The electronic structure of WTe${}_{2}$ and of orthorhombic $\gamma $-MoTe${}_{2}$ are theoretically claimed to contain pairs of Weyl type-II points associated with linearly touching electron and hole pockets. A series of recent angle-resolved photoemission spectroscopy (ARPES) experiments claims to observe general agreement with these predictions. Here, the authors report a de Haas–van Alphen study on $\gamma $-MoTe${}_{2}$ which, in contrast to ARPES and predictions, finds a simpler and more isotropic Fermi surface. Upon the guidance of ARPES, they show here that it is possible to explain the observed Fermi surface geometry by an independent displacement of the theoretical electron and hole bands relative to the Fermi level. However, this $a\phantom{\rule{0}{0ex}}d$ $h\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}c$ procedure, previously used by a number of groups, suppresses the crossings between electron and hole bands and hence the predicted Weyl type-II points.

[Phys. Rev. B 96, 165134] Published Tue Oct 17, 2017

]]>Ferromagnets perturbed by time-dependent fields are technologically important examples of nonequilibrium physics. Using kinetic Monte Carlo simulations and droplet theory of magnetization reversal, the authors explain recent experiments on ordered uniaxial ferromagnetic films driven by oscillating fields. The associated dynamic phase transition at a critical field period is known to be in the equilibrium Ising universality class. Consistent with experiments, the authors find that the response to a constant bias field in the supercritical period limit differs from an equilibrium ferromagnet above criticality in that, instead of a wide central susceptibility maximum, side bands are observed, which are shown to result from stochastic resonance.

[Phys. Rev. B 96, 134306] Published Fri Oct 13, 2017

]]>Periodically driven systems have recently been shown to host topological phases that are inherently dynamical in character, opening up a new arena in which to explore topological physics. One important group of such phases, known as Floquet topological insulators, arise in systems of free fermions and exhibit protected topological edge modes analogous to the edge modes of static topological insulators. In this work, the authors use methods from K theory to provide a complete topological classification of Floquet topological phases of this kind. The main result is a periodic table for Floquet topological insulators, which may be viewed as a time-dependent extension of the periodic table of topological insulators and superconductors originally introduced by Alexei Kitaev.

[Phys. Rev. B 96, 155118] Published Fri Oct 13, 2017

]]>$\alpha $-RuCl${}_{3}$ is the most promising candidate for the long-sought materialization of the Kitaev model, where compasslike exchange interactions lead to a gapless spin liquid with emergent Majorana fermion excitations. Prerequisite for these expectations is that the symmetry-imposed noncubic crystal field is smaller compared to the spin-orbit coupling and does not perturb significantly the description of the electronic ground state in terms of the ${J}_{\text{eff}}$ = 1/2 state. Here, the authors combine polarization-dependent x-ray absorption spectroscopy, a technique that gives direct access to the crystal field of a material, and full multiplet calculations to determine the size and sign of the noncubic crystal-field splitting. The results demonstrate that the “electronically cubic conditions” and the ${J}_{\text{eff}}$ = 1/2 ground state for Ru in $\alpha $-RuCl${}_{3}$ are fulfilled.

[Phys. Rev. B 96, 161107(R)] Published Thu Oct 12, 2017

]]>The authors present a unified perspective on a large class of one-dimensional symmetry-protected topological phases. Characterizing critical points between such phases leads to a conjecture of a topological lower bound on the central charge in terms of the phase-specific edge mode dimension.

[Phys. Rev. B 96, 165124] Published Thu Oct 12, 2017

]]>Multiband $\mathbf{k}\cdot \mathbf{p}$ Hamiltonians near the high symmetric points in the Brillouin zones of two-dimensional (2D) materials are widely used to predict and understand the unique transport and optical properties of 2D materials. Based on the crystal symmetry of 2D materials, the authors develop a multiband $\mathbf{k}\cdot \mathbf{p}$ Hamiltonian for monolayer GaSe and InSe using the theory of invariants. The authors establish the relationship between the crystal lattice symmetry and the Hamiltonian matrix elements, accounting for spin-orbit interaction and strain. The work provides a systematic way to construct the relevant multiband $\mathbf{k}\cdot \mathbf{p}$ Hamiltonian for various monolayer and few-layer materials.

[Phys. Rev. B 96, 155430] Published Wed Oct 11, 2017

]]>Phonon-assisted tunneling plays a crucial role in sub-10-nanometer gate-length devices. Here, the authors present a first-principles special thermal displacement method that bridges the models used independently in molecular electronics and in device simulations. The method is conceptually simple and much more efficient than any existing first-principles transport method, enabling modeling of realistic rectifiers and transistors with account of electron-phonon coupling. The results are in excellent agreement with both experiments and state-of-the-art perturbation theory calculations discussed in the paper, demonstrating that the method is an appealing design tool for next-generation devices and nanomaterials.

[Phys. Rev. B 96, 161404(R)] Published Wed Oct 11, 2017

]]>Monocrystalline SnSe, which is a binary semiconductor, has recently been demonstrated to be a high-performance thermoelectric material with a record-high figure of merit of 2.6 at high temperature (923 K). Using angle-resolved photoemission spectroscopy, the authors probed the full electronic structure of SnSe. They observed three convergent hole bands with small energy differences in their band tops and relatively small in-plane effective masses, in agreement with $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ calculations. These bands are critical for the enhancement of the Seebeck coefficient, while preserving the high electrical conductivity of the material. Thus, they represent a key factor contributing to the high figure of merit in SnSe.

[Phys. Rev. B 96, 165118] Published Tue Oct 10, 2017

]]>Quantum liquid crystals are strongly correlated quantum many-body systems with broken rotational symmetry but with partial or complete translational symmetry. Inspired by the classical theory for hexatic liquid crystals, which deals with the thermal unbinding of dislocations in 2D solids, the present work advances the idea that 3D solids can melt quantum mechanically via unbinding, or rather Bose-Einstein condensation, of dislocation worldsheets, resulting in quantum analogs of nematic, smectic, and columnar liquid crystals. In these phases, the phonons, as represented by two-form dual gauge fields, develop a mass gap reflecting the loss of shear rigidity.

[Phys. Rev. B 96, 165115] Published Mon Oct 09, 2017

]]>The development of nanoscale optical devices requires high-quality nanocavities to mediate the optical feedback. Any fabrication method will generate imperfections that may induce light loss, limiting the device performance. However, in some cases such disorder may enable new functionalities as, for example, in state-of-the art photonic-crystal waveguides where localization originates from the random multiple scattering of light. Understanding the different mechanisms leading to this type of localization is crucial to exploit disorder as a resource as well as to design structures which are more robust against disorder.

[Phys. Rev. B 96, 144201] Published Fri Oct 06, 2017

]]>Three-dimensional Kitaev models – generalizations of the well known exactly solvable Kitaev honeycomb model – exhibit a rich variety of quantum spin liquid phases. Their fractionalized excitations are Majorana fermions that may form topological band structures. These can be enriched by lattice symmetries, as shown in the present work. In particular, the authors propose two new instances of Kitaev spin liquids that exhibit nodal chains and 3D Dirac nodes in the bulk. Such crystalline Kitaev spin liquids are direct generalizations of crystalline topological insulators, albeit in an interacting spin system. A potential material realization in metal-organic frameworks is also discussed.

[Phys. Rev. B 96, 155107] Published Fri Oct 06, 2017

]]>Orthorhombic perovskites provide a perfect playground for investigations of novel magnetic phenomena, such as ferroelectricity, ultrafast spin reorientation, colossal magnetoresistance, and many others. Here, the authors combined magnetization and neutron scattering measurements with crystalline electrical field calculations to characterize the magnetic properties of a DyScO${}_{3}$ single crystal. They establish that Dy moments have a strong Ising-like single-ion anisotropy with a doublet ground state and an enormous value of effective moment. This results in a significant dipolar interaction, which strongly suppresses conventional transverse fluctuations and makes magnetic excitations invisible to neutron spectroscopy.

[Phys. Rev. B 96, 144407] Published Thu Oct 05, 2017

]]>The electronic edge states of MoS${}_{2}$ nanoparticles are important for the catalytic activity of the particles and could also have potential for photocatalysis. Here, the authors explore computationally optical properties of MoS${}_{2}$ monolayer nanoflakes. These nanostructures show a rich size-dependent optical response below the absorption onset of the infinite monolayer. The authors present a detailed analysis of the emerging optical absorption resonances revealing their edge-state origins. In particular, metallic edge states support localized edge plasmons, and overall, resonances are sensitive to changes in the edge-atom configuration.

[Phys. Rev. B 96, 155407] Published Thu Oct 05, 2017

]]>Fracton phases are three dimensional topological phases in which quasiparticle excitations are restricted to move along planes, lines, or not to move at all. In this work, we show how such phases can be understood in terms of parton constructions, and in the process we find a new type-II model with fractal logical operators.

[Phys. Rev. B 96, 165105] Published Thu Oct 05, 2017

]]>The self-learning Monte Carlo (SLMC) method speeds up the Monte Carlo simulation by designing and training an effective model to propose efficient global updates. We implement the continuous-time quantum Monte Carlo algorithm for quantum impurity models in the framework of SLMC. We introduce and train a diagram generating function (DGF) to model the probability distribution of field configurations in the continuous imaginary time at all orders of diagrammatic expansion. By using the DGF to propose global updates, we show that the self-learning continuous-time Monte Carlo method can significantly reduce the computational complexity of the simulation.

[Phys. Rev. B 96, 161102(R)] Published Tue Oct 03, 2017

]]>Magnon-polarons are coherently mixed quasiparticles – half magnon, half phonon – and are generated by magnetoelasticity. Though predicted by Kittel long ago, their effect on spin transport was discovered only recently in yttrium iron garnet (YIG) using optical and local spin Seebeck effect (SSE) experiments. In the latter, magnon-polarons are manifest as resonant peaks in the SSE signal as a function of magnetic field. Here, the authors show that they also show up in the $n\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l$ SSE, with a surprising twist: measured nonlocally, the magnon-polaron peak turns into a dip. The authors show that this crossover is a consequence of the magnon physics underlying the SSE: thermal generation and diffusive backflow of magnons in YIG compete, which can generate any sign for the magnon-polaron anomaly in nonlocal experiments.

[Phys. Rev. B 96, 104441] Published Fri Sep 29, 2017

]]>Randomly interacting Majorana fermions governed by the Sachdev-Ye-Kitaev model have recently been shown to exhibit profound links to quantum chaos and black holes. Here, the authors propose a solid-state implementation of the model that exploits hardware components that are now widely pursued for topological quantum computation – namely, an array of Majorana wires interfaced with a disordered quantum dot that mediates the desired interactions. This work highlights a potential pathway for using table-top condensed-matter systems to probe novel concepts from high-energy physics and quantum information.

[Phys. Rev. B 96, 121119(R)] Published Fri Sep 29, 2017

]]>The ferrimagnetic insulator yttrium iron garnet (YIG) has been widely used in microwave and spintronic devices. Anomalous features in spin Seeback effect voltages have been observed in Pt/YIG and attributed to magnon-phonon coupling. Using inelastic neutron scattering to map out low-energy spin waves and acoustic phonons in YIG as a function of increasing magnetic field, the authors find that the magnon-phonon interaction enhances the hybridized scattering intensity at the magnon-phonon dispersion crossing points in zero field. These results go against the expectations of conventional spin-lattice coupling, calling for a different explanation of the magnon-phonon interactions and the resulting magnon polarons in YIG.

[Phys. Rev. B 96, 100406(R)] Published Wed Sep 27, 2017

]]>In addition to the familiar Maxwell terms, the Lagrangian for a three-dimensional U(1) gauge theory can have an extra “theta” term which has two effects: attaching electric charge to magnetic monopoles in the bulk, and generating a Chern-Simons term on the boundary. This work studies theta terms in the context of higher rank tensor U(1) gauge theories, which host gapless higher-spin gauge modes and subdimensional charge excitations. There are multiple different varieties of theta terms, enforcing different types of charge attachment in the bulk. These terms also lead to generalized tensor Chern-Simons theories on the boundary, bringing about the existence of two-dimensional fracton phases.

[Phys. Rev. B 96, 125151] Published Wed Sep 27, 2017

]]>The half-Heusler superconductors such as YPtBi are multiorbital systems with both strong spin-orbit coupling and broken inversion symmetry. The spin-orbit coupling allows for the formation of Cooper pairs with effective spins larger than unity, which introduces novel pairing states. Some of these states display Fermi surfaces of Bogoliubov quasiparticles. On the other hand, the absence of inversion symmetry leads to topologically protected nodes associated with specific surface features as, for example, flat bands. The authors consider two likely pairing symmetries for this material, one that breaks time-reversal symmetry and one that does not, and find a rich variety of bulk nodes and surface states.

[Phys. Rev. B 96, 094526] Published Tue Sep 26, 2017

]]>In the tunnel magneto-Seebeck effect, a temperature gradient generates an electric voltage over a nonmagnetic tunnel barrier between two magnetic films that depends on the relative orientation of the magnetizations. This effect has been modeled in the past by the single-electron picture as implemented in band structure calculations. Here, the authors depart from this interpretation and investigate the thermoelectric response induced by electron-magnon interactions. The importance of collective magnetic effects found in the present study challenges the conventional single-electron model for the thermopower of magnetic tunneling junctions.

[Phys. Rev. B 96, 094429] Published Mon Sep 25, 2017

]]>Here, the authors discover the unexpected important role of intrinsic atomic donor spin-orbit coupling (SOC) in driving the relaxation of coupled-donor electron states in silicon-28, which is a promising host material for semiconductor qubits, despite the commonly perceived weak strength of atomic SOC in this material. The only existing spin-relaxation mechanism considered in this context before was the relaxation of two-electron states induced by hyperfine interaction with the donor nuclear spins. The new intrinsic SOC mechanism becomes important when the donors are closely spaced as is the case in silicon quantum computing architectures. This mechanism may also shed important new light on the well-studied problem of electron spin resonance in highly doped silicon.

[Phys. Rev. B 96, 115444] Published Mon Sep 25, 2017

]]>The recent observation of excitons in highly excited states has allowed the study of Rydberg physics in cuprous oxide crystals. This opens the unique possibility to elaborate on similarities and differences between Rydberg atoms and Rydberg excitons, as done here experimentally and theoretically for quantities related to the splitting of multiplets with particular principal quantum number $n$ in zero and finite electric or magnetic fields. The authors find characteristic scaling laws with $n$ with some discrepancies between excitons and atoms, but surprisingly, they find mostly similarities, even though the underlying physics, such as the complex valence band structure, are quite different.

[Phys. Rev. B 96, 125142] Published Fri Sep 22, 2017

]]>Thermal phase transitions are successfully described by fluctuations of an appropriate order parameter, which further allows classification of the transition into first or second order. However, some quantum analogs of thermal phase transitions defy this description and identifying the mechanisms behind these unconventional processes will fundamentally improve our understanding of matter. Here, the authors provide a thorough study of such a transition, which has been experimentally accessed in graphene: the emergence of a Kekulé valence bond solid. Additional gapless quantum fluctuations of Dirac electrons change the nature of this transition from first to second order. Using a nonperturbative functional renormalization group approach, the authors advance the state-of-the-art estimates for the corresponding critical exponents and predict measurable corrections to scaling.

[Phys. Rev. B 96, 115132] Published Wed Sep 20, 2017

]]>Sr${}_{2}$IrO${}_{4}$ is considered as a strong spin-orbit coupled equivalent of high-temperature superconducting cuprates. Recent experiments have most notably shown that upon electron doping a pseudogap emerges in Sr${}_{2}$IrO${}_{4}$ and that magnonlike excitations survive the rapid disappearance of long-range magnetic order. However, due to the relatively small crystal sizes and the large Ir neutron cross section, information on charge excitations and magnetism has mostly been gathered using synchrotron-based spectroscopic methods. Here, the authors use Raman scattering on electron-doped Sr${}_{2-x}$La${}_{x}$IrO${}_{4}$ single crystals to carry out a temperature and doping study of the lattice and magnetic excitations, and observe the emergence upon doping of an electron-hole continuum that couples to the phonons. Even in metallic samples, fluctuating magnetic moments and mobile carriers therefore coexist, again in close analogy with the doped cuprates.

[Phys. Rev. B 96, 115138] Published Wed Sep 20, 2017

]]>An elegant way to probe the mysterious “pseudogap” state in cuprates is to suppress superconductivity with strong magnetic fields and work at low temperature. Then, impurity scattering dominates, which simplifies calculations. A number of theories thus successfully explain the sudden change in carrier number observed experimentally. To tell these theories apart, the work here provides new predictions. First, it shows how electron pockets cause strong signatures in the specific heat and Seebeck coefficient. Second, it addresses inconsistencies in scattering rate approximations for the Hall and the Seebeck effects. Third, it shows the influence of the Van Hove singularity on these quantities.

[Phys. Rev. B 96, 125139] Published Wed Sep 20, 2017

]]>Nanoscale magnetic phase separation in dysprosium has been revealed by aberration-corrected Lorentz microscopy. The in-plane magnetization distribution of the coexisting magnetic phases has been visualized successfully by combining high-resolution Lorentz microscopy with the transport-of-intensity equation method. The phase separation of the ferromagnetic and the helical antiferromagnetic (HAFM) phases results in the formation of static magnetic solitons at around 130 K in the absence of external magnetic field. External fields of up to about 11 kOe induce nanoscale phase separation of the HAFM, the distorted HAFM, and the fan phases at 142 K. The developed technique for the high-resolution observation of fine magnetic structures is a valuable addition to the modern toolbox of magnetic materials research.

[Phys. Rev. B 96, 100405(R)] Published Tue Sep 19, 2017

]]>In addition to the celebrated Affleck-Ludwig entropy originating from the open boundaries of the path-integral manifold, recent research has shown that the entropy correction on nonorientable manifolds such as the Klein bottle is also a universal characterization of critical systems with an emergent conformal field theory (CFT). Here, the authors show that the Klein bottle entropy can be interpreted as a boundary effect by transforming the Klein bottle into an orientable manifold, whereby the Klein bottle entropy bears a resemblance to the Affleck-Ludwig entropy. The authors then propose a generic scheme to extract these universal boundary entropies from lattice models. Numerical results for the $q$-state Potts model are compared with the CFT predictions.

[Phys. Rev. B 96, 115136] Published Tue Sep 19, 2017

]]>To address the long-standing question of double phase transitions in the superconducting state of U${}_{1-x}$Th${}_{x}$Be${}_{13}$, the authors study the quasiparticle excitations and magnetic response using a monocrystalline sample. The high-resolution heat-capacity and magnetization data demonstrate that the lower-temperature transition is between two superconducting states with different gap symmetries. Quite surprisingly, the $C(T,H,p\phantom{\rule{0}{0ex}}h)$ data strongly suggest that the superconducting gap is fully open over the Fermi surface in U${}_{0.97}$Th${}_{0.03}$Be${}_{13}$. The present thermodynamic results entirely overturn a widely believed idea that nodal quasiparticle excitations occur in uranium heavy-fermion superconductivity with broken time-reversal-symmetry.

[Phys. Rev. B 96, 100505(R)] Published Mon Sep 18, 2017

]]>Time crystals are proposed states of matter that spontaneously break time translation symmetry. Despite much recent interest, there is no settled definition for such states and existing definitions only tangentially refer to the well-established foundations of spontaneous symmetry breaking. We offer a definition of time crystals, which treats time translation much like any other symmetry and follows the traditional recipe for defining Wigner symmetries and order parameters. Using our definition, we find: (i) systems with time independent Hamiltonians should not exhibit time translation symmetry breaking, and (ii) the many-body localized $\pi $ spin glass/Floquet time crystal can be viewed as breaking both a global internal symmetry and the time translation symmetry, as befits the two aspects of its name.

[Phys. Rev. B 96, 115127] Published Mon Sep 18, 2017

]]>Materials with Rashba spin-orbit coupling have long been known to exhibit interesting transport properties due to their unique spin-momentum locking. However, the topology of the dispersion itself has an even more striking impact on electron-impurity scattering, which is only observable at ultralow energies. The authors investigate the universal properties of 2D electron scattering off of circularly symmetric potentials in this regime by deriving nonperturbative approximations for the low-energy scattering quantities, including the $T$-matrix, $S$-matrix, and scattering cross section. As the energy of the electron approaches the bottom of the lowest spin-split band, a number of unusual features appear: the low-energy $S$ and $T$ matrices become independent of the potential, all partial waves contribute equally, and the total scattering cross section exhibits quantized plateaus.

[Phys. Rev. B 96, 125304] Published Mon Sep 18, 2017

]]>In ordinary superconductors the two collective modes of the complex order parameter are usually strongly resilient to the spectroscopic observation. The amplitude or Higgs mode is weakly coupled to the electromagnetic field and it overlaps with the continuum of single-particle excitations at 2$\mathrm{\Delta}$, which buries its signal. The phase or Goldstone mode is optically inert, and it is boosted to the plasma frequency by Coulomb interactions. In the present work, terahertz experiments and theoretical calculations are combined in order to demonstrate the existence of an alternative route for the optical visibility of the Goldstone mode in a nanostructured granular Al thin film. The Coulomb screening due to the grains self-capacitance and the intrinsic inhomogeneity of the local superfluid stiffness due to shell effects lead to finite-frequency subgap absorption of the phase modes, showing that nanograins arrays are a promising setting to control the Goldstone mode via optical means.

[Phys. Rev. B 96, 094514] Published Fri Sep 15, 2017

]]>Magnetoelectric materials are characterized by a coupling of electric and magnetic degrees of freedom, which can be brought about when magnetic ordering breaks inversion symmetry. This is the case in LiCoPO${}_{4}$, which displays one of the largest magnetoelectric effects among transition metal compounds. Using neutron diffraction in combination with state-of-the-art high-field magnet technology, the authors of this paper study the magnetic structures and phase diagram of LiCoPO${}_{4}$ in fields up to 25.9 T. It is shown that the known magnetoelectric response is tied to commensurate magnetic order, while an incommensurate cycloid phase allows electric polarization along a direction yet to be probed experimentally.

[Phys. Rev. B 96, 104420] Published Fri Sep 15, 2017

]]>Na${}_{2}$IrO${}_{3}$ is a promising material to realize Kitaev physics in nature. Even though it shows long-range magnetic order of antiferromagnetic zigzag type at low temperature, the Kitaev interaction has been predicted to be significant. The origin of zigzag order, however, is currently still debated. Here, the authors suggest that adding charge fluctuations to the Kitaev-Heisenberg (KH) model represents an interesting explanation of zigzag antiferromagnetism. Moreover, their phenomenological three-band Hubbard model puts this material into a larger context as it combines topological insulator physics for weak electron interactions and KH physics for strong ones.

[Phys. Rev. B 96, 121110(R)] Published Thu Sep 14, 2017

]]>The ferromagnetic quantum phase transition (FMQPT) in clean metals has attracted much interest since a first-order transition is commonly observed. The orthorhombic Ising ferromagnet URhGe provides an excellent opportunity to study the FMQPT, because its Curie temperature can be tuned to zero by applying a magnetic field $H$ parallel to the $b$ axis, perpendicular to the spontaneous moment that aligns along the $c$ axis. Here, the authors perform high-precision angle-resolved magnetization measurements on URhGe to investigate the FMQPT with $H$ applied near the $b$ axis. They find a clear first-order transition below a tricritical point (TCP) located above 4 K, and determine the detailed profiles of the wing structure of the first-order transition in the $T$-${H}_{b}$-${H}_{c}$ phase diagram. The obtained wing structure is consistent with the theoretical expectation that three second-order transition lines merge tangentially at TCP.

[Phys. Rev. B 96, 094411] Published Mon Sep 11, 2017

]]>In addition to conventional acoustic properties (mass density and compressibility), recent research has shown that inhomogeneous acoustic media may require coupling tensors between effective volume strain and momentum fields. These coupling tensors imply conversion between monopolar and dipolar motion in the effective material response. Intrinsic coupling between strain and momentum is known as Willis coupling in elastodynamics, and it is analogous to bianisotropy in electromagnetism. This paper investigates the microscale and mesoscale effects that lead to Willis coupling, studies the proper homogenization of acoustic metamaterials with non-negligible Willis coupling, and demonstrates the impact of Willis coupling on defining physically meaningful effective material parameters.

[Phys. Rev. B 96, 104303] Published Mon Sep 11, 2017

]]>In the thermal Hall and Nernst effects, electric and heat currents, respectively, are induced perpendicular to an applied temperature gradient. Usually, such a response requires the application of a magnetic field. In contrast, here the authors establish that in Weyl semimetals – semimetals with a linear conical energy dispersion around a set of points called Weyl nodes – the Nernst effect can be realized without magnetic field, provided that the Weyl cones are tilted. This is the anomalous Nernst effect. It is significantly altered by the tilt of the Weyl cones, with Fermi surface contributions becoming important. Two experimental setups that would allow measuring both responses are discussed.

[Phys. Rev. B 96, 115202] Published Fri Sep 08, 2017

]]>The authors demonstrate room-temperature multiferroic (ferroelectric-ferromagnetic) behavior in lead titanate, in which about 30% of the titanium is substituted by palladium. Palladium is an unusual element with an instability to ferromagnetism under some special conditions. Typically, it has not been easy to incorporate it into perovskite oxides – the most common structure for ferroelectric device materials. Although palladium is expensive, commercial devices like ferroelectric memories often use very thin films, for which the added cost of palladium is negligible. Thus, the present study combines basic new physics of the first multiferroic palladium compounds with potential commercial devices.

[Phys. Rev. B 96, 104104] Published Thu Sep 07, 2017

]]>Symmetry-enriched topological phases (SET) exhibit both long-range entanglement and intricate interplay with global symmetries, such as anyon permutation symmetry or fractionalization of quantum numbers, going beyond the Landau paradigm of symmetry breaking. Here, the authors study two-dimensional SET phases based on the novel concept of symmetric local unitary transformations. Using the fixed-point structure of wave functions, they construct systematically exactly solvable models that can describe generic nonchiral SET phases. The technique applies to on-site as well as spatial symmetries and, furthermore, to anomalous symmetries that can only be realized on the surface of three-dimensional symmetry-protected phases.

[Phys. Rev. B 96, 115107] Published Wed Sep 06, 2017

]]>In high-temperature superconductors, the superconductivity appears on the verge of Mott localization and is mediated by collective electronic excitations. Here, the authors present a general theoretical framework (TRILEX) that combines the ideas of DMFT and spin-fluctuation theory, and is shown to capture both the $d$-wave superconductivity and the Mott insulating phase in the Hubbard model. The emphasis is laid on the local scattering between the electrons and the effective long-range bosonic degrees of freedom. The renormalized electron-boson vertex is obtained from a quantum Monte Carlo solution of a self-consistent single-impurity effective model. Therefore, the calculation is of significantly lighter weight compared to the minimal cluster DMFT required for the same purpose. Additionally, both the level and flavor of the approximation are variable, in a fully controlled manner.

[Phys. Rev. B 96, 104504] Published Tue Sep 05, 2017

]]>In iron-based superconductors, the interplay between nematicity, magnetism, and superconductivity is still not fully understood. Experimentally, the isoelectronic series FeSe${}_{1-x}$S${}_{x}$ provides an ideal playground to tune different electronic ground states and to selectively investigate their mutual interplay. Here, the authors use high-resolution angle-resolved photoemission spectroscopy to compare the electronic structure of different single crystals in the tetragonal phase. They find that in the absence of nematicity and magnetism, the substantial reduction of electronic correlations correlates with the decrease of the superconducting transition temperature from FeSe to FeS.

[Phys. Rev. B 96, 121103(R)] Published Tue Sep 05, 2017

]]>In the search for exciting new physics and the design of next-generation devices, gated two-dimensional heterostructures are becoming omnipresent. As the fabrication and characterization techniques in this field improve, first-principles methods must follow suit. The authors present here an implementation of density functional perturbation theory, tailored to simulate the electronic and vibrational properties of two-dimensional heterostructures in the field-effect configuration. They apply the method to gated graphene and show that while the field effect activates the coupling between electrons and flexural phonons, this coupling is strongly screened by the electrons of doped graphene.

[Phys. Rev. B 96, 075448] Published Thu Aug 31, 2017

]]>The family of transition-metal dichalcogenides provides a rich platform for studying the interplay between the crystal structure and electronic properties in strongly correlated electron systems. Applying external pressure as a tuning parameter, the authors observed a striking correlation between the critical temperature ${T}_{c}$ for superconductivity and the strength of the Se-Se bonds in the high-pressure pyrite phase of PdSe${}_{2}$. The bond length of the Se${}_{2}$ dimer was identified as the main parameter for controlling the superconductivity in the pyrite structure. Strong pressure-induced enhancement of ${T}_{c}$ up to its maximum value of 13.1 K cannot, however, be explained solely by phonon softening, and this implies the relevance of additional factors influencing ${T}_{c}$. Indeed, electronic band calculations reveal the presence of topologically nontrivial states in the pyrite structure.

[Phys. Rev. B 96, 060509(R)] Published Wed Aug 30, 2017

]]>Magnetic tunnel junctions have seen active development for spin-transfer-torque based magnetic random access memory (STT-MRAM), a new technology attractive for low-power applications, especially when embedded in semiconductor logic processors. Such efforts open new frontiers for materials and device physics involving interface magnetic states and materials properties, such as an unexpected dependence of spin-torque switching efficiency on a tunnel junction’s resistance-area product. This paper examines the observation and some possible causes. An experiment-based understanding will improve device performance for wider adaptation, while at the same time deepening the knowledge about magnetic excitations and materials physics.

[Phys. Rev. B 96, 064437] Published Wed Aug 30, 2017

]]>In frustrated magnets, competing interactions give rise to many nearly degenerate low-energy states. For classical Heisenberg antiferromagnets the spin spiral ground states form a degenerate manifold in momentum space that, somewhat unexpectedly, looks like a Fermi surface. Here, the authors demonstrate an exact classical-to-quantum correspondence that allows one to map the spin spiral surfaces of classical spin systems to the Fermi surfaces of free-fermion models.

[Phys. Rev. B 96, 085145] Published Wed Aug 30, 2017

]]>Odd-parity magnetic multipole order, where time-reversal and space-inversion symmetries are spontaneously broken, may open a new paradigm in the fields of multipole physics, multiferroics, and spintronics. BaMn${}_{2}$As${}_{2}$ is a new candidate material. Here, the authors theoretically identify that the seemingly simple antiferromagnetic order in BaMn${}_{2}$As${}_{2}$ is actually magnetic hexadecapole order mixed with magnetic quadrupole order. Interestingly, doping hole carriers realize an itinerant odd-parity magnetic multipole order. Emergent electromagnetic responses, such as magnetoelectric effect, antiferromagnetic Edelstein effect, and electric current-induced nematicity, are clarified. In particular, the current-induced nematicity is a characteristic response of an itinerant magnetic hexadecapole state and indicates a novel phase, the “magnetopiezoelectric metal”.

[Phys. Rev. B 96, 064432] Published Mon Aug 28, 2017

]]>Recently, a reevaluation of particle-hole symmetry in fermionic quantum Hall systems has led to a wealth of new insights about quantum Hall phases and connections between quantum Hall physics and other topological phases as well as high-energy physics. Similar progress may be possible in the bosonic case if such a symmetry exists, unlike the fermionic case such a symmetry is not exact. Here, the authors provide the first evidence for the existence of an emergent bosonic particle-hole symmetry.

[Phys. Rev. B 96, 075148] Published Wed Aug 23, 2017

]]>In one-dimensional electron systems, cooperative effects are expected to cause very specific electronic instabilities. Atomic nanowires, formed via self-organized growth on semiconductor surfaces, represent viable physical realizations for the study of such fascinating one-dimensional quantum states. The system in the focus of this work, Si(553)-Au, is created by Au adsorption on a stepped silicon substrate. It features two different chain types, interspersed with each other: Au chains on the terraces, and Si step edges that are subject to the formation of spin chains. By combining high-resolution scanning tunneling microscopy and local tunneling spectroscopy, the authors reveal the complex interplay between the two distinct wire architectures. The interaction is effectively “one-way” in that the Si step edges respond to the Au chains, but not vice versa. As a consequence, the symmetry of the system is lowered as the parity of the Si chains is broken. This fundamental effect creates two different configurations of chains with opposite directionality.

[Phys. Rev. B 96, 081406(R)] Published Tue Aug 22, 2017

]]>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

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