The 100 most-cited articles in Physical review. B./Physical review. B, 2019

The atomic cluster expansion is developed as a complete descriptor of the local atomic environment, including multicomponent materials, and its relation to a number of other descriptors and potentials is discussed. The effort for evaluating the atomic cluster expansion is shown to scale linearly with the number of neighbors, irrespective of the order of the expansion. Application to small Cu clusters demonstrates smooth convergence of the atomic cluster expansion to meV accuracy. By introducing nonlinear functions of the atomic cluster expansion an interatomic potential is obtained that is comparable in accuracy to state-of-the-art machine learning potentials. Because of the efficient convergence of the atomic cluster expansion relevant subspaces can be sampled uniformly and exhaustively. This is demonstrated by testing against a large database of density functional theory calculations for copper.

Non-Hermitian systems can exhibit a counterintuitive phenomenon where a single local boundary or disorder modifies the entire spectrum, no matter how large the system is. In such cases, all bulk modes become localized ``skin'' modes, and usual bulk topological invariants no longer correctly predict topological boundary modes. Generalizing Laughlin's gauge argument to complex fluxes, the authors derive a geometrical approach for the exact determination of the skin-mode spectrum. They also devise a new topological criterion for non-Hermitian particle-hole symmetric Hamiltonians based on complex analysis.

Building upon the concepts of band representations and their relation to localized Wannier functions, the authors systematically study charge fractionalization in higher-order topological insulators. They show that, when protected by ${C}_{n}$ rotation symmetries, charge can be fractionally quantized in units of $e/n$. The authors build topological indices for ${C}_{n}$-symmetric higher-order topological insulators in class AI. Due to an algebraic structure of the topological classification of these phases, these indices apply even to fragile phases, which lack a Wannier description, but for which a ``generalized'' Wannier description can be formulated.

An on-the-fly force field generation method is developed and applied to liquid-solid phase transitions. The method allows the machine to automatically self-learn interatomic potentials during molecular dynamics simulations and to generate force fields with the distinctive chemical precision of first-principles methods. Applications show that more than 99% of the expensive first-principles calculations are bypassed, and molecular dynamics simulations are accelerated by more than two orders of magnitude already during learning, with many more orders during production runs.

Quantum information tends to spread out in interacting systems, leading to thermalization as characterized by the volume-law entropy of entanglement, while local measurements that extract classical information of the state tend to interrupt the propagation of information and reduce the entanglement entropy. Can entanglement sustain repeated measurements? The authors address this question in a toy hybrid-circuit model, where qubits undergo both unitary time evolution and sporadic local measurements. The phase diagram features a stable volume-law phase of entanglement for weak measurements and an area-law phase for strong measurements. The two domains in the diagram are separated by a single second-order phase transition. Characterization of both phases as well as of the critical point are presented.

This paper develops a comprehensive theory of symmetry fractionalization together with the properties of symmetry defects in topologically ordered phases of matter in two spatial dimensions. To do this, the authors also introduce the full set of data, consistency conditions, and equivalences for a mathematical theory, known as a G-crossed braided tensor category, that characterizes the algebraic braiding and fusion properties of symmetry defects. This theoretical framework can completely characterize and classify symmetry-enriched topological phases of matter in the presence of arbitrarily strong interactions in quantum many-body systems in two spatial dimensions.

Starting from a state of low quantum entanglement, local unitary time evolution increases the entanglement of a quantum many-body system. In contrast, local projective measurements disentangle degrees of freedom and decrease entanglement. We study the interplay of these competing tendencies by considering time evolution combining both unitary and projective dynamics. We begin by constructing a toy model of Bell pair dynamics which demonstrates that measurements can keep a system in a state of low, i.e., area-law, entanglement, in contrast with the volume-law entanglement produced by generic pure unitary time evolution. While the simplest Bell pair model has area-law entanglement for any measurement rate, as seen in certain noninteracting systems, we show that more generic models of entanglement can feature an area-to-volume law transition at a critical value of the measurement rate, in agreement with recent numerical investigations. As a concrete example of these ideas, we analytically investigate Clifford evolution in qubit systems which can exhibit an entanglement transition. We…

The competition between the kinetic energy of electrons in a crystal and the inter-electron Coulomb repulsion underlies many fascinating phenomena in quantum materials. Superlattices formed from moir\'e patterns in graphene structures have emerged as tunable platforms for such physics. This paper shows that in many graphene moir\'e structures not only are the electronic bands narrow enough for Coulomb interaction to be important, but they are also topologically nontrivial. This setting of a strong correlated partially filled topological band promises to lead to many novel phenomena.

Classifications of symmetry-protected topological phases provide a framework to understand systematically the physical properties and potential applications of topological systems. Here, the authors derive a comprehensive 38-fold topological classification of non-Hermitian systems with generic non-Hermitian symmetry classes. Two independent generalizations of Kramers' degeneracy to the non-Hermitian setting are presented. The nature of the non-Hermitian topological invariants obtained in this classification is explained through worked-out examples, thus providing a framework for the experimental design and engineering of non-Hermitian symmetry-protected topological systems.

Interlayer magnetic interactions between two-dimension van der Waals layers are often thought to be weak or even negligible and their mechanism is largely unexplored. Here, the authors propose a mechanism for these interactions in a CrI${}_{3}$ bilayer, based on the concept of covalent-like quasibonding, which offers significant alteration of interlayer wave-function hybridization but small energetic difference between different stacking configurations. While a near-collinear coupling between two parallel spin-polarized I 5$p$ orbitals prefers interlayer ferromagnetism, a noncollinear one favors antiferromagnetism, which is tunable by surmounting a tiny energy barrier between two stacking orders. This explains experimental observations and suggests a strategy for designing interlayer magnetism.

We propose a methodology for crystal structure prediction that is based on the evolutionary algorithm USPEX and the machine-learning interatomic potentials actively learning on-the-fly. Our methodology allows for an automated construction of an interatomic interaction model from scratch, replacing the expensive density functional theory (DFT) and giving a speedup of several orders of magnitude. Predicted low-energy structures are then tested on DFT, ensuring that our machine-learning model does not introduce any prediction error. We tested our methodology on prediction of crystal structures of carbon, high-pressure phases of sodium, and boron allotropes, including those that have more than 100 atoms in the primitive cell. All the the main allotropes have been reproduced, and a hitherto unknown 54-atom structure of boron has been predicted with very modest computational effort.

Correlated insulators and superconductivity have been observed in "magic-angle" twisted bilayer graphene, when the nearly flat bands close to neutrality are partially filled. While a momentum-space continuum model accurately describes these flat bands, interaction effects are more conveniently incorporated in tight-binding models. We have previously shown that no fully symmetric tight-binding model can be minimal, in the sense of capturing just the flat bands, so extended models are unavoidable. Here, we introduce a family of tight-binding models that capture the flat bands while simultaneously retaining all symmetries. In particular, we construct three concrete models with five, six, or ten bands per valley and per spin. These models are also faithful, in that the additional degrees of freedom represent energy bands further away from neutrality, and they serve as optimal starting points for a controlled study of interaction effects. Furthermore, our construction demonstrates the "fragile topology" of the nearly flat bands; i.e., the obstruction to constructing exponentially…

Asymmetric coupling amplitudes effectively create an imaginary gauge field, which induces a non-Hermitian Aharonov-Bohm (AB) effect. Nonzero imaginary magnetic flux invalidates the bulk-boundary correspondence and leads to a topological phase transition. However, the way of non-Hermiticity appearance may alter the system topology. By alternatively introducing the non-Hermiticity under symmetry to prevent nonzero imaginary magnetic flux, the bulk-boundary correspondence recovers and every bulk state becomes extended; the bulk topology of the Bloch Hamiltonian predicts the (non)existence of edge states and topological phase transition. These are elucidated in a non-Hermitian Su-Schrieffer-Heeger model, where chiral inversion symmetry ensures the vanishing of imaginary magnetic flux. The average value of Pauli matrices under the eigenstate of chiral-inversion-symmetric Bloch Hamiltonian defines a vector field; the vorticity of topological defects in the vector field is a topological invariant. Our findings are applicable in other non-Hermitian systems. We first uncover the roles played…

Non-Hermiticity from nonreciprocal hoppings has been shown recently to demonstrate the non-Hermitian skin effect (NHSE) under open boundary conditions (OBCs). Here we study the interplay of this effect and the Anderson localization (AL) in a nonreciprocal quasiperiodic lattice, dubbed nonreciprocal Aubry-Andr\'e model, and a rescaled transition point is exactly proved. The nonreciprocity can induce not only NHSEs but also the asymmetry in localized states, characterized by two Lyapunov exponents. Meanwhile, this transition is also topological, in the sense of a winding number associated with complex eigenenergies under periodic boundary conditions (PBCs), establishing a bulk-bulk correspondence. This interplay can be realized straightforwardly by an electrical circuit with only linear passive RLC components instead of elusive nonreciprocal ones, showing the transport of a continuous wave undergoes a transition between insulating and amplifying. This paradigmatic scheme can be immediately accessed in experiments even for more nonreciprocal models and will definitely inspire the study…

We present a computational study of antiferromagnetic transition in ${\mathrm{RuO}}_{2}$. The rutile structure with the magnetic sublattices coupled with $\ensuremath{\pi}/2$ rotation leads to a spin-polarized band structure in the antiferromagnetic state, which gives rise to a $d$-wave modulation of the Fermi surface in the spin-triplet channel. We argue a finite spin conductivity that changes sign in the $ab$ plane is expected in ${\mathrm{RuO}}_{2}$ because of this band structure. We analyze the origin of the antiferromagnetic instability and link it to presence of a nodal line close to the Fermi level.

Metasurfaces based on resonant subwavelength photonic structures enable novel methods of wavefront control and light focusing, underpinning a new generation of flat-optics devices. Recently emerged all-dielectric metasurfaces exhibit high-quality resonances underpinned by the physics of bound states in the continuum that drives many interesting concepts in photonics. Here we suggest an approach to explain the physics of bound photonic states embedded in the radiation continuum. We study dielectric metasurfaces composed of planar periodic arrays of Mie-resonant nanoparticles (``meta-atoms'') which support both symmetry protected and accidental bound states in the continuum, and employ the multipole decomposition approach to reveal the physical mechanism of the formation of such nonradiating states in terms of multipolar modes generated by isolated meta-atoms. Based on the symmetry of the vector spherical harmonics, we identify the conditions for the existence of bound states in the continuum originating from the symmetries of both the lattice and the unit cell. Using this formalism…

We show that weak measurements can induce a quantum phase transition of interacting many-body systems from an ergodic thermal phase with a large entropy to a nonergodic localized phase with a small entropy, but only if the measurement strength exceeds a critical value. We demonstrate this effect for a one-dimensional quantum circuit evolving under random unitary transformations and generic positive operator-valued measurements of variable strength. As opposed to projective measurements describing a restricted class of open systems, the measuring device is modeled as a continuous Gaussian probe, capturing a large class of environments. By employing data collapse and studying the enhanced fluctuations at the transition, we obtain a consistent phase boundary in the space of the measurement strength and the measurement probability, clearly demonstrating a critical value of the measurement strength below which the system is always ergodic, irrespective of the measurement probability. These findings provide guidance for quantum engineering of many-body systems by controlling their…

Random unitary circuits are powerful minimal models for chaotic nonequilibrium evolution. Here, the authors establish an exact mapping between dynamical observables in random circuits and a statistical mechanical model of interacting spins, and use it to study systematically the entanglement generated by a random circuit consisting of local gates. A domain wall line tension in this spin model sets the entanglement growth rate. By a careful analysis of the interaction between domain walls, the authors derive explicitly the Kardar-Parisi-Zhang equation that governs the evolution of second Renyi entropy and identify a phase transition of the growth-rate function.

The standard equilibrium Green--Kubo and nonequilibrium molecular dynamics (MD) methods for computing thermal transport coefficients in solids typically require relatively long simulation times and large system sizes. To this end, we revisit here the homogeneous nonequilibrium MD method by Evans [Phys. Lett. A 91, 457 (1982)] and generalize it to many-body potentials that are required for more realistic materials modeling. We also propose a method for obtaining spectral conductivity and phonon mean-free path from the simulation data. This spectral decomposition method does not require lattice dynamics calculations and can find important applications in spatially complex structures. We benchmark the method by calculating thermal conductivities of three-dimensional silicon, two-dimensional graphene, and a quasi-one-dimensional carbon nanotube and show that the method is about one to two orders of magnitude more efficient than the Green--Kubo method. We apply the spectral decomposition method to examine the long-standing dispute over thermal conductivity convergence vs divergence in…

The energy charging of a quantum battery is analyzed in an open quantum setting, where the interaction between the battery element and the external power source is mediated by an ancilla system (the quantum charger) that acts as a controllable switch. Different implementations are analyzed putting emphasis on the interplay between coherent energy pumping mechanisms and thermalization.

Emergence of exceptional points in two dimensions is one of the remarkable phenomena in non-Hermitian systems. We here elucidate the impacts of symmetry on the non-Hermitian physics. Specifically, we analyze chiral symmetric correlated systems in equilibrium where the non-Hermitian phenomena are induced by the finite lifetime of quasiparticles. Intriguingly, our analysis reveals that the combination of symmetry and non-Hermiticity results in topological degeneracies of energy bands which we call symmetry-protected exceptional rings (SPERs). We observe the emergence of SPERs by analyzing a non-Hermitian Dirac Hamiltonian. Furthermore, by employing the dynamical mean-field theory, we demonstrate the emergence of SPERs in a correlated honeycomb lattice model whose single-particle spectrum is described by a non-Hermitian Dirac Hamiltonian. We uncover that the SPERs survive even beyond the non-Hermitian Dirac Hamiltonian, which is related to a zeroth Chern number. The argument of symmetry protection also holds for three dimensions, elucidating the presence of a symmetry-protected…

The recent discovery of correlated insulating states and superconductivity in twisted bilayer graphene has opened the door to a new approach to designing correlated materials by controlling the twist angles. In this study, the authors propose a large class of graphene-based moir\'e heterostructures which similarly exhibit flat bands at a sequence of magic angles whose pattern is analytically related to the bilayer magic angles. The proposed setup may be experimentally more stable compared to the bilayer case and features the coexistence of a flat band with Dirac dispersing bands, potentially leading to qualitatively different types of flat-band induced correlated phases.

Recently, there has been rapid progress in classifying topological band structures by examining possible configurations, arising through consideration of the constraints of the underlying lattice symmetry. Apart from topologically stable and trivial bands, these pursuits have identified fragile topologies that can be trivialized by additional bands. The authors present a general and directly implementable tool to diagnose the full topological characterization of bands using Wilson flows. As a result, they find a general relation and unambiguous representation of these novel topological concepts.

We propose that the electronic structure of twisted bilayer graphene (TBG) can be understood as Dirac fermions coupled with opposite pseudomagnetic fields generated by the moir\'e pattern. The two low-energy flat bands from each monolayer valley originate from the two zeroth pseudo Landau levels of Dirac fermions under such opposite effective magnetic fields, which have opposite sublattice polarizations and carry opposite Chern numbers $\ifmmode\pm\else\textpm\fi{}1$, giving rise to helical edge states in the gaps below and above the low-energy bulk bands near the first magic angle. We argue that small Coulomb interactions would split the eightfold degeneracy (including valley and physical spin) of these zeroth pseudo Landau levels, and may lead to insulating phases with nonvanishing Chern numbers at integer fillings. Besides, we show that all the high-energy bands below or above the flat bands are also topologically nontrivial in the sense that for each valley the sum of their Berry phases is quantized as $\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}$. Such quantized Berry phases…

Separating electron and phonon components in thermal conductivity is imperative for understanding thermal transport in metals and highly desirable in many applications. In this work, we predict the mode-dependent electron and phonon thermal conductivities of 18 different metals at room temperature from first principles. Our first-principles predictions, in general, agree well with those available experimental data. For phonon thermal conductivity, we find that it is in the range of 2--18 W/mK, which accounts for 1%--40% of the total thermal conductivity. It is also found that the phonon thermal conductivities in transition metals and transition-intermetallic compounds (TICs) are non-negligible compared to noble metals due to the high phonon group velocities of the former. We further show that the electron-phonon coupling effect on phonon thermal conductivity in transition metals and intermetallic compounds is stronger than that of nobles, which is attributed to the larger electron-phonon coupling constant with a high electron density of states within the Fermi window and high phonon…

Non-Hermitian Hamiltonians, which describe a wide range of dissipative systems, and higher-order topological phases, which exhibit novel boundary states on corners and hinges, comprise two areas of intense current research. Here we investigate systems where these frontiers merge and formulate a generalized biorthogonal bulk-boundary correspondence, which dictates the appearance of boundary modes at parameter values that are, in general, radically different from those that mark phase transitions in periodic systems. By analyzing the interplay between corner/hinge, edge/surface, and bulk degrees of freedom we establish that the non-Hermitian extensions of higher-order topological phases exhibit an even richer phenomenology than their Hermitian counterparts and that this can be understood in a unifying way within our biorthogonal framework. Saliently this works in the presence of the non-Hermitian skin effect, and also naturally encompasses genuinely non-Hermitian phenomena in the absence thereof.

A recent experiment on a 51-atom Rydberg-blockaded chain observed anomalously long-lived temporal oscillations of local observables after quenching from an antiferromagnetic initial state. This coherence is surprising as the initial state should have thermalized rapidly to infinite temperature. In this Rapid Communication, we show that the experimental Hamiltonian exhibits nonthermal behavior across its entire many-body spectrum, with similar finite-size scaling properties as models proximate to integrable points. Moreover, we construct an explicit small local deformation of the Hamiltonian which enhances both the signatures of integrability and the coherent oscillations observed after the quench. Our results suggest that a parent proximate integrable point controls the early-to-intermediate time dynamics of the experimental system. The distinctive quench dynamics in the parent model could signal an unconventional class of integrable system.

Magic angle twisted bilayer graphene (MATBG) is an exciting new system for flat electronic bands and correlated physics. However, heterostrains are ubiquitous in the TBG samples, and their role on the electronic properties has not been clear. Here, the authors show that heterostrains generically broaden the flat bands in MATBG. This explains many puzzling results from transport and scanning tunneling spectroscopy experiments. Furthermore, one can utilize the heterostrain to engineer highly flexible moir\'e flat bands in bilayer transition metal dichalcogenides, suggesting another platform to be exploited.

Recently, with advances in acoustic metamaterial science, the possibility of sound attenuation using subwavelength structures, while maintaining permeability to air, has been demonstrated. However, the ongoing challenge addressed herein is the fact that among such air-permeable structures to date, the open area represents only small fraction of the overall area of the material. In the presented paper in order to address this challenge, we first demonstrate that a transversely placed bilayer medium with large degrees of contrast in the layers' acoustic properties exhibits an asymmetric transmission, similar to the Fano-like interference phenomenon. Next, we utilize this design methodology and propose a deep-subwavelength acoustic metasurface unit cell comprising nearly 60% open area for air passage, while serving as a high-performance selective sound silencer. Finally, the proposed unit-cell performance is validated experimentally, demonstrating a reduction in the transmitted acoustic energy of up to 94%. This ultra-open metamaterial design, leveraging a Fano-like interference,…

Symmetry plays a paramount role for the classification of phases of matter occurring in nature. Combining symmetry analysis with concepts from topology that allow us to identify global and robust physical properties, new phases of quantum matter, such as topological insulators and semimetals, have been discovered in recent years. Here, the authors analyze in general and exemplify the importance of symmetries to the dissipative counterparts of topological semimetals in non-Hermitian systems, thus introducing the notion of symmetry-protected non-Hermitian nodal phases.

The use of artificial neural networks to represent quantum wave functions has recently attracted interest as a way to solve complex many-body problems. The potential of these variational parametrizations has been supported by analytical and numerical evidence in controlled benchmarks. While approaching the end of the early research phase in this field, it becomes increasingly important to show how neural-network states perform for models and physical problems that constitute a clear open challenge for other many-body computational methods. In this paper, we start addressing this aspect, concentrating on a presently unsolved model describing two-dimensional frustrated magnets. Using a fully convolutional neural network model as a variational ans\"atz, we study the frustrated spin-1/$2{J}_{1}\text{\ensuremath{-}}{J}_{2}$ Heisenberg model on the square lattice. We demonstrate that the resulting predictions for both ground-state energies and properties are competitive with, and often improve upon, existing state-of-the-art methods. In a relatively small region in the parameter space,…

Nonlinear optical responses are a crucial probe of physical systems including periodic solids. In the absence of electron-electron interactions, they are calculable with standard perturbation theory starting from the band structure of Bloch electrons, but the resulting formulas are often large and unwieldy, involving many energy denominators from intermediate states. This work gives a Feynman diagram approach to calculating nonlinear responses. This diagrammatic method is a systematic way to perform perturbation theory, which often offers shorter derivations and also provides a natural interpretation of nonlinear responses in terms of physical processes. Applying this method to second-order responses concisely reproduces formulas for the second-order-harmonic shift current. We then apply this method to third-order responses and derive formulas for third-order-harmonic generation and self-focusing of light, which can be directly applied to tight-binding models. Third-order responses in the semiclasscial regime include a Berry curvature quadrupole term, whose importance is discussed…

Non-Hermiticity makes possible novel topological phenomena, which are notallowed in Hermitian systems. The authors show that topologically nontrivial band degeneracies can appear as exceptional surfaces in non-Hermitian systems with parity--time and parity--particle-hole symmetries. It is shown that when parity--time or parity--particle-hole symmetry is present, $d$-dimensional non-Hermitian systems can have $(d\ensuremath{-}1)$-dimensional exceptional surfaces. This work suggests new topological phases with nodal band structures protected by non-Hermitian topology and symmetry.

Motivated by the discovery of the nickelate superconductor Nd${}_{0.8}$Sr${}_{0.2}$NiO${}_{2}$, the authors construct an effective low-energy Hamiltonian for the parent compound NdNiO${}_{2}$, which is found to be strongly correlated. While high-T${}_{c}$ superconductivity emerges near the Mott-insulating phase in the cuprates, NdNiO${}_{2}$ is not a Mott insulator, because the 3$d$ ${x}^{2}$-${y}^{2}$ band of nickel is self-doped. The authors propose a route for the design of novel nickelates where self-doping is suppressed. If synthesized, such nickelates would provide a playground for the exploration of high-T${}_{c}$ superconductivity.

An effective semianalytical method for analyzing the Cartesian multipole contributions in light transmission and reflection spectra of flat metasurfaces composed of identical nanoparticles is developed and demonstrated. The method combines numerical calculation of metasurface reflection and transmission coefficients with their multipole decompositions. The developed method is applied for the multipole analysis of reflection and transmission spectra of metasurfaces composed of silicon nanocubes or nanocones. In the case of nanocubes, we numerically demonstrate a ``lattice invisibility effect,'' when light goes through the metasurface almost without amplitude and phase perturbations with the simultaneous excitation of nanoparticles' multipole moments. The effect is realized due to destructive interference between the fields generated by the basic multipole moments of nanoparticles in the backward and forward directions. For metasurfaces composed of conical nanoparticles, we show that their transmission coefficient does not depend on illumination direction. In contrast, the reflection…

Universal features in patterns of entanglement of quantum states provide valuable information-theoretical insights into complex quantum many-body phases and dynamics. This work uncovers a novel class of phase transition between weakly- and strongly- entangled quantum states, described holographically at the boundary of a random tensor network. The univeral scaling structure of these transitions is explored via an exact mapping onto a classical spin model, which also opens the door to efficient numerical methods to study these complex quantum phenomena.

The surface states of a $d$-dimensional conventional or first-order topological state of matter reside on a ($d$-1) dimensional boundary. However, its $n$th order incarnation, commonly known as a higher-order topological (HOT) phase, accommodates ($d$-$n$) dimensional boundary states, with corner and hinge modes standing as their representatives. Here, the authors introduce a general principle of systematically constructing the hierarchy of HOT phases by exploiting the symmetry of the system and the corresponding Clifford algebra. This procedure is applicable for insulating as well as gapless phases, and paves the route toward realizing a fourth-order nodal-loop semimetal, devoid of any surface bound state.

By performing density functional theory based first-principles calculations, the electronic, vibrational, elastic, and piezoelectric properties of two dynamically stable crystal phases of monolayer Janus MoSTe, namely $1H$-MoSTe and $1{T}^{\ensuremath{'}}$-MoSTe, are investigated. Vibrational frequency analysis reveals that the other possible crystal structure, $1T$-MoSTe, of this Janus monolayer does not exhibit dynamical stability. The $1H$-MoSTe phase is found to be an indirect band-gap semiconductor while $1{T}^{\ensuremath{'}}$-MoSTe is predicted as small-gap semiconductor. Notably, in contrast to the direct band-gap nature of monolayers $1H\ensuremath{-}{\mathrm{MoS}}_{2}$ and $1H\ensuremath{-}{\mathrm{MoTe}}_{2}, 1H$-MoSTe is found to be an indirect gap semiconductor driven by the induced surface strains on each side of the structure. The calculated Raman spectrum of each structure shows unique character enabling us to clearly distinguish the stable crystal phases via Raman measurements. The systematic piezoelectric stress and strain coefficient analysis reveals that…

We study the effect of electron-acoustic phonon interactions in twisted bilayer graphene on resistivity in the high-temperature transport and superconductivity in the low-temperature phase diagram. We theoretically show that twisted bilayer graphene should have an enhanced and strongly twist-angle dependent linear-in-temperature resistivity in the metallic regime with the resistivity magnitude increasing as the twist angle approaches the magic angle. The slope of the resistivity versus temperature could approach one hundred ohm per kelvin with a strong angle dependence, but with a rather weak dependence on the carrier density. This higher-temperature density-independent linear-in-$T$ resistivity crosses over to a ${T}^{4}$ dependence at a low density-dependent characteristic temperature, becoming unimportant at low temperatures. This angle-tuned resistivity enhancement arises from the huge increase in the effective electron-acoustic phonon coupling in the system due to the suppression of graphene Fermi velocity induced by the flat-band condition in the moir\'e superlattice system.…

Higher-order topological insulators belong to a recently discovered class of materials where nontrivial topology results in boundary states with a dimensionality two or more below that of the bulk. This work realizes a higher-order topological insulator in a circuit model. By using nonlinear elements, the authors demonstrate dynamical switching between topological and trivial phases as well as controllable interactions between topologically protected states. The topological nature of the model is exhibited by direct extraction of the invariants from the measured boundary Green's functions.

Vertical stacking of ultrathin two-dimensional materials via weak van der Waals (vdW) interactions is identified as an important technique for tuning the physical properties and designing viable products for nanoelectronics, spintronics, and renewable energy source applications. The geometry, electronic, and photocatalytic properties of vdW heterostructures of GeC and Janus transition metal dichalcogenides $M\mathrm{SSe}$ ($M$ = Mo, W) monolayers are investigated by performing first-principles calculations. Two different possible models of GeC-$M\mathrm{SSe}$ heterostructures are presented with an alternative order of chalcogen atoms at opposite surfaces in $M\mathrm{SSe}$. The most favorable stacking pattern of both models is dynamically and energetically feasible. A direct type-II band alignment is obtained in both models of understudy heterobilayer systems. The spin orbit coupling (SOC) effect causes considerable Rashba spin splitting in both $M\mathrm{SSe}$ monolayers. In particular, a greater Rashba spin polarization is demonstrated in model 1 (GeC-WSSe) than model 2…

The implementation of a single magnon level quantum manipulation is one of the fundamental targets in quantum magnetism with a significant practical relevance for precision metrology, quantum information processing, and quantum simulation. Here we demonstrate theoretically the feasibility of using a hybrid ferromagnet-superconductor quantum system to prepare a single magnon source based on magnon blockade effects. By numerically solving the quantum master equation, we show that the second-order correlation function of the magnon mode depends crucially on the relation between the qubit-magnon coupling strength and the driving detuning, and simultaneously signatures of the magnon blockade appear only under quite stringent conditions of a cryogenic temperature. In addition to providing perception into the quantum phenomena of magnon, the study of magnon blockade effects will help to develop novel technologies for exploring the undiscovered magnon traits at the quantum level and may find applications in designing single magnon emitters.

Non-Hermitian Hamiltonians---proposed to describe dissipative systems where the particles have a finite lifetime---exhibit many puzzling properties, strongly contrasting their Hermitian counterparts. In particular, their spectra exhibit extreme sensitivity to boundary conditions and a piling up of all states at the boundary, signalling a breakdown of the conventional bulk-boundary correspondence. The authors study these systems using transfer matrices, whereby they can explain as well as predict the aforementioned features without resorting to numerical computations. This analytical approach allows for a deeper understanding of the topological properties of non-Hermitian systems.

Quantum batteries are quantum-mechanical systems with many degrees of freedom which can be used to store energy and that display fast charging. The physics behind fast charging is still unclear. Is it just due to the collective behavior of the underlying interacting many-body system, or does it have its roots in the quantum-mechanical nature of the system itself? In this work we address these questions by studying three examples of quantum-mechanical many-body batteries with rigorous classical analogs. We find that the answer is model dependent and, even within the same model, depends on the value of the coupling constant that controls the interaction between the charger and the battery itself.

Flat electronic bands can accommodate a plethora of interaction-driven quantum phases, since kinetic energy is quenched therein and electronic interactions therefore prevail. Twisted bilayer graphene, near the so-called ``magic angles'', features slow Dirac fermions close to the charge-neutrality point that persist up to high energies. Starting from a continuum model of slow but strongly interacting Dirac fermions, we show that with increasing chemical doping away from the charge-neutrality point, a time-reversal symmetry breaking, valley pseudospin-triplet, topological $p+ip$ superconductor gradually sets in, when the system resides at the brink of an antiferromagnetic ordering (due to Hubbard repulsion), in qualitative agreement with recent experimental findings. The $p+ip$ paired state exhibits quantized spin and thermal Hall conductivities, and polar Kerr and Faraday rotations. Our conclusions should also be applicable for other correlated two-dimensional Dirac materials.

A magnetic bimeron is a pair of two merons and can be understood as the in-plane magnetized version of a skyrmion. Here we theoretically predict the existence of single magnetic bimerons as well as bimeron crystals, and compare the emergent electrodynamics of bimerons with their skyrmion analogues. We show that bimeron crystals can be stabilized in frustrated magnets and analyze what crystal structure can stabilize bimerons or bimeron crystals via the Dzyaloshinskii-Moriya interaction. We point out that bimeron crystals, in contrast to skyrmion crystals, allow for the detection of a pure topological Hall effect. By means of micromagnetic simulations, we show that bimerons can be used as bits of information in in-plane magnetized racetrack devices, where they allow for current-driven motion for torque orientations that leave skyrmions in out-of-plane magnets stationary.

We show that flat bands can be categorized into two distinct classes, that is, singular and nonsingular flat bands, by exploiting the singular behavior of their Bloch wave functions in momentum space. In the case of a singular flat band, its Bloch wave function possesses immovable discontinuities generated by the band-crossing with other bands, and thus the vector bundle associated with the flat band cannot be defined. This singularity precludes the compact localized states from forming a complete set spanning the flat band. Once the degeneracy at the band crossing point is lifted, the singular flat band becomes dispersive and can acquire a finite Chern number in general, suggesting a new route for obtaining a nearly flat Chern band. On the other hand, the Bloch wave function of a nonsingular flat band has no singularity, and thus forms a vector bundle. A nonsingular flat band can be completely isolated from other bands while preserving the perfect flatness. All one-dimensional flat bands belong to the nonsingular class. We show that a singular flat band displays a novel…

Flat bands with extremely narrow bandwidths on the order of a few millielectron volts can appear in twisted multilayer graphene systems for appropriate system parameters. Here we investigate the electronic structure of a twisted bi-bilayer graphene, or twisted double bilayer graphene, to find the parameter space where isolated flat bands can emerge as a function of twist angle, vertical pressure, and interlayer potential differences. We find that in twisted bi-bilayer graphene the bandwidth is generally flatter than in twisted bilayer graphene by roughly up to a factor of 2 in the same parameter space of twist angle $\ensuremath{\theta}$ and interlayer coupling $\ensuremath{\omega}$, making it in principle simpler to tailor narrow bandwidth flat bands. Application of vertical pressure can enhance the first magic angle in minimal models at $\ensuremath{\theta}\ensuremath{\sim}1.{05}^{\ensuremath{\circ}}$ to larger values of up to $\ensuremath{\theta}\ensuremath{\sim}1.{5}^{\ensuremath{\circ}}$ when $P\ensuremath{\sim}2.5$ GPa, where…

Quantum networks hold the promise of secure communication, guaranteed by fundamental laws of physics. Building such networks, however, requires quantum nodes that can relay this quantum information. Here, the authors address practical considerations necessary for building a quantum network node using silicon-vacancy color centers (SiVs) in diamond. Specifically, they describe improvements in nanophotonic cavity design and fabrication, as well as the roles that strain, spin bath, and heating have in performing spin-photon and spin-spin entanglement experiments with SiVs at 100-mK temperatures.

The Spirit framework is designed for atomic-scale spin simulations of magnetic systems with arbitrary geometry and magnetic structure, providing a graphical user interface with powerful visualizations and an easy-to-use scripting interface. An extended Heisenberg-type spin-lattice Hamiltonian including competing exchange interactions between neighbors at arbitrary distances, higher-order exchange, Dzyaloshinskii-Moriya and dipole-dipole interactions is used to describe the energetics of a system of classical spins localized at atom positions. A variety of common simulation methods are implemented including Monte Carlo and various time evolution algorithms based on the Landau-Lifshitz-Gilbert (LLG) equation of motion. These methods can be used to determine static ground-state and metastable spin configurations, sample equilibrium and finite-temperature thermodynamical properties of magnetic materials and nanostructures, or calculate dynamical trajectories including spin torques induced by stochastic temperature or electric current. Methods for finding the mechanism and rate of…

Recent studies reveal that four-phonon scattering can be important in determining thermal conductivities of solids. However, these studies have been focused on materials where the thermal conductivity $\ensuremath{\kappa}$ is dominated by acoustic phonons, and the role of four-phonon scattering, although significant, is still smaller than that of three-phonon scattering at room temperature. In this work, taking AlSb as an example for which optical phonons contribute significantly to the thermal conductivity under the three-phonon picture, we demonstrate that four-phonon scattering can be more important than three-phonon scattering as it diminishes optical phonon thermal transport and therefore significantly reduces the thermal conductivities of materials. Also, our calculations show that four-phonon scattering can diminish the isotope effect on $\ensuremath{\kappa}$. Specifically, including four-phonon scattering reduces the room-temperature $\ensuremath{\kappa}$ of isotopically pure BAs from 3322 W/mK to 1721 W/mK, a $48%$ reduction. Four-phonon scattering reduces the…

Weyl semimetals exhibit anomalous electronic transport due to the presence of topological band crossings, called Weyl nodes. In this work, experimental and theoretical results show that EuCd${}_{2}$As${}_{2}$ in a magnetic field is what Bernevig has termed the hydrogen atom of a Weyl semimetal, i.e. one with a single pair of Weyl nodes at the Fermi level and without overlapping electron bands. The discovery opens the door to a wide range of exotic physics, predicted for Weyl fermions in the solid state.

We study the electronic band structure and the topological properties of the twisted double bilayer graphene, or a pair of AB-stacked bilayer graphenes rotationally stacked on top of each other. We consider two different arrangements, AB-AB and AB-BA, which differ in the relative orientation. For each system, we calculate the energy band and the valley Chern number using the continuum Hamiltonian. We show that the AB-AB and the AB-BA have similar band structures, while the Chern numbers associated with the corresponding bands are completely different. In the absence of the perpendicular electric field, in particular, the AB-AB system is a trivial insulator when the Fermi energy is in a gap, while the AB-BA is a valley Hall insulator. Also, the lowest electron and hole bands of the AB-AB are entangled by the symmetry protected band touching points, while they are separated in the AB-BA. In both cases, the perpendicular electric field immediately opens an energy gap at the charge neutral point, where the electron branch becomes much narrower than the hole branch, due to the…

The long-sought quantum spin liquid phase of the triangular antiferromagnet has been concealed by various effects, including structural disorder, geometrical distortion, and Dzyaloshinskii-Moriya anisotropy. NaYbO${}_{2}$ has emerged as a model material that overcomes such obstacles and has been found to show the generic physics of a quantum spin liquid in a triangular antiferromagnet. In the experiments here, NaYbO${}_{2}$ shows persistent spin dynamics, continuous magnetic excitations emanating from the $K$ point of the Brillouin zone, and absence of a spin gap down to the lowest measured temperatures, suggesting that the spin-liquid state is gapless.

A theoretical exploration of exotic properties in open quantum many-body systems requires an efficient search of the nonequillibrum stationary states. Aiming to accelerate the process, the authors develop here a variational method, in which the ansatz for the mixed states is based on the restricted Boltzmann machine, motivated by its high expressive power shown in various recent works. It is demonstrated that the ansatz successfully simulates dissipative spin systems both in one and two dimensions.

We employ a quantum Liouville equation with relaxation to model the recently observed anomalous Hall effect in graphene irradiated by an ultrafast pulse of circularly polarized light. In the weak-field regime, we demonstrate that the Hall effect originates from an asymmetric population of photocarriers in the Dirac bands. By contrast, in the strong-field regime, the system is driven into a nonequilibrium steady state that is well described by topologically nontrivial Floquet-Bloch bands. Here, the anomalous Hall current originates from the combination of a population imbalance in these dressed bands together with a smaller anomalous velocity contribution arising from their Berry curvature. This robust and general finding enables the simulation of electrical transport from light-induced Floquet-Bloch bands in an experimentally relevant parameter regime and creates a pathway to designing ultrafast quantum devices with Floquet-engineered transport properties.

The high-pressure phases and superconductivity of ${\mathrm{CaYH}}_{12}$ have been explored by using a particle swarm optimization structure prediction methodology in combination with first-principles calculations. Our results show that ${\mathrm{CaYH}}_{12}$ becomes stable with a cubic $Fd\overline{3}m$ structure above 170 GPa, where metal atoms form body-centered-cubic (bcc) lattices and hydrogens occupy all the tetrahedral interstices of these bcc lattices, completing sodalitelike cages. The electron-phonon coupling calculations indicate that the $Fd\overline{3}m$ structure is a potential high-temperature superconductor, with a calculated ${T}_{c}$ of 258 K at 200 GPa. Our current study provides a possibility for searching new high-${T}_{c}$ superconductors in ternary hydrides.

Here, the authors report a thorough study of the evolution of structural, magnetic, and electrical properties of MnBi${}_{2\ensuremath{-}x}$Sb${}_{x}$Te${}_{4}$ single crystals. This investigation reveals an intimate correlation between the structural, magnetic, and transport properties. Partial substitution of Bi by Sb is an effective approach to tuning both the magnetism and the Fermi level, which are two key components of magnetic topological insulators. This work provides an important guide for exploring the topological properties in MnBi${}_{2}$Te${}_{4}$-based materials. The effect of doping on the magnetic properties also suggests complex competing interactions, which should be considered when tuning the magnetic properties.

We introduce an approach to compute the spectra of reduced density matrices for local quantum unitary circuits of finite depth and infinite width. Suppose the time-evolved state under the circuit is a matrix-product state with bond dimension $D$; then the reduced density matrix of a half-infinite system has the same spectrum as an appropriate $D\ifmmode\times\else\texttimes\fi{}D$ matrix acting on an ancilla space. We show that reduced density matrices at different spatial cuts are related by quantum channels acting on the ancilla space. This quantum channel approach allows for efficient numerical evaluation of the entanglement spectrum and R\'enyi entropies and their spatial fluctuations at finite times in an infinite system. We benchmark our numerical method on random unitary circuits, where many analytic results are available, and also show how our approach analytically recovers the behavior of the kicked Ising model at the self-dual point. We study various properties of the spectra of the reduced density matrices and their spatial fluctuations in both the random and…

High-quality single crystals of ${\mathrm{NaYbSe}}_{2}$, which resembles a perfect triangular-lattice antiferromagnet without intrinsic disorder, are investigated by magnetization and specific heat, as well as the local probe techniques nuclear magnetic resonance (NMR) and electron spin resonance. The low-field measurements confirm the absence of any spin freezing or long-range magnetic order down to 50 mK, which suggests a quantum spin liquid ground (QSL) state with gapless excitations. Instability of the QSL state is observed upon applying magnetic fields. For the $H\ensuremath{\perp}c$ direction, a field-induced magnetic phase transition is observed above 2 T from the ${C}_{\mathrm{p}}(T)$ data, agreeing with a clear $\frac{{M}_{s}}{3}$ plateau of $M(H)$, which is associated with an up-up-down spin arrangement. For the $H\ensuremath{\parallel}c$ direction, a field-induced transition could be evidenced at a much higher field range (9--21 T). The $^{23}\mathrm{Na}$ NMR measurements provide microscopic evidence of field-induced ordering for both directions. A reentrant behavior of…

The layered van der Waals antiferromagnet MnBi${}_{2}$Te${}_{4}$ has been predicted previously to realize the first intrinsic magnetic topological insulator. Here, the authors report spin- and angle-resolved photoemission experiments for the MnBi${}_{2}$Te${}_{4}$(0001) surface, revealing a surface state in the bulk band gap and providing evidence for the interplay between magnetic exchange interaction and spin-orbit coupling in the surface electronic structure. MnBi${}_{2}$Te${}_{4}$ thus constitutes a promising candidate to exploit the interplay of topological states and magnetic order in spintronic device concepts.

The collective and quantum behavior of many-body systems may be harnessed to achieve fast charging of energy storage devices, which have been recently dubbed quantum batteries. In this paper, we present an extensive numerical analysis of energy flow in a quantum battery described by a disordered quantum Ising chain Hamiltonian, whose equilibrium phase diagram presents many-body localized (MBL), Anderson localized (AL), and ergodic phases. We demonstrate that (i) the low amount of entanglement of the MBL phase guarantees much better work extraction capabilities, measured by the ergotropy, than the ergodic phase and (ii) interactions suppress temporal energy fluctuations in comparison with those of the noninteracting AL phase. Finally, we show that the statistical distribution of values of the optimal charging time is a clear-cut diagnostic tool of the MBL phase.

The electronic properties of twisted bilayer graphene are being intensely investigated. The flat bands emerging at the first ``magic angle'' are responsible for anomalous insulating behavior at half-filling and superconductivity. Using state-of-the-art $a\phantom{\rule{0}{0ex}}b\phantom{\rule{0.333em}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ calculations, it is shown that the energy gaps and band dispersion are captured correctly only when the atomic corrugation, arising as a result of the interlayer interaction, is considered. The atomic corrugation shows a wave profile with maximum atomic out-of-plane displacements of about 0.1 angstrom.

We study a new class of non-Hermitian topological phases in three dimensions in the absence of any symmetry, where the topological robust band degeneracies are Hopf-link exceptional lines. As a concrete example, we investigate the non-Hermitian band structures of nodal line semimetals under non-Hermitian perturbations, where the Fermi surfaces can transit from one-dimensional nodal lines to two-dimensional twisting surfaces with Hopf-link boundaries when the winding number defined along the nodal line is $\ifmmode\pm\else\textpm\fi{}1$. The linking numbers of these linked exceptional line phases are also proposed, based on the integral of Chern-Simons form over the Brillouin zone.

Proximity effects in two-dimensional heterostructures allow to transform nonmagnetic materials into synthetic magnets that do not involve magnetic ions. Based on density functional theory calculations, the present study predicts a sizeable proximity exchange interactions, induced by ferromagnetic CrI${}_{3}$, in transition metal dichalcogenides. Remarkably, the exchange coupling depends qualitatively on the twist angle, while being also tunable by an electric field. The authors establish the short-range character of the proximity effect and make specific predictions for the excitonic level splittings, due to the induced exchange coupling.

We study non-Bloch bulk-boundary correspondence in a non-Hermitian Su-Schieffer-Heeger model in a domain wall configuration where the left and right bulks have different parameters. Focusing on the case where chiral symmetry is still conserved, we show that non-Hermitian skin effects of bulk states persist in the system, while the definition of the non-Bloch winding number of either bulk depends on parameters on both sides of the boundary. Under these redefined non-Bloch topological invariants, we confirm non-Bloch bulk-boundary correspondence under the domain wall configuration, which exemplifies the impact of boundary conditions in non-Hermitian topological systems.

Geometrical moir\'e patterns, generic for almost aligned bilayers of two-dimensional crystals with similar lattice structure but slightly different lattice constants, lead to zone folding and miniband formation for electronic states. Here, we show that moir\'e superlattice (mSL) effects in $\mathrm{MoSe}{}_{2}/\mathrm{WS}{}_{2}$ and $\mathrm{MoTe}{}_{2}/\mathrm{MoSe}{}_{2}$ heterobilayers that feature alignment of the band edges are enhanced by resonant interlayer hybridization, and anticipate similar features in twisted homobilayers of transition-metal dichalcogenides (TMDs), including examples of narrow minibands close to the actual band edges. Such hybridization determines the optical activity of interlayer excitons in TMD heterostructures, as well as energy shifts in the exciton spectrum. We show that the resonantly hybridized exciton energy should display a sharp modulation as a function of the interlayer twist angle, accompanied by additional spectral features caused by umklapp electron-photon interactions with the mSL. We analyze the appearance of resonantly enhanced mSL…

Non-Hermitian extensions of the Anderson and Aubry-Andr\'e-Harper models are attracting considerable interest as platforms to study localization phenomena, metal-insulator, and topological phase transitions in disordered non-Hermitian systems. Most of the available studies, however, resort to numerical results, while few analytical and rigorous results are available owing to the extraordinary complexity of the underlying problem. Here we consider a parity-time symmetric extension of the Aubry-Andr\'e-Harper model, undergoing a topological metal-insulator phase transition, and provide rigorous analytical results of energy spectrum, symmetry breaking phase transition, and localization length. In particular, by extending to the non-Hermitian realm the Thouless result relating localization length and density of states, we derive an analytical form of the localization length in the insulating phase, showing that---like in the Hermitian Aubry-Andr\'e-Harper model---the localization length is independent of energy.

The Su-Schrieffer-Heeger (SSH) model on a two-dimensional square lattice exhibits a topological phase transition which is related to the Zak phase determined by bulk band topology. The strong modulation of electron hopping causes nontrivial charge polarization even in the presence of inversion symmetry. The energy band structures and topological edge states have been calculated numerically in previous studies. Here, however, the full energy spectrum and explicit form of wave functions for two-dimensional bulk and one-dimensional ribbon geometries of the SSH model are analytically derived using the wave mechanics approach. Explicit analytic representations of wave functions provide the information of parity for each subband, localization length, and critical point of the topological phase transition in the SSH ribbon. IThe dimensional crossover of the topological transition point for the SSH model from one to two dimensions is also shown.

Nonreciprocal non-Hermitian higher-order topological skin states are realized in $L\phantom{\rule{0}{0ex}}C$ electric circuits with diodes. Here, the authors give examples with rhombus geometry of the nonreciprocal honeycomb system and rhombohedron geometry of the diamond lattice system. In the accompanying image, red balls represent zero-energy topological corner states.

A Weyl point is topologically stable in three dimensions, but is typically vulnerable in two dimensions of space, especially when spin-orbit coupling (SOC) and magnetic ordering are considered. Here, the authors predict a state, labeled as a Weyl half-semimetal, which hosts a stable magnetic Weyl point in two dimensions in the presence of SOC. It is simultaneously a Weyl semimetal and a half metal, such that the low-energy electrons are fully spin-polarized two-dimensional Weyl fermions. A specific material, the monolayer PtCl${}_{3}$, is proposed to realize this state.

Twisted bilayer graphene is one of the most promising low-dimensional materials, showing both superconducting and insulating behavior. This is understood to be due to the occurrence of extremely flat bands at the magic angles, which enhances the residual interaction effects. Here, the authors show that the most common understanding of this problem neglects two important effects, the deformation of the graphene layers and the modification of electronic hopping between the atoms of the layers. A method to extract a continuum model in the presence of these effects is presented.

We extend the standard Su-Schrieffer-Heeger (SSH) model to include long-range hopping amplitudes and disorder, and analyze how the electronic and topological properties are affected. We show that long-range hopping can change the symmetry class and the topological invariant, while diagonal and off-diagonal disorder lead to Anderson localization. Interestingly, we find that the Lyapunov exponent $\ensuremath{\gamma}(E)$ can be linked in two ways to the topological properties in the presence of disorder---either due to the different response of midgap states to increasing disorder or due to an extra contribution to $\ensuremath{\gamma}$ due to the presence of edge modes. Finally, we discuss its implications in realistic transport measurements.

Unidirectional propagation of spin waves is key to the realization of magnonic logic devices. Here, the authors experimentally demonstrate excitations of unidirectional exchange spin waves by a nanoscale magnetic grating fabricated on an ultrathin yttrium iron garnet film. The authors explain and model the nearly perfect unidirectional excitation by the chirality of the magnetodipolar interactions. Reversal of the magnetic configurations changes the direction of the exchange spin waves. Such results raise the prospect of a chiral magnonic logic without the need for fragile surface states.

We explore different ways to simplify the evaluation of the smooth overlap of atomic positions (SOAP) many-body atomic descriptor [Bart\'ok et al., Phys. Rev. B 87, 184115 (2013).]. Our aim is to improve the computational efficiency of SOAP-based similarity kernel construction. While these improved atomic descriptors can be used for general characterization and interpolation of atomic properties, their main target application is accelerated evaluation of machine-learning-based interatomic potentials within the Gaussian approximation potential (GAP) framework [Bart\'ok et al., Phys. Rev. Lett. 104, 136403 (2010)]. We achieve this objective by expressing the atomic densities in an approximate separable form, which decouples the radial and angular channels. We then express the elements of the SOAP descriptor (i.e., the expansion coefficients for the atomic densities) in analytical form given a particular choice of radial basis set. Finally, we derive recursion formulas for the expansion coefficients. This new SOAP-based descriptor allows for tenfold speedups compared to previous…

A two-dimensional carbon allotrope, Stone-Wales graphene, is identified in stochastic group and graph constrained searches and systematically investigated by first-principles calculations. Stone-Wales graphene consists of well-arranged Stone-Wales defects, and it can be constructed through a ${90}^{\ensuremath{\circ}}$ bond rotation in a $\sqrt{8}\ifmmode\times\else\texttimes\fi{}\sqrt{8}$ supercell of graphene. Its calculated energy relative to graphene, +149 meV/atom, makes it more stable than the most competitive previously suggested graphene allotropes We find that Stone-Wales graphene (SW-graphene) based on a $\sqrt{8}$ supercell is more stable than those based on $\sqrt{9}\ifmmode\times\else\texttimes\fi{}\sqrt{9},\phantom{\rule{0.28em}{0ex}}\sqrt{12}\ifmmode\times\else\texttimes\fi{}\sqrt{12}$, and $\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13}$ supercells, and is a ``magic size'' that can be further understood through a simple ``energy splitting and inversion'' model. The calculated vibrational properties and molecular dynamics of SW-graphene confirm that it is…

According to quantum statistical mechanics, generic many-body systems decohere and thermalize regardless of their initial state. A recent experiment found a surprising exception to this rule, wherein an otherwise thermalizing system maintains its coherence only for a particular initial condition, a phenomenon dubbed ``quantum many-body scarring.'' Here, the authors propose and provide numerical evidence for a physical mechanism underlying the observed phenomena: Bose-Einstein condensation, wherein macroscopic numbers of quasiparticles occupy a single quantum state. While this phenomenon is well known at equilibrium, its emergence in a strongly interacting, far-from-equilibrium system is striking and unexpected. The physical principle explored in this work can be applied to other systems to reveal new examples of quantum many-body scars.

Motivated by the discovery of near-room-temperature superconductivity in the sodalite-like clathrate hydride ${\mathrm{LaH}}_{10}$, we report ab initio calculations of the superconducting properties of two closely related hydrides ${\mathrm{YH}}_{6}$ and ${\mathrm{YH}}_{10}$ for which an even higher ${T}_{\mathrm{c}}$ has been predicted. Using fully anisotropic Migdal-Eliashberg theory with Coulomb corrections, we find almost isotropic superconducting gaps, resulting from a uniform distribution of the coupling over states of both Y and H sublattices. The Coulomb screening is rather weak, resulting in a Morel-Anderson pseudopotential ${\ensuremath{\mu}}^{*}=0.11$, at odds with claims of unusually large ${\ensuremath{\mu}}^{*}$ in lanthanum hydrides. The corresponding critical temperatures at 300 GPa exceed room temperature (${T}_{\mathrm{c}}=290$ and 310 K for ${\mathrm{YH}}_{6}$ and ${\mathrm{YH}}_{10}$), in agreement with a previous isotropic-gap calculation. We estimate anharmonic effects to be weak. The different response of these two compounds to external pressure along with a…

The authors report an experimental demonstration of an opposite anticrossing effect in a photon-magnon coupled system consisting of a YIG and an inverted pattern of split-ring resonator (ISRR) structure in a planar geometry. The dispersion shape of the anticrossing effect is modifiable by changing the position/orientation of the ISRR split gap relative to the microstrip-line axis. This experimental observation, coupled with analytical calculations of various dispersion types of anticrossing, shows the potential and flexibility of photon-magnon systems for exploring emergent phenomena and light-matter interaction applications.

Realization of pseudo-Hermiticity is proposed in a cavity magnonics system consisting of two magnon modes coupled to a microwave cavity. By varying the coupling strengths between the magnon modes and the cavity mode, the authors show the existence of a third-order exceptional point in the parameter space, in addition to the usual second-order exceptional point known for a system with parity-time symmetry. This work provides an experimentally feasible scheme to realize pseudo-Hermiticity and a third-order exceptional point in a hybrid quantum system.

Recently, topological quantum states of non-Hermitian systems, exhibiting rich, new exotic states, have attracted great attention in condensed-matter physics. As for the demonstration, most of non-Hermitian topological phenomena previously focused on are in one- and two-dimensional systems. Here, we investigate three-dimensional non-Hermitian nodal-line semimetals in the presence of a particle gain-and-loss perturbation. It is found that this perturbation will split the original nodal ring into two exceptional rings (ERs). The topological nature of the bulk electronic structure is characterized by two different topological invariants, namely, the vorticity and the winding number defined for a one-dimensional loop in momentum space, both of which are shown to take half-integer (integer) values when an odd (even) number of ERs thread through the loop. The conventional bulk-surface correspondence in non-Hermitian nodal-line semimetals is found to break down, where the surface zero-energy flat bands are no longer bounded by projections of bulk ERs. Alternatively, a macroscopic fraction…

Among the recently widely studied van der Waals layered magnets $\mathrm{Cr}{X}_{3}$ ($X=\mathrm{Cl}$, Br, I), $\mathrm{CrC}{\mathrm{l}}_{3}$ monolayer (ML) is particularly puzzling: it is shown by experiments to have only an in-plane magnetic easy axis and, furthermore, all previous first-principles calculation results contradict this. Through systematic first-principles calculations, we unveil that the in-plane shape anisotropy that dominates over the weak perpendicular magnetocrystalline anisotropy is responsible for the in-plane magnetic easy axis of $\mathrm{CrC}{\mathrm{l}}_{3}$ ML. To tune the in-plane ferromagnetism of $\mathrm{CrC}{\mathrm{l}}_{3}$ ML into the desirable perpendicular one, we propose substituting Cr with isovalent tungsten (W). We find that $\mathrm{CrWC}{\mathrm{l}}_{6}$ has a strong perpendicular magnetic anisotropy and a high Curie temperature up to 76 K. Our work not only gives insight into understanding the two-dimensional ferromagnetism of van der Waals MLs but also sheds new light on engineering their performances for nanodevices.

When considering transition-metal dichalcogenides (TMDCs) in van der Waals heterostructures for periodic ab initio calculations, usually, lattice mismatch is present, and the TMDC needs to be strained. In this study we provide a systematic assessment of biaxial strain effects on the orbital, spin-orbit, and optical properties of the monolayer TMDCs using ab initio calculations. We complement our analysis with a minimal tight-binding Hamiltonian that captures the low-energy bands of the TMDCs around the $K$ and ${K}^{\ensuremath{'}}$ valleys. We find characteristic trends of the orbital and spin-orbit parameters as a function of the biaxial strain. Specifically, the orbital gap decreases linearly, while the valence (conduction) band spin splitting increases (decreases) nonlinearly in magnitude when the lattice constant increases. Furthermore, employing the Bethe-Salpeter equation and the extracted parameters, we show the evolution of several exciton peaks, with biaxial strain, on different dielectric surroundings, which are particularly useful for interpreting experiments studying…

Newly synthesized Janus transition-metal dichalcogenides MXY ($M=\mathrm{Mo}$, W; $X\ensuremath{\ne}Y=\mathrm{S}$, Se, Te) possess intrinsic Rashba spin splitting and out-of-plane dipole moment due to the breaking of mirror symmetry. Taking WSSe as an example, we present a first-principles investigation of the structural stability and electronic properties of mono-, bi-, and multilayer MXY. Results show that S atoms contribute more than Se atoms in the valence-band maximum at the $\mathrm{\ensuremath{\Gamma}}$ point, which can be greatly affected by interlayer interactions. The high-symmetry AA\ensuremath{'} stacking is still the most stable pattern, but there are various orders of chalcogen atomic layers in each stacking. The most preferred order of two adjacent layers is S-Se-Se-S, followed by Se-S-Se-S. The Se-S-Se-S--ordered WSSe bilayer is found to have significant layer splitting due to the net dipole moment, which has great potential for solar cells. Layer-dependent Rashba splittings exist in asymmetry-ordered WSSe bilayers, that can be tuned by changing the interlayer…

Topology has recently emerged as a powerful tool in condensed matter physics to engineer materials whose macroscopic properties are robust to imperfections. Indeed, rather than measuring the ``topological invariants'' associated with the band structure of a material, the existence of such invariants is often inferred from observations of disorder robustness. Here, the authors directly measure such a topological invariant by building a ``topological circuit'' that exhibits Weyl points, one of the paradigmatic topological features of 3$d$ band structures. The exaggerated size of the unit cell in the circuit allows for measurement of the complete band structure and Berry curvature of the circuit's bands, and thus a direct measurement of the chiral charge of Weyl points.

Recent theoretical studies have extended the Berry phase framework to account for higher electric multipole moments; quadrupole and octupole topological phases have been proposed. Although the two-dimensional quantized quadrupole insulators have been demonstrated experimentally, octupole topological phases have not previously been observed experimentally. Here we report on the experimental realization of a classical analog of the octupole topological insulator in the electric circuit system. Three-dimensional topolectrical circuits for realizing such topological phases are constructed experimentally. We observe octupole topological states protected by the topology of the bulk, which are localized at the corners. Our results provide conclusive evidence of a form of robustness against disorder and deformation, which is characteristic of octupole topological insulators. Our study opens a route toward higher-order topological phenomena in three dimensions and paves the way for employing topolectrical circuitry to study complex topological phenomena.

Anharmonic vibrational effects in solids, although essential for the finite-temperature properties, remain difficult to model via first-principles calculations. The authors detail a systematic information-theory based approach: compressive sensing lattice dynamics. The conventional lattice dynamics theory is recast into a form, suited for complex multicomponent compounds with large unit cells. With $L$1 regularization, the anharmonic force constants can be obtained efficiently using modest sets of training structures. The approach therefore allows for easier access to finite-temperature properties of complex solids.

We apply the compressive sensing lattice dynamics method to calculate phonon dispersion for crystalline solids. While existing methods such as frozen phonon, small displacement, and linear response are routinely applied for phonon calculations, they are considerably more expensive or cumbersome to apply to certain solids, including structures with large unit cells or low symmetry, systems that require more expensive electronic structure treatment, and polar semiconductors/insulators. In the latter case, we propose an approach based on a corrected long-range force constant model with proper treatment of the acoustic sum rule and the symmetric on-site force constant matrix. Our approach is demonstrated to be accurate and efficient for these systems through case studies of NaCl, ${\mathrm{CeO}}_{2}$, ${\mathrm{Y}}_{3}{\mathrm{Al}}_{5}{\mathrm{O}}_{12}$, and ${\mathrm{La}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$.

Electron spins in silicon have long coherence times and are a promising qubit platform. However, electric field noise in semiconductors poses a challenge for most single- and multiqubit operations in quantum-dot spin qubits. We investigate the dependence of low-frequency charge noise spectra on temperature and aluminum-oxide gate dielectric thickness in Si/SiGe quantum dots with overlapping gates. We find that charge noise increases with aluminum-oxide thickness. We also find strong dot-to-dot variations in the temperature dependence of the noise magnitude and spectrum. These findings suggest that each quantum dot experiences noise caused by a distinct ensemble of two-level systems, each of which has a nonuniform distribution of thermal activation energies. Taken together, our results suggest that charge noise in Si/SiGe quantum dots originates at least in part from a nonuniform distribution of two-level systems near the surface of the semiconductor.

A new method enables $a\phantom{\rule{0}{0ex}}b\phantom{\rule{0.333em}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ calculations of large and small polarons on the same footing. The authors develop the theory and computational methodology required to determine the ground-state energy and wave functions of electron and hole polarons in semiconductors and insulators, without performing explicit supercell calculations. As initial applications, the authors investigate polarons in LiF and Li${}_{2}$O${}_{2}$.

Unlike the ordinary Hall effect, the nonlinear Hall effect (NLHE) can occur in time-reversal-invariant metals lacking inversion symmetry. The authors uncover that two disorder-mediated corrections to the NLHE, namely, the nonlinear side jump and the nonlinear skew scattering, can be as large as the intrinsic Berry curvature dipole contribution. These findings will help to understand recent experiments searching for NLHE in various materials. They also offer a description of the role of disorder in current rectification in materials without inversion symmetry that could be relevant for wireless energy harvesting technology.

Tungsten will be used as a plasma-facing material in fusion power reactors, where the absorption of high-energy neutrons leads to permanent damage in the crystal structure. A comprehensive understanding of the atom-level damage in tungsten has been limited by the slowness of quantum simulations and the insufficient accuracy of classical simulations. This study bridges the gap between the two by developing a machine-learning interatomic potential that allows the simulation of tungsten in extreme environments with quantum accuracy.

We analyze spin and orbital angular momenta in monochromatic acoustic wave fields in a homogeneous medium. Despite being purely longitudinal (curl-free), inhomogeneous acoustic waves generically possess nonzero spin angular momentum density caused by the local rotation of the vector velocity field. We show that the integral spin of a localized acoustic wave vanishes in agreement with the spin-0 nature of longitudinal phonons. We also show that the helicity or chirality density vanishes identically in acoustic fields. As an example, we consider nonparaxial acoustic Bessel beams carrying well-defined integer orbital angular momentum, as well as nonzero local spin density, with both transverse and longitudinal components. We describe the nontrivial polarization structure in acoustic Bessel beams and indicate a number of observable phenomena, such as nonzero energy density and purely circular transverse polarization in the center of the first-order vortex beams.

As a crucial part in thermal management, interfacial thermal transport is still not well understood. In this paper, we employ the newly developed modal nonequilibrium molecular dynamics to study the Si/Ge interfacial thermal transport and clarify several long-standing issues. We find that the few atomic layers at the interface are dominated by interfacial modes, which act as a bridge that connects the bulk Si and Ge modes. Such bridging effect boosts the inelastic transport to contribute more than 50% to the total thermal conductance even at room temperature. The apparent inelastic transport can even allow effective four-phonon processes across the interface when the mass difference between the two materials is large. Surprisingly, optical phonon modes can contribute equal or more thermal conductance than the acoustic modes due to the bridging effect. From the modal temperature analysis, we find that the phonon modes are in strong thermal nonequilibrium near the interface, which impedes the thermal transport. The widely used Landauer approach that does not consider the phonon…

The recent discovery of the antiferromagnetic (AFM) topological insulator (TI) ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ has triggered great research efforts on exploring novel magnetic topological physics. Based on first-principles calculations, we find that the manipulation of magnetic orientation and order not only significantly affects material symmetries and orbital hybridizations, but also results in variant magnetic topological phases in ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$. We thus predict a series of topological quantum phase transitions that are magnetically controllable in the material, including phase transitions from an AFM TI to an AFM mirror topological crystalline insulator, from a type-II to type-I topological Weyl semimetal, and from an axion insulator to a Chern insulator. The findings open opportunities for future research and applications of magnetic topological materials.

We develop an approach to design, engineer, and measure band structures in a synthetic crystal composed of electric circuit elements. Starting from a nodal analysis of a circuit lattice in terms of currents and voltages, our Laplacian formalism for synthetic matter allows us to investigate arbitrary tight-binding models in terms of wave-number-resolved Laplacian eigenmodes, yielding an admittance band structure of the circuit. For illustration, we model and measure a honeycomb circuit featuring a Dirac cone admittance bulk dispersion as well as flat band admittance edge modes at its bearded and zigzag terminations. We further employ our circuit band analysis to measure a topological phase transition in the topolectrical Su-Schrieffer-Heeger circuit.

In spite of decades of work it has remained unclear whether or not superradiant quantum phases, referred to here as photon condensates, can occur in equilibrium. In this Rapid Communication, we first show that when a nonrelativistic quantum many-body system is coupled to a cavity field, gauge invariance forbids photon condensation. We then present a microscopic theory of the cavity quantum electrodynamics of an extended Falicov-Kimball model, showing that, in agreement with the general theorem, its insulating ferroelectric and exciton condensate phases are not altered by the cavity and do not support photon condensation.

Wurzite GaN possesses a low hole mobility, preventing its mass adoption for applications in high-power transistors. The authors perform predictive $a\phantom{\rule{0}{0ex}}b\phantom{\rule{0.333em}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ calculations of the phonon-limited mobility of GaN and find that hole mobilities as high as 120 cm${}^{2}$/Vs at room temperature could potentially be achieved by reversing the sign of the crystal-field splitting via uniaxial compressive strain or biaxial tensile strain. The experimental realization of this proposal will contribute to the development of high-power electronics based on GaN.

First-principles calculations show that electric fields applied to ferromagnets generate spin currents flowing perpendicularly to the electric field. Reduced symmetry in these ferromagnets enables a wide variety of such spin currents. However, the total spin current is approximately the sum of a magnetization-independent spin Hall current and an anisotropic spin anomalous Hall current. Intrinsic spin currents are not subject to dephasing, enabling their spin polarizations to be misaligned with the magnetization and allowing for the magnetization-independent spin Hall effect. The spin Hall conductivity and spin anomalous Hall conductivities of transition-metal ferromagnets are comparable to those found in heavy metals, opening different avenues for efficient spin current generation in spintronic devices.

Polycrystalline samples of ${\mathrm{NaYbO}}_{2}$ are investigated by bulk magnetization and specific-heat measurements, as well as by nuclear magnetic resonance (NMR) and electron spin resonance (ESR) as local probes. No signatures of long-range magnetic order are found down to 0.3 K, evidencing a highly frustrated spin-liquid-like ground state in zero field. Above 2 T, signatures of magnetic order are observed in thermodynamic measurements, suggesting the possibility of a field-induced quantum phase transition. The $^{23}\mathrm{Na}$ NMR relaxation rates reveal the absence of magnetic order and persistent fluctuations down to 0.3 K at very low fields and confirm the bulk magnetic order above 2 T. The $H\ensuremath{-}T$ phase diagram is obtained and discussed along with the existing theoretical concepts for layered spin-$\frac{1}{2}$ triangular-lattice antiferromagnets.