the actual calculation of the topological invariants of individual bands throughout the topological phase diagram. Along with insulating topological phases, Weyl-semimetal phases have also been identified in this material.

[Phys. Rev. B 96, 035124] Published Fri Jul 14, 2017

]]>It it has been found in recent years that a spin current can be generated when a ferromagnetic (FM) thin film is excited with a femtosecond laser pulse. Here, the authors use a noncollinear magnetic bilayer to investigate both the absorption and generation of these optical spin currents. It is demonstrated that the very local absorption of the spin current near the injection interface of a FM layer allows for terahertz spin wave excitation in such noncollinear magnetic bilayers, and advance of much relevance for the upcoming field of ultrafast magnonics.

[Phys. Rev. B 96, 014417] Published Thu Jul 13, 2017

]]>Piezoelectric effects can strongly affect the position and shape of the electron and hole wave functions in compound semiconductors quantum dots (QDs). From the point of view of theoretical semiconductor physics, this is a well-documented notion. From an experimental point of view, however, the extreme sensitivity of the electronic properties of QDs to tiny variations of their shape, size, and composition makes it very challenging to single out the effect of piezoelectricity, which is often neglected in the analysis of experimental data. Here, the authors demonstrate that externally induced piezoelectric fields can be used for wave function engineering and to even force an inversion of the exciton built-in dipole moment in the very same QD. Detailed calculations based on k.p theory disclose that the inversion of the exciton dipole is driven by the nonlinear terms of the piezoelectric fields. These findings shine new light on the role and the possible exploitation of piezoelectric effects in semiconductor QDs.

[Phys. Rev. B 96, 045414] Published Thu Jul 13, 2017

]]>Whether two-dimensional tensor networks, such as the so-called projected entangled pair states (PEPS), have the potential to describe a genuine quantum critical point or quantum critical phases separating two ordered phases is of much interest for the understanding of frustrated magnets and strongly correlated systems. Here, the authors identify a specific family of SU(2)-invariant PEPS, which provides excellent variational energies for the ${J}_{1}-{J}_{2}$ frustrated Heisenberg model at large frustration. Using a full-update infinite-PEPS scheme directly in the thermodynamic limit, the authors provide evidence of diverging correlation lengths in both the singlet and triplet channels. It is argued that such PEPS give a qualitative description of the critical point or critical phase of the ${J}_{1}-{J}_{2}$ model.

[Phys. Rev. B 96, 014414] Published Wed Jul 12, 2017

]]>How fast does quantum information spread in isolated quantum systems? The authors address this question using out-of-time order correlation functions and find that information spreads behind a power law front in disordered spin chains, with a characteristic exponent, which is related to the exponent of the power law growth of the entanglement entropy. The authors also observe that disorder inhibits information spreading and leads to unsaturated Lieb-Robinson bounds.

[Phys. Rev. B 96, 020406(R)] Published Wed Jul 12, 2017

]]>The nodal-line semimetal stands out as a paradigmatic representative of a gapless topological phase of matter that supports linearly dispersing quasiparticles around an isolated closed loop in the bulk and accommodates drumhead shaped surface states. The author demonstrates an intriguing possibility of realizing either charge or spin order on the surface of this system for arbitrarily weak Coulomb repulsion. When onsite Hubbard repulsion dominates, each surface becomes ferromagnetic with its moment pointing in opposite directions on complementary faces, which yields a global antiferromagnet. As shown numerically, surface ordering via proximity effect can in turn induce charge or spin order in the bulk at weak coupling, making the proposed scenario for spontaneous symmetry breaking relevant for materials, such as Ca${}_{3}$P${}_{2}$ and PbTaSe${}_{2}$, specifically in a thin-film geometry.

[Phys. Rev. B 96, 041113(R)] Published Wed Jul 12, 2017

]]>$I\phantom{\rule{0}{0ex}}d\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l\phantom{\rule{0.333em}{0ex}}s\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}g\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}h$ is a fundamental property describing the upper bound on a material’s strength in the absence of crystal defects. Similarly, the nature of the failure at the ideal strength can be related to the $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c\phantom{\rule{0.333em}{0ex}}d\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}y$ of metals and alloys. In this work, analytical expressions are derived for the ideal strength and intrinsic ductility with knowledge of only the second-order and third-order elastic constants. This analytical approach greatly improves the fundamental understanding of mechanical behavior in advanced materials.

[Phys. Rev. B 96, 014105] Published Mon Jul 10, 2017

]]>Two-dimensional few-layered materials exhibit unique and exotic electronic properties when compared to their bulk counterparts. Here, the authors report the role of dimensionality on the electronic properties of a van der Waals GaSe/graphene heterostructure. The strong thickness dependence of the GaSe band gap is demonstrated experimentally using scanning tunneling spectroscopy. The GaSe band gap decreases as the number of layers increases due to quantum confinement effects. The particular GaSe ‘Mexican hat’ band structure is further explored by angle-resolved photoemission spectroscopy and by theoretical calculations. Interestingly, an electron transfer from graphene to GaSe has also been detected. It results in $n$-type doped thin GaSe films, opposite to the $p$-type character GaSe bulk.

[Phys. Rev. B 96, 035407] Published Fri Jul 07, 2017

]]>The nonequilibrium dynamics of isolated many-body quantum systems leads generically to the emergence of standard thermal ensembles. However, some models, denoted as integrable, can display nontrivial steady states with a large memory of the initial state. This is due to the presence of conserved quantities constraining the unitary dynamics. Here, the authors show that in interacting integrable spin chains there exist also conserved operators that are responsible for ballistic spin transport. Such conserved operators highly influence both the equilibrium and nonequilibrium physics: at equilibrium they are responsible for a nonvanishing spin Drude weight even at half filling, while in the nonequilibrium case they are responsible for out-of-equilibrium steady states characterized by persistent spin currents or dynamically expanding magnetic domains.

[Phys. Rev. B 96, 020403(R)] Published Thu Jul 06, 2017

]]>Carrier mobility in graphene can be degraded by roughness. Here, the authors have measured the roughness of a suspended graphene layer encapsulated with hexagonal boron nitride (hBN) by diffraction in the transmission electron microscope. The 12-pm roughness measured is close to that found for carbon atoms within graphite. The simulations here show that encapsulation localizes flexural phonons in the hBN layers, leading to a suppression of roughness in the graphene. These results could lead to new strategies for device fabrication in applications requiring ultimately high performance, and show that layer roughness in artificially fabricated van der Waals heterostructures approaches that in naturally occurring bulk crystals.

[Phys. Rev. B 96, 014101] Published Wed Jul 05, 2017

]]>One of the hallmarks of clean, $d$-wave superconductivity is a linear temperature dependence of superfluid density in the low-temperature limit. Here, the authors show that weak scattering, combined with a realistic model of the cuprate Fermi surface, allows the linear behavior of superfluid density to persist in the presence of disorder even in the regime where the superfluid density and the transition temperature are strongly suppressed from their clean-limit values. This leads to a superfluid density that is both correlated with the transition temperature and linear in temperature, consistent with recent experiments on overdoped La${}_{2-x}$Sr${}_{x}$CuO${}_{4}$.

[Phys. Rev. B 96, 024501] Published Wed Jul 05, 2017

]]>The authors present magnetic moment measurements of a levitated graphene nanoplatelet, observing two mechanisms of interaction with a magnetic field, one diamagnetic and one due to the particle’s rotation. These observations are made possible by gyroscopically stabilizing the nanoplatelet using radio-frequency electric fields, achieving control of the rotation frequency up to 100 MHz while it is held in an electric field trap at high vacuum. Levitation avoids substrate effects and clamping losses entirely, while gyroscopic stabilization provides unparalleled torque sensitivity. The method is expected to be generalizable to other two-dimensional materials and nanoparticles and may be incorporated into optomechanics experiments.

[Phys. Rev. B 96, 035402] Published Wed Jul 05, 2017

]]>Recently, the picture of type-II energy dispersion with broken Lorentz invariance has been adapted from Weyl to Dirac fermions. It is demonstrated in PdTe${}_{2}$ in this paper by extensive evidence from electrical transport, de Haas–van Alphen oscillations, first-principles calculations, and angle-resolved photoemission spectroscopy. The type-II Dirac cone looks like a pinnacle in momentum space. It may be cut by the Fermi level with the result of a pair of Dirac pockets (of $p$/$n$ type), whose sizes change dramatically with the Fermi level. The nontrivial Berry phase of the hole pocket is also displayed. This also means a nonvanishing density of states in the whole Dirac cones, which makes PdTe${}_{2}$ an improved platform for possible topological superconductors and Majorana fermions

[Phys. Rev. B 96, 041201(R)] Published Wed Jul 05, 2017

]]>The authors demonstrate that in nonmagnetic crystals with threefold or sixfold symmetry, such as trigonal Te or hexagonal NbSi${}_{2}$, three linear band crossings (Weyl nodes) of the same chirality can merge at a high-symmetry point on the symmetry axis, forming a triple Weyl node with a cubic off-axis dispersion. If time reversal invariance is broken, the triple node in NbSi${}_{2}$ is displaced from the symmetry point along the axis, while its off-axis dispersion becomes quadratic. In Te, however, it splits into three linear Weyl nodes, since threefold symmetry is unable to stabilize triple Weyl nodes in the absence of time reversal symmetry.

[Phys. Rev. B 96, 045102] Published Wed Jul 05, 2017

]]>The spin dynamics of electrons at the metal-to-insulator transitionis a surprisingly complex problem in semiconductor physics, where many claims to complete solution have often been made in the past. In this work, the authors employ magnetotransport measurements in $n$-doped GaAs and identify unambiguously the underlying electron scattering mechanisms in a contiguous set of specially designed $n$-doped bulk semiconductor structures. They deduce the doping dependent scattering angles and combine these findings with highly accurate optical measurements of the spin dynamics. This complementary analysis allows for a complete quantitative modeling of the electron spin relaxation over the entire range from weakly interacting to fully delocalized electrons.

[Phys. Rev. B 96, 045201] Published Wed Jul 05, 2017

]]>Silicon quantum dots represent a promising platform for quantum computing, in which the spin states of single electrons can be used as qubits. Here, the authors examine the effect of the intervalley spin-orbit coupling on the resonance spectrum of a single qubit and propose a new type of universal two-qubit operation that utilizes the small g-factor difference between two exchange-coupled quantum dots. Both findings have significant implications for the development of larger-scale spin qubit systems.

[Phys. Rev. B 96, 045302] Published Wed Jul 05, 2017

]]>After two decades of intense investigation, the pseudogap phase of cuprate superconductors still remains a puzzle. The authors explore it by measuring the electrical resistivity and Hall coefficient of the cuprate Nd-LSCO in high magnetic fields for closely spaced doping rates across the pseudogap critical point. They report two main observations. First, both types of measurement indicate a drop of carrier density from 1+$p$ to $p$ upon entering the pseudogap phase; the process is best characterized as a crossover as a function of temperature at fixed doping and as a transition as a function of doping at zero temperature. Second, the mobility is unaffected by the transition. These findings, quantitively consistent with the cases of YBCO and LSCO, appear to be a universal signature of the pseudogap.

[Phys. Rev. B 95, 224517] Published Thu Jun 29, 2017

]]>The Dzyaloshinskii-Moriya interaction is vital in stabilizing the magnetic phases of chiral magnets, but how does this interaction manifest itself in the magnetic excitation spectrum? The authors perform high resolution time-domain terahertz spectroscopy experiments and time-of-flight neutron scattering experiments to investigate the low-energy magnetic response of the newly discovered insulating and magnetoelectric chiral magnet Cu${}_{2}$OSeO${}_{3}$. Transmission and polarimetry experiments performed as a function of magnetic field uncover new magnetic excitations, detect the topologically nontrivial skyrmion phase, and reveal unexpected magnon dynamics. These observations are consistent with the presence of an exceptionally strong Dzyaloshinskii-Moriya interaction in this material and highlight the need for its inclusion if the magnetic response of such chiral magnets is to be completely understood..

[Phys. Rev. B 95, 235155] Published Thu Jun 29, 2017

]]>In this article, the authors investigate the interplay between the Peierls instability and magnetism in members of the $R$NiC${}_{2}$ family, where $R$=Nd, Pr, Ce. Transport, magnetic, specific heat, and galvanomagnetic properties are studied. In NdNiC${}_{2}$, the charge density wave state is partially suppressed by an antiferromagnetic transition. The application of a strong enough magnetic field induces a ferromagnetic transition, accompanied by further destruction of the charge density wave and by a metastable lattice transformation. PrNiC${}_{2}$ shows the opposite behavior in that, although the Peierls instability is initially weakened by the Zeeman splitting of the conduction bands, the magnetic anomaly restores and even enhances the nesting conditions.

[Phys. Rev. B 95, 235156] Published Thu Jun 29, 2017

]]>The authors demonstrate a novel ultrahigh-resolution, minimally invasive technique for the detection of the domain-wall position within magnetic nanostructures. Based on the anomalous Nernst effect, the technique is suitable for a wide range of spintronic nanodevices implementing perpendicular-anisotropy materials. A thermal gradient generated on-chip is used to drive charge carriers, which are in turn deflected by the sample’s magnetization. Domain-wall positions within the device can be monitored with a resolution of 20 nm by measurement of the resulting Nernst voltages.

[Phys. Rev. B 95, 220410(R)] Published Tue Jun 27, 2017

]]>The interpretation of scanning tunneling microscopy (STM) data on poorly metallic materials is challenging: one needs to take into account the partial penetration of the electric field into the sample, the so-called tip-induced band bending (TIBB). This effect is well known from STM experiments on semiconductors, but rarely discussed for correlated-electron systems. The authors find that in the lightly doped Mott insulator (Sr${}_{1-x}$La${}_{x}$)${}_{2}$IrO${}_{4}$, TIBB is at the root of apparent discrepancies in the gap values measured by photoemission, optical measurements, and STM. They develop a model to extract the intrinsic Mott gap of the system, leading to agreement with results from other techniques.

[Phys. Rev. B 95, 235141] Published Fri Jun 23, 2017

]]>Large classes of matter have been systematized before by using a pair of groups to classify all spontaneous symmetry breaking phases or by using a pair of groups plus projective representations to classify all gapped quantum phases of one-dimensional boson and fermion systems with various symmetries. In this paper, the authors go beyond this and use a pair of groups plus three braided fusion categories and a chiral central charge in order to classify gapped quantum phases of two-dimensional boson and fermion systems with various finite symmetries. This allows, among other things, to generate many group tables of two-dimensional gapped phases.

[Phys. Rev. B 95, 235140] Published Thu Jun 22, 2017

]]>The Kitaev quantum spin liquid on a honeycomb lattice has received much interest in magnetism, mainly because of the possibility of emergence of novel excitations called Majorana quasiparticles. The quest is under way to realize this exotic state in actual materials. One of the promising materials is the layered honeycomb material RuCl${}_{3}$, for which the signatures of the Majorana quasiparticles have been reported. Here, the authors investigate the magnetic thermal conductivity of RuCl${}_{3}$ along with bulk thermodynamic measurements. They find that in the temperature range on the order of the Kitaev-coupling strength anomalous heat conduction takes place, which correlates with the growth of the magnetic specific heat. The result is consistent with the propagation of magnetic excitations, likely itinerant Majorana quasiparticles, driven by Kitaev coupling.

[Phys. Rev. B 95, 241112(R)] Published Thu Jun 22, 2017

]]>Organic light-emitting diodes convert electricity to light through the recombination of positive and negative charges. This process is fundamentally dependent on the spin of the carriers, and can therefore be influenced by a magnetic field. We show that magnetic-field-induced changes to the conductivity of a diode structure made of a conducting polymer can be resolved down to the scale of some 20 parts per billion. This extreme sensitivity to magnetic fields allows us to resolve field changes of less than 100 nanotesla, which is almost two orders of magnitude less than Earth’s magnetic field.

[Phys. Rev. B 95, 241407(R)] Published Thu Jun 22, 2017

]]>A team of experimentalists and theorists proposes a scalable protocol for quantum computation based on topological superconductors.

[Phys. Rev. B 95, 235305] Published Wed Jun 21, 2017

]]>The real benefit of antiferromagnets is that they can enable terahertz spintronic circuits. Spin pumping is a versatile tool for generating pure spin currents and probing spin dynamics. The high antiferromagnetic resonance frequencies represent a challenge for experimental detection, but magnetic fields can reduce these resonance frequencies. The authors compute the inverse spin Hall voltages resulting from dynamical spin excitations. They suggest practical opportunities that will significantly enhance the spin pumping and inverse spin Hall voltage for the uniaxial antiferromagnets MnF${}_{2}$ and FeF${}_{2}$.

[Phys. Rev. B 95, 220408(R)] Published Tue Jun 20, 2017

]]>Topological crystalline materials are emergent topological phases due to crystalline space group symmetry. They are either gapful or gapless in the bulk, while hosting topological states at the boundary. Here, the authors define topological crystalline materials rigorously on the basis of a mathematical theory, known as twisted equivariant K-theory. Abstract mathematical ideas, such as the Mayer-Vietoris sequence and module structure, are explained in terms of band theory. The formulation is applicable to bulk gapful topological crystalline insulators/superconductors and their gapless boundary and defect states as well as to bulk gapless topological materials, such as Weyl and Dirac semimetals or nodal superconductors. The authors present a complete classification of topological crystalline surface states and band insulators protected by 17 wallpaper groups in the absence of time-reversal invariance, which may support topological states beyond simple Dirac fermions.

[Phys. Rev. B 95, 235425] Published Mon Jun 19, 2017

]]>Resonance effects lead to surprising and beautiful phenomena in many areas of physics. Here, the authors identify resonance points as key to understanding the breakdown and revival of strong zero modes in parafermionic clock models. Through analytic and numerical methods, they have been able to determine the behavior at and away from resonance points, proving many statements about the asymptotic behavior of these systems. In particular, they show that there exists special points in parameter space where strong zero modes can exist and, as such, topological degeneracy is preserved at all energies.

[Phys. Rev. B 95, 235127] Published Thu Jun 15, 2017

]]>In this paper, the author examines coherent transport of levitons through a quantum dot system in the Kondo regime. He demonstrates the repeated emergence of the Kondo resonance in the nonequilibrium regime where the Fermi sea is driven by optimal periodic Lorentzian pulses. This work suggests practical opportunities to design quasiparticle excitations in interacting electron systems by engineering time-dependent fields.

[Phys. Rev. B 95, 241302(R)] Published Thu Jun 15, 2017

]]>An individual Mn atom coupled to a heavy hole confined in a semiconductor quantum dot forms a hybrid spin with a large magnetic anisotropy. In this work, the authors study the dynamics of such hybrid spin in a charge tunable II-VI semiconductor quantum dot. The hole-Mn energy levels and the positively charged exciton form an ensemble of optical $\mathrm{\Lambda}$ systems that can be independently addressed. Analyzing the dynamics of the resonant photoluminescence of the $\mathrm{\Lambda}$ systems, they identify an efficient relaxation channel for the coupled hole-Mn spins driven by the interplay of the hole-Mn exchange interaction and the coupling to acoustic phonons.

[Phys. Rev. B 95, 245308] Published Tue Jun 13, 2017

]]>Topological phases are exotic states of matters which do not order locally yet exhibit a global ordering in their entanglement, making their characterization a challenging task. In this work, the authors construct an explicit holographic duality between topological phases in the bulk and the structure of the entanglement spectrum at its boundary, thereby establishing a one-to-one correspondence between different bulk topological orders and symmetry-breaking and symmetry-protected orderings of the entanglement. This allows classification and detection of topological phases holographically through “entanglement spectroscopy” at the boundary, and construction of order parameters that measure the condensation and confinement of anyons, and thus permits characterization of the nature of topological phase transitions.

[Phys. Rev. B 95, 235119] Published Mon Jun 12, 2017

]]>The detection of single microwave photons remains a difficult challenge because of their low energy. The authors demonstrate that an aluminum superconducting double dot can be tuned to a regime in which a photon creating a single quasiparticle pair changes the quantum capacitance of the device. By using high-bandwidth reflectometry techniques, they observe the absorption of photons in real time. They also exploit the band structure of the device to tune the frequencies to which it is sensitive and use this to carry out spectroscopy on the ambient black-body radiation environment in their dilution refrigerator.

[Phys. Rev. B 95, 235413] Published Fri Jun 09, 2017

]]>Thermoelectric effects can be surprisingly large in superconductor-ferromagnet tunnel junctions when the superconductor is subject to a spin-splitting field. The spin splitting can be provided by an applied magnetic field, but can also be created by exchange coupling to a ferromagnetic insulator. Here, it is shown that thermoelectric effects in superconductor-ferromagnet tunnel junctions can be enhanced by boosting the spin splitting with an intrinsic exchange field provided by a ferromagnetic insulator. These findings could lead to more precise local electron thermometry or efficient microrefrigerators.

[Phys. Rev. B 95, 224505] Published Wed Jun 07, 2017

]]>Weyl semimetals are materials with topological defects in their band structure. They arise through mechanisms similar to vortex–anti-vortex pair creation, forming a hedgehog-structure in momentum space that is described by a four-dimensional energy-momentum Dirac cone. The authors show that, under pressure, the Dirac cone can be tilted in the same way as the relativistic light cone is tilted on approach to the black hole horizon. Following through with this analogy, at the defined horizon, time and space reversal may trigger unconventional phenomena, including the possibility for time travel. For Weyl semimetals under pressure, this means the creation of a new topological hyperbolic phase, which is anisotropic and highly active to electromagnetic radiation. Electrons may propagate across a multilayer structure made of Weyl semimetals analogous to light. Such a multilayer sandwich may have a negative refractive index for electron waves and, therefore, it may act as an electron-focusing Veselago lens. In this case, it would become possible to confine electrons to an extremely narrow beam. In the context of applications, this effect could be used to improve the resolution of the scanning electron microscopy technique, by constructing tunneling tips made up of Weyl semimetal multilayers.

[Phys. Rev. B 95, 214103] Published Mon Jun 05, 2017

]]>Low-temperature thermal conductivity measurements in the ferrimagnetic insulator Cu${}_{2}$OSeO${}_{3}$ reveal an unprecedentedly large magnonic contribution below T~10K, far exceeding (by nearly two orders of magnitude) that observed previously in any other ferromagnet or ferrimagnet. Features predicted by theory more than 50 years ago are identified, including ballistic behavior with ${\kappa}_{m}\propto {T}^{2}$, and, possibly, Poiseuille flow, wherein the magnon mean-free path exceeds the specimen dimensions as momentum conserving scattering occurs more frequently than scattering by resistive processes.

[Phys. Rev. B 95, 224407] Published Mon Jun 05, 2017

]]>The authors determine the fate of single-particle Weyl excitations in the presence of short-range disorder. Using analytic and numerical methods, the authors show that weakly disordered Weyl excitations are not ballistic and acquire an exponentially small but nonzero quasiparticle lifetime due to rare regions of the random disorder potential. As a result, the Green function near the Weyl quasiparticle peaks remains analytic and the avoided quantum critical point renormalizes the single-particle excitation spectrum at finite momentum.

[Phys. Rev. B 95, 235101] Published Thu Jun 01, 2017

]]>Defining thermal analogs of electrical components is required for the demanding task of managing heat in devices. The authors introduce a novel strategy to achieve a thermal transistor by utilizing thermal fluctuations. They identify the absorption of heat through inelastic transitions as the main limitation of existing proposals and solve this problem by partitioning the device so that the conductor is coupled to the heat source via an auxiliary system whose fluctuations act as a switch for heat flow. This can be achieved even with arbitrarily low heat injection, which results in extremely large amplification coefficients. This concept may open up a new avenue for the efficient manipulation of heat by heat and, more generally, for the field of noise-assisted thermal devices.

[Phys. Rev. B 95, 241401(R)] Published Thu Jun 01, 2017

]]>The Kitaev model is a spin model on a honeycomb lattice with bond-dependent anisotropic interactions whose ground state is an exactly solvable example of a quantum spin liquid. Interest in this model has been growing rapidly due to the potential for realization of this artificial model in insulating materials with strong spin-orbit interactions. One such example is the layered honeycomb material $\alpha $-RuCl${}_{3}$. Although this material orders magnetically rather than realizing a spin liquid ground state, there have been suggestions that suppression of the magnetic order through application of magnetic field could reveal a quantum spin liquid phase in this material. In this work, the authors examine the temperature-field phase diagram of $\alpha $-RuCl${}_{3}$ using neutron diffraction and bulk thermodynamic measurements. They found that the magnetic order disappears at magnetic fields above 8 Tesla, and a magnetic gap showing quantum critical scaling opens up in the high field phase.

[Phys. Rev. B 95, 180411(R)] Published Wed May 31, 2017

]]>The nature of the “hidden order” phase of URu${}_{2}$Si${}_{2}$ continues to defy understanding, despite three decades of intense research. Nonetheless, progress has been made in identifying key properties of the “hidden order” phase, including the observation of spin correlations indicative of Fermi surface nesting by neutron scattering. Here, the authors show that these spin correlations appear in the pressure-induced antiferromagnetic phase of URu${}_{2}$Si${}_{2}$, with remarkably similar properties. The persistence of these magnetic correlations suggests a significant kinship between these phases that had not been previously appreciated.

[Phys. Rev. B 95, 195171] Published Wed May 31, 2017

]]>Orbital degrees of freedom are a key ingredient in unconventional physics, including colossal magnetoresistance (CMR). When ordered, orbital arrangements can be characterized using conventional crystallographic approaches. Yet CMR emerges from states of orbital disorder, for which the experimental signature is much more ambiguous. Here, the authors study the CMR parent compound LaMnO${}_{3}$, using total scattering to understand its orbital order/disorder transition. They find a discontinuous change in local structure that indicates a fundamental change in the type of orbital arrangement at the transition. The analysis highlights the difficulty of discriminating between local structural models when static and dynamic disorder are strongly coupled.

[Phys. Rev. B 95, 174107] Published Tue May 30, 2017

]]>Nonequilibrium systems under periodic driving realize novel topological phases that cannot be achieved in equilibrium systems. One unique feature of periodically driven systems is that they can host a purely dynamical symmetry that involves time translation. This work explores a new class of Floquet topological phases protected by one realization of such dynamical symmetry, i.e., “time glide symmetry”, which is defined by a combination of reflection and time translation. Lattice models with time glide symmetric driving that are introduced show stable gapless surface states along with nontrivial topological numbers defined with time glide symmetry. In addition, a general classification theory of time glide symmetric topological phases is obtained by using a Clifford algebra approach.

[Phys. Rev. B 95, 195155] Published Fri May 26, 2017

]]>Finding suitable nonlocal order parameters that distinguish various symmetry-protected topological (SPT) phases is an important subject in view of experimental and numerical detection of SPT phases. By “simulating” the generating manifold of cobordism group in the operator formalism, the authors here propose nonlocal operations as diagnoses for SPT phases protected by point group symmetries. The nonlocal operations involve “partial point group transformations”, which are obtained by point group transformations restricted to a spatial subregion on a given ground-state wave function. Through analytical and numerical calculations, the authors show that the complex $U(1)$ phase of the ground state expectation value of such partial point group transformations may serve as an order parameter for those SPT phases. The examples in the paper include the ${Z}_{8}$ and ${Z}_{16}$ invariants of topological superconductors protected by inversion symmetry in $(1+1)$ and $(3+1)$ dimensions, respectively, as well as the lens space topological invariants in $(2+1)$-dimensional fermionic topological phases.

[Phys. Rev. B 95, 205139] Published Thu May 25, 2017

]]>As one of the most common waves in daily life, scalar sound is not easy to control by external fields since it lacks the intrinsic degrees of freedom such as the charge and spin in electrons. Here, the authors present an experimental observation of acoustic vortex states in sonic crystals, where the vortex chirality forms a good carrier of information for sound. By selectively exciting such novel states, they have fabricated a lattice array of sound vortices, and controlled the vortex chirality according to the operating frequency or incident direction of external sound. Furthermore, a peculiar beam-splitting behavior is observed, where the two spatially separated sound beams carry opposite vortex chirality, as manifested in the image here.

[Phys. Rev. B 95, 174106] Published Tue May 23, 2017

]]>Symmetry-protected topological (SPT) phases (e.g., the well known topological insulators) are a class of energetically gapped condensed matter systems that exhibit interesting topological properties only in the presence of certain global symmetries. While a great understanding of noninteracting fermionic SPT phases has been achieved recently, interacting SPT phases, in particular those that exist only in strongly interacting systems, are much less understood in general. Here, the authors study strongly interacting fermionic SPT phases in two spatial dimensions with finite Abelian unitary symmetries and provide a potentially complete classification of them.

[Phys. Rev. B 95, 195147] Published Mon May 22, 2017

]]>Dielectric antennas possess a complex multipolar response comprising both electric and magnetic resonances. These various modes can be selectively enhanced or suppressed using illumination beams of varying symmetries. However, such demonstrations are currently limited to single-particle antennas. Here, the authors address this issue with a theoretical study of dielectric dimer antennas illuminated by cylindrical vector beams (CVBs). They excite and study hybridized multipolar resonances in both horizontal and vertical dimer systems and show how the different beam symmetries of CVBs can selectively excite electric and magnetic modes in vertical systems, as well as couple to dark multipolar modes. The authors design an antenna system that yields unprecedented magnetic field enhancement and establishes the dielectric dimer antenna as a useful test bed for further experimental investigation.

[Phys. Rev. B 95, 201111(R)] Published Mon May 22, 2017

]]>Despite two decades of intense research, the origin of ultrafast demagnetization remains disputed. Here, the authors employ resonant magnetic x-ray reflectivity to follow simultaneously magnetization and structural dynamics at the BESSY femtoslicing source. They find that significant changes in nonmagnetic x-ray reflectivity accompany the subpicosecond demagnetization, which can be modeled as a variation of film thickness. Their simulations further show that the higher photon flux and energy resolution provided by x-ray free electron lasers will yield decisive data to differentiate between different mechanisms proposed to govern ultrafast demagnetization dynamics.

[Phys. Rev. B 95, 184422] Published Thu May 18, 2017

]]>The theoretical suggestion and the subsequent experimental evidence for the existence of discrete time crystals (DTCs) constitutes one of the most exciting recent fundamental advances in out-of-equilibrium (OOE) quantum physics. These are OOE phases in which time translation symmetry is spontaneously broken alongside the symmetry breaking familiar from static equilibrium systems. In this work, the authors study the fragility of this phenomenon by considering the effect of an external environment on the $\pi $ spin glass, the paradigmatic model of a disordered DTC, which had hitherto only been considered in completely isolated systems.

[Phys. Rev. B 95, 195135] Published Wed May 17, 2017

]]>Interfaces between solids play a key role in heat conduction by phonons in applications ranging from heat dissipation in light-emitting diodes to waste heat recovery with thermoelectric devices. Despite their importance, our understanding of thermal phonon transport across interfaces lags far behind that of electrons and photons, with basic parameters such as transmission coefficients being largely unknown. In this work, the authors demonstrate an experimental approach that enables the measurement of phonon transmission coefficients at solid interfaces using experiments and $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ computation. This work provides a unique microscopic window into the microscopic processes governing interfacial transport of terahertz-frequency thermal phonons that could impact numerous applications.

[Phys. Rev. B 95, 205423] Published Wed May 17, 2017

]]>The subgap excitations known as Andreev levels govern the low-energy physics of hybrid superconductor-semiconductor nanostructures, and lie at the origin of topological superconductivity. A quantitative understanding of their scaling with respect to relevant parameters is, however, still limited. Here, the authors address this issue for the prototypical case of a quantum dot coupled to a superconductor. Experimentally, the authors demonstrate the ability to controllably tune the hybridization of the dot and the superconductor. In addition, they develop a methodology to reliably extract the relevant parameters by numerically adjusting the Anderson model to the experimental data. As a result, they carefully study the scaling of Andreev levels within the parameter space and obtain an experimental phase diagram of the system. Their results show an impressive quantitative agreement with the theory.

[Phys. Rev. B 95, 180502(R)] Published Mon May 15, 2017

]]>Quantum annealers are computing machines that utilize quantum effects to solve hard optimization problems. Understanding the circumstances under which quantum annealers are more efficient than other optimization methods is a challenging open problem. A clue to this conundrum comes from studying the complexity of quantum annealers. In general, the complexity of a physical system relates to how efficiently the system can be simulated by classical algorithms. One then asks: if some class of quantum annealers are hard to simulate classically, are they also more powerful optimization machines? Here, the authors study quantum annealers belonging to two different complexity classes: the so-called “stoquastic” systems, which can be efficiently simulated with classical algorithms, and “nonstoquastic” systems, currently with no efficient classical treatment. Applied to a prototypical optimization problem, the authors observe that, on average, the stoquastic systems perform better on easier instances of the problem, while the more complex nonstoquastic annealers show significant advantage when applied to the hardest instances. The authors conjecture that the performance of the nonstoquastic Hamiltonians is closely related to their internal structure during the annealing process and elucidate how properties such as magnetic frustration can play a key role in devising more powerful quantum annealers.

[Phys. Rev. B 95, 184416] Published Mon May 15, 2017

]]>Transparent conducting oxides are used in a wide variety of applications, including solar cells and displays. Recently, the perovskite oxide BaSnO${}_{3}$ was demonstrated to be a superior material, with a carrier mobility that is 30 times larger than that of the prototypical perovskite oxide, SrTiO${}_{3}$. This outstanding value has opened up prospects for applications in advanced electronic devices. Using accurate first-principles calculations and Boltzmann transport theory, the authors perform a careful and detailed numerical analysis of the LO phonon and ionized impurity scattering mechanisms to elucidate their impact on mobility. They also compute the Hall factor explicitly, enabling a direct comparison to experimental reports of Hall mobilities for bulk and thin films. The analysis provides insights into the nature of the dominant mechanisms that limit mobility in state-of-the-art samples, and will aid the design of perovskite oxides with targeted transport properties.

[Phys. Rev. B 95, 205202] Published Mon May 15, 2017

]]>Periodically driven Floquet systems can host dynamical phases, including a range of exotic topological phases that have no static analogs. This work presents a homotopy approach to the study of driven systems, which treats unitary evolutions as paths in the space of unitary operators. By considering loop evolutions in this space, the authors obtain a topological classification, free of uncertainties and ambiguities about the long-time robustness of of behavior in specific physical models. Two classes of Floquet symmetry-protected topological phase are identified, characterized by unitary evolutions that act trivially in the bulk but nontrivially at the boundary of an open system. The first class is captured by an explicit group cohomology construction; it has the remarkable property of converting a trivial boundary state into a nontrivial symmetry-protected topological state. The second class exhibits anomalous counter propagating Hilbert-space transport at the boundary; it lies beyond the cohomology construction.

[Phys. Rev. B 95, 195128] Published Fri May 12, 2017

]]>In laser ultrasonics, ultrashort light pulses generate coherent acoustic pulses of picosecond duration via multiple possible physical mechanisms, involving optoacoustic conversion processes. These wide-band GHz acoustic signals are optically detected at the sample surfaces by ultrafast time-delayed probe light pulses. When the coherent acoustic pulses in GaAS are detected via the Brillouin scattering of probe light pulses of 400 nm wavelength, certain spectral components of the acoustic pulses remain invisible. The theoretical analysis relates this observation to the existence of zeros in the spectral transformation function of acousto-optic conversion. The phenomenon of zero sensitivity of the optical reflectivity to coherent acoustic phonons of particular frequencies is rather general and depends on both the probe light wavelength and the evaluated material. These findings substantially contribute to picosecond ultrasonics and laser-based nondestructive testing.

[Phys. Rev. B 95, 184302] Published Thu May 11, 2017

]]>In the past decade, topological materials have been continuously attracting the interest of the condensed-matter physics community because of their unique band structures and transport properties. Recently, ZrTe${}_{5}$ is becoming a promising platform to study topological phase transitions, as it could possibly be a 3D Dirac semimetal, a 3D weak topological insulator (TI), or a 3D strong TI, which are distinguished by whether there is a finite band gap and whether there is a topological surface state (TSS). This paper performs a systematic high-momentum-resolution photoemission study on ZrTe${}_{5}$ using 6 eV photon energy. The conduction and valence bands near $\mathrm{\Gamma}$ are measured with a gap between 18 and 29 meV. The gap size is smaller than former studies. This work also examines the spectral difference at different photon energies, which is attributed to final-state effects and the 3D nature of the band structure. By doing so, the authors confirm the gap is not due to some specific out-of-plane momentum, and conclude that there is no TSS. The final-state interpretation also reconciles the discrepancies of previous studies regarding the existence of the TSS. Hence, the results are consistent with ZrTe${}_{5}$ being a 3D weak topological insulator.

[Phys. Rev. B 95, 195119] Published Wed May 10, 2017

]]>Here, the authors investigate the possibility of building up nonequilibrium spin accumulation in a ferromagnetic thin film composed of a metallic alloy of nickel and iron, commonly known as permalloy. Analogously to thermoelectric generators, a temperature gradient in a ferromagnet is known to generate a distribution of accumulated electron spins over macroscopic distances, which may be used as an efficient source of spin-polarized electrons in future spintronic devices. To enable the detection and quantitative determination of this imbalance of electron spins, the authors have developed an entirely optical technique, based on the magneto-optical Kerr effect, using a Sagnac interferometer microscope with a spatial resolution of one micrometer and an absolute polarization angle resolution of ten nanoradians. This new technique is not only robust against electrical artifacts, but allows the user to measure previously inaccessible experimental geometries where the magnetization is not restricted to lying in the plane of the film.

[Phys. Rev. B 95, 180401(R)] Published Mon May 08, 2017

]]>Here, the authors argue that the recent experimental discovery of Majorana zero modes in topological superconductors may be the key to realizing an elusive state of matter, the odd-frequency superconductor, in which Cooper pairing is entirely dynamical in nature. Via an exactly soluble model, it is shown that when Majorana zero modes couple to a spin-polarized metallic system, equal-spin $s$-wave odd-frequency pairing is generically induced in the spin-polarized metal. The key properties of odd-frequency superconductivity, such as a vanishing static superconducting order parameter and a paramagnetic Meissner effect, are demonstrated in this system by explicit calculations.

[Phys. Rev. B 95, 184506] Published Mon May 08, 2017

]]>Due to its highly tunable unconventional superconductivity, FeSe is one of the most intriguing materials among the iron-based superconductors. In addition, it exhibits a tetragonal to orthorhombic “nematic” phase transition in the absence of static magnetic order. This article presents a set of tight-binding parameters, optimized directly with respect to experimental angle-resolved photoemission spectroscopy (ARPES) data obtained at 100 K. It thus provides a quantitatively accurate model of the electronic structure of the tetragonal phase of FeSe and, hence, a coherent foundation for the understanding of the unique physical properties of this system. The authors go on to predict a feature that has so far remained unnoticed in simpler DFT-based tight-binding-model analyses, namely, a significant temperature dependence of the chemical potential in FeSe. This is confirmed by performing ARPES on FeSe single crystals, whereby a 25-meV rigid chemical potential shift is detected across the entire Brillouin zone over the temperature range between 100 K and 300 K. The finding has important implications for any future theoretical models of nematicity and superconductivity in FeSe and related materials.

[Phys. Rev. B 95, 195111] Published Mon May 08, 2017

]]>Antiferromagnets are rapidly gaining importance as crucial ingredients in many applications. They are abundant in nature and they are robust against externally applied fields. Rather usefully, their spin resonances often lie within the THz regime, which makes them ideal candidates for optical studies. In this paper, the authors present a thorough experimental and theoretical exploration of the optical spin excitation in antiferromagnetic NiO. Based on a phenomenological theory, they derive expressions for the optically induced magnetization via the inverse Faraday effect and the inverse Cotton-Mouton effect. Light polarization is conserved by pumping and probing along the optical axis of the material, facilitating the comparison between theory and experiment. Those agree amazingly well, making possible the identification of the driving mechanism behind the ultrafast magnon excitations. Moreover, the authors succeed in obtaining information about the otherwise elusive spin-domain distribution in NiO and in showing that the energy transfer into the magnon mode is about three orders of magnitude more efficient via the inverse Cotton-Mouton effect than via the inverse Faraday effect.

[Phys. Rev. B 95, 174407] Published Fri May 05, 2017

]]>Tunneling of an electron from one composite fermion liquid into another in a bilayer system offers a unique spectroscopic probe into the short-distance high-energy physics of this highly nontrivial strongly correlated state. The authors identify the interlayer exciton responsible for the maximum current, and find excellent quantitative agreement with the experimentally measured energy as well as its dependence on an in-plane magnetic field. They also predict that the spin-polarization transitions of the fractional quantum Hall states as a function of the Zeeman energy will be marked by discontinuous jumps in the energy of this exciton.

[Phys. Rev. B 95, 195105] Published Wed May 03, 2017

]]>The topological phase of a one-dimensional triplet superconductor support fractionalized edge states described by Majorana fermions. Generically, these edge states are not completely decoupled. Their wave functions decay exponentially, and the tails induce a coupling that decays exponentially with the length of the system. This is not the whole story, however. In an appropriate parameter range (if the pairing is smaller than the hopping in the simplest model, the Kitaev model), the Majorana wave functions oscillate on top of decaying, and these oscillations induce exact zero modes, at which the Majorana edge states are rigorously decoupled. In the present paper, motivated by the observation of ground-state level crossings in chains of cobalt adatoms, which, up to a Jordan-Wigner transformation, are described by the same model, the authors explicitly calculate the Majorana edge-state wave functions, and they fully characterize the number and positions of the level crossings induced by the wave-function oscillations.

[Phys. Rev. B 95, 174404] Published Tue May 02, 2017

]]>Chiral magnetism plays an increasingly important role in condensed matter investigations driven by the discovery of exotic spin textures, such as the topologically protected chiral skyrmions that can form a lattice under magnetic fields. The nature and universality of the phase diagram in cubic chiral magnets has been the subject of numerous experimental and theoretical investigations, including the transition to the helimagnetic state. In MnSi, the archetypal system in this family, this transition is of first order and involves a precursor phase, where strong chiral fluctuating correlations build up. This work presents an experimental investigation of the structural and dynamical aspects of the phase transition in Fe${}_{1-x}$Co${}_{x}$Si, a system that belongs to the same family as MnSi but with the additional possibility to tune important physical interactions and parameters through variation of the Fe and Co concentration. In this system, the combination of small-angle neutron scattering and neutron spin echo spectroscopy uncovers that the scenario of the transition is qualitatively very different from that in MnSi. This goes beyond what can be expected from a comparison of the relevant length scales and thus challenges the validity of a universal approach to the helimagnetic transition in cubic chiral magnets.

[Phys. Rev. B 95, 144433] Published Fri Apr 28, 2017

]]>The strong spin-spin exchange interaction in some low-dimensional magnetic materials can give rise to a high group velocity and thermal conductivity contribution from magnons, which are energy quanta of collective spin excitations. Examples are the incommensurate layered compounds (Sr,Ca,La)${}_{14}$Cu${}_{24}$O${}_{41}$, with strong antiferromagnetic interaction along the ladders. The effects of grain boundaries and defects on quasi-one-dimensional magnon transport in these compounds are not well understood. Here, the authors report the microstructures and anisotropic thermal transport properties of textured Sr${}_{14}$Cu${}_{24}$O${}_{41}$. TEM clearly reveals nanolayered grains and the presence of dislocations and planar defects. The thermal conductivity contribution and mean free paths of magnons in the textured samples are evaluated with the use of a kinetic model for one-dimensional magnon transport, and found to be suppressed significantly compared to single crystals at low temperatures. The experimental results can be explained by a one-dimensional magnon-defect scattering model, provided that the magnon–grain boundary scattering mean free path in the anisotropic magnetic structure is smaller than the average length of these nanolayers along the $c$ axis. The finding suggests low transmission coefficients for energy-carrying magnons across grain boundaries.

[Phys. Rev. B 95, 144310] Published Thu Apr 27, 2017

]]>Recently, it was suggested that metastable spin supercurrents (spin superfluidity) are possible in the magnon condensate observed in yttrium iron garnet (YIG) magnetic films under strong magnon pumping. In YIG, magnetic anisotropy is rather weak, and this material can be treated as an isotropic ferromagnet. Its order-parameter space is a sphere of radius equal to the spontaneous magnetization $M$. A current state maps on the equatorial circumference on the sphere, and topology of the sphere allows us to continuously transform the equatorial circumference to the point. This rules out metastable current states. Only easy-plane anisotropy reducing the order-parameter space from the sphere to the equatorial circumference makes current states topologically stable. However, by pumping magnons to the isotropic ferromagnet in a magnetic field, it is possible to support a strongly nonequilibrium state with fixed average ${M}_{z}$. This confines the precessing spin to an “easy plane” of dynamical rather than topological origin and makes metastable spin currents possible. But final judgment requires evaluation of the Landau superfluidity criterion as done in the present work. The conclusion is that spin superfluidity in YIG films is possible in principle, although the recently published claim of its observation is not justified.

[Phys. Rev. B 95, 144432] Published Thu Apr 27, 2017

]]>The tenfold way classification of noninteracting topological insulators represents a great success towards understanding topological states of matter. The question the authors try to address is: can all possible topological insulators be represented by the prototypes in the tenfold way classification (perhaps enriched with some extra symmetries), or are there exceptions fundamentally different from those prototypes? The Hopf insulator was considered as a special type of TI that is not obviously represented by any prototype in the tenfold way classification, but its stability and classification were never clarified. In this work, the authors identify a generalized particle-hole symmetry that gives the Hopf insulator and its higher-dimensional analog a rigorous definition and classification. Moreover, they provide a very heuristic understanding of the minimal models for the 3$d$ and 4$d$ Hopf insulators, based on which possible experimental realization of a Hopf insulator can be found.

[Phys. Rev. B 95, 161116(R)] Published Thu Apr 27, 2017

]]>Recently, there has been a surge of interest in effects arising from the interplay between the electron-electron interaction and spin-orbit coupling (SOC). One such effect is a novel type of collective spin excitations (chiral spin waves), which are oscillations of the magnetization that exist even without an external magnetic field. However, a typical experimental setup in a semiconductor heterostructure includes a magnetic field as well as both Rashba and Dresselhaus types of SOC. In this case, the spectrum of chiral spin waves becomes fairly complicated, with some branches running into or splitting off the continuum of spin-flip excitations. The authors show that this complicated physics can be understood by exact mapping of the quantum kinetic equation for a two-dimensional Fermi liquid onto an effective one-dimensional tight-binding model. This mapping is a useful tool that helps us to understand the nature of the collective modes in a Fermi liquid of arbitrary type.

[Phys. Rev. B 95, 165140] Published Wed Apr 26, 2017

]]>Experiments with ultracold fermionic atoms in optical lattices have attracted much interest in condensed matter physics as they serve as a model for electrons in solids. While experiments now allow the observation of nonequilibrium transport processes with single-site resolution, the theoretical description, especially for strongly correlated fermions, remains very challenging. Standard methods, such as the density matrix renormalization group (DMRG) and nonequilibrium Green functions (NEGF), have been primarily applied to one-dimensional systems and weak to moderate coupling, respectively. Here, the authors perform a detailed comparison of DMRG and NEGF and demonstrate that both can be benchmarked against each other. They observe complementary applicability ranges suggesting that a combination of both will allow to substantially expand the range and duration of first-principles quantum dynamics simulations for fermionic lattice systems.

[Phys. Rev. B 95, 165139] Published Tue Apr 25, 2017

]]>This paper reports on the observation of the magnetic quantum ratchet effect in (Cd,Mn)Te- and CdTe-based quantum well structures with an asymmetric lateral dual grating gate superlattice subjected to an external magnetic field ($B$), applied normal to the quantum well plane. An electric current excited by terahertz laser radiation shows 1/$B$-periodic oscillations with amplitude much larger than the photocurrent at zero magnetic field. It is shown that the photocurrent generation is due to the combined action of a spatially periodic lateral potential and the spatially modulated radiation due to the near-field effects of light diffraction. The magnitude and direction of the photocurrent are determined by the degree of the lateral asymmetry controlled by the variation of voltages applied to the individual gates. The observed magneto-oscillations with enhanced photocurrent amplitude result from Landau quantization. For (Cd,Mn)Te-based structures at low temperatures, a beating-like pattern of the oscillations is observed, caused by the interplay of Zeeman and Landau splitting. A theoretical analysis, considering the magnetic quantum ratchet effect in the framework of a semiclassical approach, describes quite well the experimental results.

[Phys. Rev. B 95, 155442] Published Mon Apr 24, 2017

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