The 100 most-cited articles in Physical review. B./Physical review. B, 2016

We show that the time-dependent variational principle provides a unifying framework for time-evolution methods and optimization methods in the context of matrix product states. In particular, we introduce a new integration scheme for studying time evolution, which can cope with arbitrary Hamiltonians, including those with long-range interactions. Rather than a Suzuki-Trotter splitting of the Hamiltonian, which is the idea behind the adaptive time-dependent density matrix renormalization group method or time-evolving block decimation, our method is based on splitting the projector onto the matrix product state tangent space as it appears in the Dirac-Frenkel time-dependent variational principle. We discuss how the resulting algorithm resembles the density matrix renormalization group (DMRG) algorithm for finding ground states so closely that it can be implemented by changing just a few lines of code and it inherits the same stability and efficiency. In particular, our method is compatible with any Hamiltonian for which ground-state DMRG can be implemented efficiently. In fact, DMRG…

Unsupervised learning is a discipline of machine learning which aims at discovering patterns in large data sets or classifying the data into several categories without being trained explicitly. We show that unsupervised learning techniques can be readily used to identify phases and phases transitions of many-body systems. Starting with raw spin configurations of a prototypical Ising model, we use principal component analysis to extract relevant low-dimensional representations of the original data and use clustering analysis to identify distinct phases in the feature space. This approach successfully finds physical concepts such as the order parameter and structure factor to be indicators of a phase transition. We discuss the future prospects of discovering more complex phases and phase transitions using unsupervised learning techniques.

We introduce a generalization of conventional lattice gauge theory to describe fracton topological phases, which are characterized by immobile, pointlike topological excitations, and subextensive topological degeneracy. We demonstrate a duality between fracton topological order and interacting spin systems with symmetries along extensive, lower-dimensional subsystems, which may be used to systematically search for and characterize fracton topological phases. Commutative algebra and elementary algebraic geometry provide an effective mathematical tool set for our results. Our work paves the way for identifying possible material realizations of fracton topological phases.

Nonuniform metasurfaces (electrically thin composite layers) can be used for shaping refracted and reflected electromagnetic waves. However, known design approaches based on the generalized refraction and reflection laws do not allow realization of perfectly performing devices: there are always some parasitic reflections into undesired directions. In this paper we introduce and discuss a general approach to the synthesis of metasurfaces for full control of transmitted and reflected plane waves and show that perfect performance can be realized. The method is based on the use of an equivalent impedance matrix model which connects the tangential field components at the two sides on the metasurface. With this approach we are able to understand what physical properties of the metasurface are needed in order to perfectly realize the desired response. Furthermore, we determine the required polarizabilities of the metasurface unit cells and discuss suitable cell structures. It appears that only spatially dispersive metasurfaces allow realization of perfect refraction and reflection of…

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

We present a comprehensive study of the band alignments of two-dimensional (2D) semiconducting materials and highlight the possibilities of forming momentum-matched type I, II, and III heterostructures, an enticing possibility being atomic heterostructures where the constituent monolayers have band edges at the zone center, i.e., $\mathrm{\ensuremath{\Gamma}}$ valley. Our study, which includes the group IV and III-V compound monolayer materials, group V elemental monolayer materials, transition-metal dichalcogenides, and transition-metal trichalcogenides, reveals that almost half of these materials have conduction and/or valence band edges residing at the zone center. Using first-principles density functional calculations, we present the type of the heterostructure for 903 different possible combinations of these 2D materials which establishes a periodic table of heterostructures.

We have investigated the exciton dynamics in transition metal dichalcogenide monolayers using time-resolved photoluminescence experiments performed with optimized time resolution. For $\mathrm{MoS}{\mathrm{e}}_{2}$ monolayer, we measure ${\ensuremath{\tau}}_{\mathrm{rad}}^{0}=1.8\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{0.16em}{0ex}}\mathrm{ps}$ at $T=7\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ that we interpret as the intrinsic radiative recombination time. Similar values are found for $\mathrm{WS}{\mathrm{e}}_{2}$ monolayers. Our detailed analysis suggests the following scenario: at low temperature $(T\ensuremath{\lesssim}50\phantom{\rule{0.16em}{0ex}}\mathrm{K})$, the exciton oscillator strength is so large that the entire light can be emitted before the time required for the establishment of a thermalized exciton distribution. For higher lattice temperatures, the photoluminescence dynamics is characterized by two regimes with very different characteristic times. First the photoluminescence intensity drops drastically with a decay time in the range of the picosecond driven by…

The magnetic anisotropy energy of tetragonally distorted disordered alloys Fe ${}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}$ is calculated by two different virtual crystal approximation methods and an averaged supercell method within the projected-augmented-wave (PAW) methodology and the magnetic force theorem. The details of the spin-orbit coupling implementation in the PAW methodology are given. We compare our results to the recent coherent potential approximation (CPA) studies, results of full potential calculations, and to the available experiments.

Coherent conversion of microwave and optical photons in the single quantum level can significantly expand our ability to process signals in various fields. Efficient up-conversion of a feeble signal in the microwave domain to the optical domain will lead to quantum-noise-limited microwave amplifiers. Coherent exchange between optical photons and microwave photons will also be a stepping stone to realize long-distance quantum communication. Here we demonstrate bidirectional and coherent conversion between microwave and light using collective spin excitations in a ferromagnet. The converter consists of two harmonic oscillator modes, a microwave cavity mode and a magnetostatic mode called the Kittel mode, where microwave photons and magnons in the respective modes are strongly coupled and hybridized. An itinerant microwave field and a traveling optical field can be coupled through the hybrid system, where the microwave field is coupled to the hybrid system through the cavity mode, while the optical field addresses the hybrid system through the Kittel mode via Faraday and inverse…

We provide direct evidence to understand the origin of low thermal conductivity of SnSe using elastic measurements. Compared to state-of-the-art lead chalcogenides $\mathrm{Pb}Q(Q=\mathrm{Te}$, Se, S), SnSe exhibits low values of sound velocity $(\ensuremath{\sim}1420\phantom{\rule{0.28em}{0ex}}\mathrm{m}/\mathrm{s})$, Young's modulus $(E\ensuremath{\sim}27.7\phantom{\rule{0.28em}{0ex}}\mathrm{GPa})$, and shear modulus $(G\ensuremath{\sim}9.6\phantom{\rule{0.28em}{0ex}}\mathrm{GPa})$, which are ascribed to the extremely weak Sn-Se atomic interactions (or bonds between layers); meanwhile, the deduced average Gr\"uneisen parameter \ensuremath{\gamma} of SnSe is as large as \ensuremath{\sim}3.13, originating from the strong anharmonicity of the bonding arrangement. The calculated phonon mean free path (l \ensuremath{\sim} 0.84 nm) at 300 K is comparable to the lattice parameters of SnSe, indicating little room is left for further reduction of the thermal conductivity through introducing nanoscale microstructures and microscale grain boundaries. The low elastic properties indicate that…

Computationally characterizing magnetic properies of novel two-dimensional (2D) materials serves as an important first step of exploring possible applications. Using density-functional theory, we show that single-layer ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$ is a potential 2D material with sufficiently low formation energy to be synthesized by mechanical exfoliation from the bulk phase with a van der Waals layered structure. In addition, we calculated the phonon dispersion demonstrating that single-layer ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$ is dynamically stable. Furthermore, we find that similar to the bulk phase, 2D ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$ exhibits a magnetic moment that originates from a Stoner instability. In contrast to other 2D materials, we find that single-layer ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$ exhibits a significant uniaxial magnetocrystalline anisotropy energy of $920\ensuremath{\mu}\mathrm{eV}$ per Fe atom originating from spin-orbit coupling. Finally, we show that applying biaxial tensile strains enhances the anisotropy energy, which reveals strong…

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

We survey the electronic structure and magnetic properties of two-dimensional (2D) $M\mathrm{P}{X}_{3}$ ($M=\text{V},\text{Cr},\text{Mn},\text{Fe},\text{Co},\text{Ni},\text{Cu},\text{Zn}$, and $X=\text{S},\text{Se},\text{Te}$) transition-metal chalcogenophosphates to shed light on their potential role as single-layer van der Waals materials that possess magnetic order. Our ab initio calculations predict that most of these single-layer materials are antiferromagnetic semiconductors. The band gaps of the antiferromagnetic states decrease as the atomic number of the chalcogen atom increases (from S to Se to Te), leading in some cases to half-metallic ferromagnetic states or to nonmagnetic metallic states. We find that the competition between antiferromagnetic and ferromagnetic states can be substantially influenced by gating and by strain engineering. The sensitive interdependence we find between magnetic, structural, and electronic properties establishes the potential of this 2D materials class for applications in spintronics.

We present a systematic study of the variables affecting the electronic and optical properties of two-dimensional (2D) crystals within ab initio $GW$ and $GW$ plus Bethe-Salpeter equation ($GW$-BSE) calculations. As a prototypical 2D transition metal dichalcogenide material, we focus our study on monolayer ${\mathrm{MoS}}_{2}$. We find that the reported variations in $GW$-BSE results in the literature for monolayer ${\mathrm{MoS}}_{2}$ and related systems arise from different treatments of the long-range Coulomb interaction in supercell calculations and convergence of $k$-grid sampling and cutoffs for various quantities such as the dielectric screening. In particular, the quasi-2D nature of the system gives rise to fast spatial variations in the screening environment, which are computationally challenging to resolve. We also show that common numerical treatments to remove the divergence in the Coulomb interaction can shift the exciton continuum leading to false convergence with respect to $k$-point sampling. Our findings apply to $GW$-BSE calculations on any low-dimensional…

Recently, first principle-based predictions of lattice thermal conductivity $\ensuremath{\kappa}$ from perturbation theory have achieved significant success. However, it only includes three-phonon scattering due to the assumption that four-phonon and higher-order processes are generally unimportant. Also, directly evaluating the scattering rates of four-phonon and higher-order processes has been a long-standing challenge. In this work, however, we have developed a formalism to explicitly determine quantum mechanical scattering probability matrices for four-phonon scattering in the full Brillouin zone, and by mitigating the computational challenge we have directly calculated four-phonon scattering rates. We find that four-phonon scattering rates are comparable to three-phonon scattering rates at medium and high temperatures, and they increase quadratically with temperature. As a consequence, $\ensuremath{\kappa}$ of Lennard-Jones argon is reduced by more than 60% at 80 K when four-phonon scattering is included. Also, in less anharmonic materials---diamond, silicon, and…

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

We present a method for rapidly generating efficient $k$-point grids for Brillouin zone integration by using a database of precalculated grids. Benchmark results on 102 randomly selected materials indicate that for well-converged calculations, the grids generated by our method have fewer than half as many irreducible $k$-points as Monkhorst-Pack grids generated using a more conventional method, significantly accelerating the calculation of properties of crystalline materials.

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

Based on first-principles phonon and finite temperature molecular dynamics calculations including spin-orbit coupling, we showed that free-standing single-layer phases of bismuth, namely buckled honeycomb and asymmetric washboard structures named as bismuthene, are stable at high temperature. We studied the atomic structure, mechanical, and electronic properties of these single-layer bismuthene phases and their bilayers. The spin-orbit coupling is found to be crucial in determining lattice constants, phonon frequencies, band gaps, and cohesion. In particular, phonons of 3D hexagonal crystal, as well as those of single-layer bismuthene phases, are softened with spin orbit coupling. By going from 3D hexagonal crystal to free-standing single-layer structures, 2D hexagonal lattice is compressed and semimetal is transformed to semiconductor as a result of confinement effect. On the contrary, by going from single-layer to bilayer bismuthenes, the lattice is slightly expanded and fundamental band gaps are narrowed. Our results reveals that interlayer coupling in multilayer and 3D Bi…

Using the Kubo formalism we have calculated the local dynamic conductivity of a bulk, i.e., three-dimensional (3D), Dirac semimetal (BDS). We obtain that at frequencies lower than Fermi energy the metallic response in a BDS film manifests in the existence of surface-plasmon polaritons, but at higher frequencies the dielectric response is dominated and it occurs that a BDS film behaves as a dielectric waveguide. At this dielectric regime we predict the existence inside a BDS film of novel electromagnetic modes, a 3D analog of the transverse electric waves in graphene. We also find that the dielectric response manifests as the wide-angle passband in the mid-infrared (IR) transmission spectrum of light incident on a BDS film, which can be used for the interferenceless omnidirectional mid-IR filtering. The tuning of the Fermi level of the system allows us to switch between the metallic and the dielectric regimes and to change the frequency range of the predicted modes. This makes BDSs promising materials for photonics and plasmonics.

The layer-dependent structural, electronic, and vibrational properties of ${\mathrm{SnS}}_{2}$ and ${\mathrm{SnSe}}_{2}$ are investigated using first-principles density functional theory (DFT). The in-plane lattice constants, interlayer distances and binding energies are found to be layer-independent. Bulk ${\mathrm{SnS}}_{2}$ and ${\mathrm{SnSe}}_{2}$ are both indirect band gap semiconductors with ${E}_{g}=2.18$ and $1.07\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$, respectively. Few-layer and monolayer 2D systems also possess an indirect band gap, which is increased to 2.41 and $1.69\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ for single layers of ${\mathrm{SnS}}_{2}$ and ${\mathrm{SnSe}}_{2}.$ The effective mass theory of 2D excitons, which takes into account the combined effect of the anisotropy, nonlocal 2D screening and layer-dependent 3D screening, predicts strong excitonic effects. The binding energy of indirect excitons in monolayer samples, ${E}_{x}\ensuremath{\sim}0.9\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$, is substantially reduced to…

Significant advances in coherence render superconducting quantum circuits a viable platform for fault-tolerant quantum computing. To further extend capabilities, highly coherent quantum systems could act as quantum memories for these circuits. A useful quantum memory must be rapidly addressable by Josephson-junction-based artificial atoms, while maintaining superior coherence. We demonstrate a superconducting microwave cavity architecture that is highly robust against major sources of loss that are encountered in the engineering of circuit QED systems. The architecture allows for storage of quantum superpositions in a resonator on the millisecond scale, while strong coupling between the resonator and a transmon qubit enables control, encoding, and readout at MHz rates. This extends the maximum available coherence time attainable in superconducting circuits by almost an order of magnitude compared to earlier hardware. Our design is an ideal platform for studying coherent quantum optics and marks an important step towards hardware-efficient quantum computing in…

Single crystals of the Kitaev spin-liquid candidate $\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$ have been studied to determine the low-temperature bulk properties, the structure, and the magnetic ground state. Refinements of x-ray diffraction data show that the low-temperature crystal structure is described by space group $C2/m$ with a nearly perfect honeycomb lattice exhibiting less than 0.2% in-plane distortion. The as-grown single crystals exhibit only one sharp magnetic transition at ${T}_{N}=7$ K. The magnetic order below this temperature exhibits a propagation vector of $k=(0,1,1/3)$, which coincides with a three-layer stacking of the $C2/m$ unit cells. Magnetic transitions at higher temperatures up to 14 K can be introduced by deformations of the crystal that result in regions in the crystal with a two-layer stacking sequence. The best-fit symmetry-allowed magnetic structure of the as-grown crystals shows that the spins lie in the $ac$ plane, with a zigzag configuration in each honeycomb layer. The three-layer repeat out-of-plane structure can be refined as a…

We experimentally demonstrate magnon Kerr effect in a cavity-magnon system where magnons in a small yttrium iron garnet (YIG) sphere are strongly but dispersively coupled to the photons in a three-dimensional cavity. When the YIG sphere is pumped to generate considerable magnons, the Kerr effect yields a perceptible shift of the cavity's central frequency and more appreciable shifts of the magnon modes. We derive an analytical relation between the magnon frequency shift and the drive power for the uniformly magnetized YIG sphere and find that it agrees very well with the experimental results of the Kittel mode. Our study paves the way to explore nonlinear effects in the cavity-magnon system.

We construct a systematic high-frequency expansion for periodically driven quantum systems based on the Brillouin-Wigner (BW) perturbation theory, which generates an effective Hamiltonian on the projected zero-photon subspace in the Floquet theory, reproducing the quasienergies and eigenstates of the original Floquet Hamiltonian up to desired order in $1/\ensuremath{\omega}$, with $\ensuremath{\omega}$ being the frequency of the drive. The advantage of the BW method is that it is not only efficient in deriving higher-order terms, but even enables us to write down the whole infinite series expansion, as compared to the van Vleck degenerate perturbation theory. The expansion is also free from a spurious dependence on the driving phase, which has been an obstacle in the Floquet-Magnus expansion. We apply the BW expansion to various models of noninteracting electrons driven by circularly polarized light. As the amplitude of the light is increased, the system undergoes a series of Floquet topological-to-topological phase transitions, whose phase boundary in the high-frequency regime is…

Oxygen vacancy $({V}_{\mathrm{O}})$ is a common native point defect that plays crucial roles in determining the physical and chemical properties of metal oxides such as ZnO. However, fundamental understanding of ${V}_{\mathrm{O}}$ is still very sparse. Specifically, whether ${V}_{\mathrm{O}}$ is mainly responsible for the $n$-type conductivity in ZnO has been still unsettled in the past 50 years. Here, we report on a study of oxygen self-diffusion by conceiving and growing oxygen-isotope ZnO heterostructures with delicately controlled chemical potential and Fermi level. The diffusion process is found to be predominantly mediated by ${V}_{\mathrm{O}}$. We further demonstrate that, in contrast to the general belief of their neutral attribute, the oxygen vacancies in ZnO are actually $+2$ charged and thus responsible for the unintentional $n$-type conductivity as well as the nonstoichiometry of ZnO. The methodology can be extended to study oxygen-related point defects and their energetics in other technologically important oxide materials.

A topological nodal-line semimetal is a state of matter with one-dimensional bulk nodal lines and two-dimensional so-called drumhead surface bands. Based on first-principles calculations and an effective $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{p}$ model, we theoretically propose the existence of topological nodal-line fermions in the ternary transition-metal chalcogenide ${\mathrm{TlTaSe}}_{2}$. The noncentrosymmetric structure and strong spin-orbit coupling give rise to spinful nodal-line bulk states which are protected by a mirror reflection symmetry of this compound. This is remarkably distinguished from other proposed nodal-line semimetals such as ${\mathrm{Cu}}_{3}\mathrm{NPb}$(Zn) in which the nodal line exists only in the limit of vanishing spin-orbit coupling and thus is not as robust. In addition, we show that the drumhead surface states in ${\mathrm{TlTaSe}}_{2}$, which are associated with the topological nodal lines, exhibit an unconventional chiral spin texture and an exotic Lifshitz transition as a consequence of the linkage among multiple…

Machine learning techniques are applied to make prediction models of the ${G}_{0}{W}_{0}$ band gaps for 270 inorganic compounds using Kohn-Sham (KS) band gaps, cohesive energy, crystalline volume per atom, and other fundamental information of constituent elements as predictors. Ordinary least squares regression (OLSR), least absolute shrinkage and selection operator, and nonlinear support vector regression (SVR) methods are applied with two levels of predictor sets. When the KS band gap by generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE) or modified Becke-Johnson (mBJ) is used as a single predictor, the OLSR model predicts the ${G}_{0}{W}_{0}$ band gap of randomly selected test data with the root-mean-square error (RMSE) of 0.59 eV. When KS band gap by PBE and mBJ methods are used together with a set of predictors representing constituent elements and compounds, the RMSE decreases significantly. The best model by SVR yields the RMSE of 0.24 eV. Band gaps estimated in this way should be useful as predictors for virtual screening of a large set of materials.

A key challenge in fabrication of superconductor (S)-semiconductor (Sm) hybrid devices is forming highly transparent contacts between the active electrons in the semiconductor and the superconducting metal. In this work, we show that a near perfect interface and a highly transparent contact can be achieved using epitaxial growth of aluminum on an InAs two-dimensional electron system. We demonstrate that this material system, Al-InAs, satisfies all the requirements necessary to reach into the topological superconducting regime by individual characterization of the semiconductor two-dimensional electron system, superconductivity of Al, and performance of S-Sm-S junctions. This exciting development might lead to a number of useful applications ranging from spintronics to quantum computing.

In recently predicted type-II Weyl semimetals, the Weyl states connect hole and electron bands that would otherwise be separated by an indirect gap. The set of points in the momentum space at which both bands touch are called Weyl points and are connected by Fermi arcs at the surface of the sample. Experimental evidence confirmed existence of these exotic fermions in MoTe${}_{2}$ and Mo${}_{x}$W${}_{1-x}$Te${}_{2}$. Here, the authors show that such states also exist in pure WTe${}_{2}$ and are very sensitive to strain. While flat undistorted portions of the sample show well defined Fermi arcs -- a clear signature of Weyl fermions -- strain due to bulking of the surface in other locations suppresses this state and destroys the Weyl state. Such high sensitivity to strain provides a simple way to tune properties of Weyl Fermions and explore the physics of these unusual objects.

The discovery of new spin-to-charge current conversion effects such as the spin Hall effect (SHE) is expanding the potential of spintronic applications. In contrast to the anomalous Hall effect (AHE), a systematic experimental study of the different mechanisms contributing to the SHE is lacking. Finding routes to maximize the SHE is not possible as long as it remains unclear which is the dominant mechanism in a material. The authors systematically study Pt, the prototypical SHE material, using the spin absorption method and find a single intrinsic spin Hall conductivity in a wide range of Pt conductivities, in good agreement with theory. By tuning the conductivity, they observe for the first time the crossover between the moderately dirty and the superclean scaling regimes of the SHE, equivalent to that obtained for the AHE. These results explain the dispersion of values in the literature and show a clear path to enhance this important effect.

Many-body localization is characterized by a slow logarithmic growth of the entanglement entropy after a global quantum quench while the local memory of an initial density imbalance remains at infinite time. We investigate how much the proximity of a many-body localized phase can influence the dynamics in the delocalized ergodic regime where thermalization is expected. Using an exact Krylov space technique, the out-of-equilibrium dynamics of the random-field Heisenberg chain is studied up to $L=28$ sites, starting from an initially unentangled high-energy product state. Within most of the delocalized phase, we find a sub-ballistic entanglement growth $S(t)\ensuremath{\propto}{t}^{1/z}$ with a disorder-dependent exponent $z\ensuremath{\ge}1$, in contrast with the pure ballistic growth $z=1$ of clean systems. At the same time, anomalous relaxation is also observed for the spin imbalance $\mathcal{I}(t)\ensuremath{\propto}{t}^{\ensuremath{-}\ensuremath{\zeta}}$ with a continuously varying disorder-dependent exponent $\ensuremath{\zeta}$, vanishing at the transition. This provides a…

A joint experimental and theoretical study is presented for the electronic structures of copper oxides including $\mathrm{C}{\mathrm{u}}_{2}\mathrm{O}$, CuO, and the metastable mixed-valence oxide $\mathrm{C}{\mathrm{u}}_{4}{\mathrm{O}}_{3}$. The optical band gap is determined by experimental optical absorption coefficient, and the electronic structure in valence and conduction bands is probed by photoemission and electron energy loss spectroscopies, respectively. The experimental results are compared with many-body $GW$ calculations utilizing an additional on-site potential for $d$-orbital energies that facilitates tractable and predictive computations. The side-by-side comparison between the three oxides, including a band insulator $(\mathrm{C}{\mathrm{u}}_{2}\mathrm{O})$ and two Mott/charge-transfer insulators (CuO, $\mathrm{C}{\mathrm{u}}_{4}{\mathrm{O}}_{3})$ leads to a consistent picture for the optical and band-structure properties of the Cu oxides, strongly supporting indirect band gaps of about 1.2 and 0.8 eV in CuO and $\mathrm{C}{\mathrm{u}}_{4}{\mathrm{O}}_{3}$,…

The application of Cartesian multipoles in irreducible representations provides the possibility to get explicit contributions of the toroidal multipole terms in the extinction and scattering power without the introduction of special form factors. In the framework of the Cartesian multipoles, we obtained multipole decomposition (up to the third order) of the induced polarization (current) inside an arbitrarily shaped scatterer (nanoparticle). The third-order decomposition includes the toroidal dipole, magnetic quadrupole, electric octupole terms, and also nonradiating terms. The corresponding multipole decomposition of the scattering cross section, taking into account the electric octupole term, is derived and compared with the multipole decomposition of the extinction cross section obtained using the optical theorem. We show that the role of multipoles in the optical theorem (light extinction) and scattering by arbitrarily shaped nanoparticles can be different. This can result in seemingly paradoxical conclusions with respect to the appearance of multipole contributions in the…

The carrier mobility of anisotropic two-dimensional semiconductors under longitudinal acoustic phonon scattering was theoretically studied using deformation potential theory. Based on the Boltzmann equation with the relaxation time approximation, an analytic formula of intrinsic anisotropic mobility was derived, showing that the influence of effective mass on mobility anisotropy is larger than those of deformation potential constant or elastic modulus. Parameters were collected for various anisotropic two-dimensional materials (black phosphorus, Hittorf's phosphorus, ${\mathrm{BC}}_{2}\mathrm{N}$, MXene, ${\mathrm{TiS}}_{3}$, and ${\mathrm{GeCH}}_{3}$) to calculate their mobility anisotropy. It was revealed that the anisotropic ratio is overestimated by the previously described method.

Recent work has shown that a variety of novel phases of matter arise in periodically driven Floquet systems. Among these are many-body localized phases which spontaneously break global symmetries and exhibit novel multiplets of Floquet eigenstates separated by quantized quasienergies. Here we show that these properties are stable to all weak local deformations of the underlying Floquet drives---including those that explicitly break the defining symmetries---and that the models considered until now occupy submanifolds within these larger ``absolutely stable'' phases. While these absolutely stable phases have no explicit global symmetries, they spontaneously break Hamiltonian-dependent emergent symmetries, and thus continue to exhibit the novel multiplet structure. The multiplet structure in turn encodes characteristic oscillations of the emergent order parameter at multiples of the fundamental period. Altogether these phases exhibit a form of simultaneous long-range order in space and time which is new to quantum systems. We describe how this spatiotemporal order can be detected in…

We develop a linear-response transport theory of diffusive spin and heat transport by magnons in magnetic insulators with metallic contacts. The magnons are described by a position-dependent temperature and chemical potential that are governed by diffusion equations with characteristic relaxation lengths. Proceeding from a linearized Boltzmann equation, we derive expressions for length scales and transport coefficients. For yttrium iron garnet (YIG) at room temperature we find that long-range transport is dominated by the magnon chemical potential. We compare the model's results with recent experiments on YIG with Pt contacts [L. J. Cornelissen et al., Nat. Phys. 11, 1022 (2015)] and extract a magnon spin conductivity of ${\ensuremath{\sigma}}_{m}=5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ S/m. Our results for the spin Seebeck coefficient in YIG agree with published experiments. We conclude that the magnon chemical potential is an essential ingredient for energy and spin transport in magnetic insulators.

Proximity orbital and spin-orbital effects of graphene on monolayer transition-metal dichalcogenides (TMDCs) are investigated from first-principles. The Dirac band structure of graphene is found to lie within the semiconducting gap of TMDCs for sulfides and selenides, while it merges with the valence band for tellurides. In the former case, the proximity-induced staggered potential gaps and spin-orbit couplings (all on the meV scale) of the Dirac electrons are established by fitting to a phenomenological effective Hamiltonian. While graphene on ${\mathrm{MoS}}_{2}$, ${\mathrm{MoSe}}_{2}$, and ${\mathrm{WS}}_{2}$ has a topologically trivial band structure, graphene on ${\mathrm{WSe}}_{2}$ exhibits inverted bands. Using a realistic tight-binding model we find topologically protected helical edge states for graphene zigzag nanoribbons on ${\mathrm{WSe}}_{2}$, demonstrating the quantum spin Hall effect. This model also features ``half-topological states,'' which are protected against time-reversal disorder on one edge only.

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

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

The magnetic structure and phase diagram of the layered ferromagnetic compound ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$ has been investigated by a combination of synthesis, x-ray and neutron diffraction, high-resolution microscopy, and magnetization measurements. Single crystals were synthesized by self-flux reactions, and single-crystal neutron diffraction finds ferromagnetic order with moments of $1.11(5){\ensuremath{\mu}}_{B}/\mathrm{Fe}$ aligned along the $c$ axis at 4 K. These flux-grown crystals have a lower Curie temperature ${T}_{\text{c}}\ensuremath{\approx}150$ K compared to crystals previously grown by vapor transport (${T}_{\text{c}}=220$ K). The difference is a reduced Fe content in the flux-grown crystals, as illustrated by the behavior observed in a series of polycrystalline samples. As Fe content decreases, so does the Curie temperature, magnetic anisotropy, and net magnetization. In addition, Hall effect and thermoelectric measurements on flux-grown crystals suggest multiple carrier types contribute to electrical transport in…

Using first-principles calculation and symmetry analysis, we propose that $\ensuremath{\theta}$-TaN is a topological semimetal having a new type of point nodes, i.e., triply degenerate nodal points. Each node is a band crossing between degenerate and nondegenerate bands along the high-symmetry line in the Brillouin zone, and is protected by crystalline symmetries. Such new type of nodes will always generate singular touching points between different Fermi surfaces and three-dimensional spin texture around them. Breaking the crystalline symmetry by external magnetic field or strain leads to various topological phases. By studying the Landau levels under a small field along the $c$ axis, we demonstrate that the system has a new quantum anomaly that we call ``helical anomaly.'

The family of two-dimensional transition metal carbides, so called MXenes, has recently found new members with ordered double transition metals ${M}_{2}^{\ensuremath{'}}{M}^{\ensuremath{''}}{\mathrm{C}}_{2}$, where ${M}^{\ensuremath{'}}$ and ${M}^{\ensuremath{''}}$ stand for transition metals. Here, using a set of first-principles calculations, we demonstrate that some of the newly added members, oxide ${M}_{2}^{\ensuremath{'}}{M}^{\ensuremath{''}}{\mathrm{C}}_{2}$ (${M}^{\ensuremath{'}}=\mathrm{Mo}$, W; ${M}^{\ensuremath{''}}=\mathrm{Ti}$, Zr, Hf) MXenes, are topological insulators. The nontrivial topological states of the predicted MXenes are revealed by the ${Z}_{2}$ index, which is evaluated from the parities of the occupied bands below the Fermi energy at time reversal invariant momenta, and also by the presence of the edge states. The predicted ${M}_{2}^{\ensuremath{'}}{M}^{\ensuremath{''}}{\mathrm{C}}_{2}{\mathrm{O}}_{2}$ MXenes show nontrivial gaps in the range of 0.041--0.285 eV within the generalized gradient approximation and 0.119--0.409 eV within the hybrid functional.…

Most spintronic devices share two features: they utilize spin-orbit coupling and they contain interfaces. While bulk spin-orbit effects are thought to be well described by phenomenological theories, interfacial spin-orbit effects are not. A theory that could describe interfacial spin-orbit effects would be useful in analyzing experiments on heavy-metal ferromagnet bilayers, which are a key feature of potential energy-efficient implementations of MRAM. To develop such a theory, the authors present the boundary conditions needed for drift-diffusion models to treat interfaces with spin-orbit coupling. Together with the drift-diffusion equations, these boundary conditions give an analytical model of spin-orbit torques caused by both the spin Hall and Rashba-Edelstein effects. A key feature of these boundary conditions is that they capture spin currents generated by interfacial spin-orbit scattering. The authors validate this phenomenological approach by comparing the results with those obtained by solving the spin-dependent Boltzmann equation. They discuss the interpretation of current…

We propose a kind of topological quantum state of semimetals in the quasi-one-dimensional (1D) crystal family ${\text{Ba}MX}_{3}$ ($M\phantom{\rule{0.28em}{0ex}}=\phantom{\rule{0.28em}{0ex}}\mathrm{V}$, Nb, or Ta; $X\phantom{\rule{0.28em}{0ex}}=\phantom{\rule{0.28em}{0ex}}\mathrm{S}$ or Se) by using symmetry analysis and first-principles calculation. We find that in ${\mathrm{BaVS}}_{3}$ the valence and conduction bands are degenerate in the ${k}_{z}=\ensuremath{\pi}/c$ plane ($c$ is the lattice constant along the $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{z}$ axis) of the Brillouin zone (BZ). These nodal points form a node surface, and they are protected by a nonsymmorphic crystal symmetry consisting of a twofold rotation about the $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{z}$ axis and a half-translation along the same $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{z}$ axis. The band degeneracy in the node surface is lifted in ${\mathrm{BaTaS}}_{3}$ by including strong spin-orbit coupling (SOC) of Ta. The node surface is reduced into 1D node lines along the high-symmetry paths…

Recently, a new ``type-II'' Weyl fermion, which exhibits exotic phenomena, such as an angle-dependent chiral anomaly, was discovered in a new phase of matter where electron and hole pockets contact at isolated Weyl points [Nature (London) 527, 495 (2015)]. This raises an interesting question about whether its counterpart, i.e., a type-II Dirac fermion, exists in real materials. Here, we predict the existence of symmetry-protected type-II Dirac fermions in a class of transition metal dichalcogenide materials. Our first-principles calculations on ${\mathrm{PtSe}}_{2}$ reveal its bulk type-II Dirac fermions which are characterized by strongly tilted Dirac cones, novel surface states, and exotic doping-driven Lifshitz transition. Our results show that the existence of type-II Dirac fermions in ${\mathrm{PtSe}}_{2}$-type materials is closely related to its structural $P\overline{3}m1$ symmetry, which provides useful guidance for the experimental realization of type-II Dirac fermions and intriguing physical properties distinct from those of the standard Dirac fermions known before.

We present density functional theory calculations of the phonon-limited mobility in $n$-type monolayer graphene, silicene, and ${\mathrm{MoS}}_{2}$. The material properties, including the electron-phonon interaction, are calculated from first principles. We provide a detailed description of the normalized full-band relaxation time approximation for the linearized Boltzmann transport equation (BTE) that includes inelastic scattering processes. The bulk electron-phonon coupling is evaluated by a supercell method. The method employed is fully numerical and does therefore not require a semianalytic treatment of part of the problem and, importantly, it keeps the anisotropy information stored in the coupling as well as the band structure. In addition, we perform calculations of the low-field mobility and its dependence on carrier density and temperature to obtain a better understanding of transport in graphene, silicene, and monolayer ${\mathrm{MoS}}_{2}$. Unlike graphene, the carriers in silicene show strong interaction with the out-of-plane modes. We find that graphene has more than an…

Using a set of first-principles calculations, we studied the electronic structures of two-dimensional transition metal carbides and nitrides, so called MXenes, functionalized with F, O, and OH. Our projected band structures and electron localization function analyses reveal the existence of nearly free electron (NFE) states in a variety of MXenes. The NFE states are spatially located just outside the atomic structure of MXenes and are extended parallel to the surfaces. Moreover, we found that the OH-terminated MXenes offer the NFE states energetically close to the Fermi level. In particular, the NFE states in some of the OH-terminated MXenes, such as $\mathrm{T}{\mathrm{i}}_{2}\mathrm{C}{(\mathrm{OH})}_{2},\mathrm{Z}{\mathrm{r}}_{2}\mathrm{C}{(\mathrm{OH})}_{2},\mathrm{Z}{\mathrm{r}}_{2}\mathrm{N}{(\mathrm{OH})}_{2},\mathrm{H}{\mathrm{f}}_{2}\mathrm{C}{(\mathrm{OH})}_{2},\mathrm{H}{\mathrm{f}}_{2}\mathrm{N}{(\mathrm{OH})}_{2},\mathrm{N}{\mathrm{b}}_{2}\mathrm{C}{(\mathrm{OH})}_{2}$, and $\mathrm{T}{\mathrm{a}}_{2}\mathrm{C}{(\mathrm{OH})}_{2}$, are partially occupied. This is in…

Magnetic skyrmions have attracted considerable attention recently for their huge potential in spintronic applications. Generally skyrmions are big compared to the atomic lattice constant, which allows for the Ginzburg-Landau type description in the continuum limit. Such a description successfully captures the main experimental observations on skyrmions in B20 compound without inversion symmetry. Skyrmions can also exist in inversion-symmetric magnets with competing interactions. Here, we derive a general Ginzburg-Landau theory for skyrmions in these magnets valid in the long-wavelength limit. We study the unusual static and dynamical properties of skyrmions based on the derived Ginzburg-Landau theory. We show that an easy axis spin anisotropy is sufficient to stabilize a skyrmion lattice. Interestingly, the skyrmion in inversion-symmetric magnets has a new internal degree of freedom associated with the rotation of helicity, i.e., the ``spin'' of the skyrmion as a particle, in addition to the usual translational motion of skyrmions (orbital motion). The orbital and spin degree of…

We report a Raman scattering study of six rare-earth orthoferrites $R{\mathrm{FeO}}_{3}$, with $R$ = La, Sm, Eu, Gd, Tb, Dy. The use of extensive polarized Raman scattering of ${\mathrm{SmFeO}}_{3}$ and first-principles calculations enable the assignment of the observed phonon modes to vibrational symmetries and atomic displacements. The assignment of the spectra and their comparison throughout the whole series allow correlating the phonon modes with the orthorhombic structural distortions of $R{\mathrm{FeO}}_{3}$ perovskites. In particular, the positions of two specific $A{}_{g}$ modes scale linearly with the two ${\mathrm{FeO}}_{6}$ octahedra tilt angles, allowing the distortion to be tracked throughout the series. At variance with literature, we find that the two octahedra tilt angles scale differently with the vibration frequencies of their respective $A{}_{g}$ modes. This behavior, as well as the general relations between the tilt angles, the frequencies of the associated modes, and the ionic radii are rationalized in a simple Landau model. The reported Raman spectra and…

We report the energetics and properties of the $\ensuremath{\beta}, \ensuremath{\alpha}$, R, $\ensuremath{\gamma}, \ensuremath{\lambda}$, and $\ensuremath{\delta}$ polymorphs of $\mathrm{MnO}{}_{2}$ within density functional theory, comparing the performance of the recently introduced SCAN functional with that of conventional exchange-correlation functionals and experiment. We find that SCAN uniquely yields accurate formation energies and properties across all $\mathrm{MnO}{}_{2}$ polymorphs. We explain the superior performance of SCAN based on its satisfaction of all known constraints appropriate to a semilocal exchange-correlation functional and its accurate representation of all types of orbital overlap.

$\ensuremath{\alpha}{\text{-RuCl}}_{3}$ has been proposed recently as an excellent playground for exploring Kitaev physics on a two-dimensional (2D) honeycomb lattice. However, structural clarification of the compound has not been completed, which is crucial in understanding the physics of this system. Here, using ab initio electronic structure calculations, we study a full three-dimensional (3D) structure of $\ensuremath{\alpha}{\text{-RuCl}}_{3}$, including the effects of spin-orbit coupling (SOC) and electronic correlations. The three major results are as follows: (i) SOC suppresses dimerization of Ru atoms, which exists in other Ru compounds such as isostructural ${\mathrm{Li}}_{2}{\text{RuO}}_{3}$, and makes the honeycomb closer to an ideal one. (ii) The nearest-neighbor Kitaev exchange interaction between the ${j}_{\mathrm{eff}}=1/2$ pseudospin strongly depends on the Ru-Ru distance and the Cl position, originating from the nature of the edge-sharing geometry. (iii) The optimized 3D structure without electronic correlations has $P\overline{3}1m$ space-group symmetry…

We introduce a topological quantum number---coined dynamical topological order parameter (DTOP)---that is dynamically defined in the real-time evolution of a quantum many-body system and represented by a momentum space winding number of the Pancharatnam geometric phase. Our construction goes conceptually beyond the standard notion of topological invariants characterizing the wave function of a system, which are constants of motion under coherent time evolution. In particular, we show that the DTOP can change its integer value at discrete times where so called dynamical quantum phase transitions occur, thus serving as a dynamical analog of an order parameter. Interestingly, studying quantum quenches in one-dimensional two-banded Bogoliubov--de Gennes models, we find that the DTOP is capable of resolving if the topology of the system Hamiltonian has changed over the quench. Furthermore, we investigate the relation of the DTOP to the dynamics of the string order parameter that characterizes the topology of such systems in thermal equilibrium.

A Boltzmann machine is a stochastic neural network that has been extensively used in the layers of deep architectures for modern machine learning applications. In this paper, we develop a Boltzmann machine that is capable of modeling thermodynamic observables for physical systems in thermal equilibrium. Through unsupervised learning, we train the Boltzmann machine on data sets constructed with spin configurations importance sampled from the partition function of an Ising Hamiltonian at different temperatures using Monte Carlo (MC) methods. The trained Boltzmann machine is then used to generate spin states, for which we compare thermodynamic observables to those computed by direct MC sampling. We demonstrate that the Boltzmann machine can faithfully reproduce the observables of the physical system. Further, we observe that the number of neurons required to obtain accurate results increases as the system is brought close to criticality.

Resonant photon modes of a 5-mm-diameter yttrium iron garnet (YIG) sphere loaded in a cylindrical cavity in the 10--30-GHz frequency range are characterized as a function of applied dc magnetic field at millikelvin temperatures. The photon modes are confined mainly to the sphere and exhibited large mode filling factors in comparison to previous experiments, allowing ultrastrong coupling with the magnon spin-wave resonances. The largest observed coupling between photons and magnons is $2g/2\ensuremath{\pi}=7.11$ GHz for a 15.5-GHz mode, corresponding to a cooperativity of $C=1.51\ifmmode\pm\else\textpm\fi{}0.47\ifmmode\times\else\texttimes\fi{}{10}^{7}$. Complex modifications, beyond a simple multioscillator model, of the photon mode frequencies were observed between 0 and 0.1 T. Between 0.4 and 1 T, degenerate resonant photon modes were observed to interact with magnon spin-wave resonances with different coupling strengths, indicating time-reversal symmetry breaking due to the gyrotropic permeability of YIG. Bare dielectric resonator mode frequencies were determined by detuning…

It has been argued that stanene has lowest lattice thermal conductivity among two-dimensional (2D) group-IV materials because of its largest atomic mass, weakest interatomic bonding, and enhanced ZA phonon scattering due to the breaking of an out-of-plane symmetry selection rule. However, we show that, although the lattice thermal conductivity $\ensuremath{\kappa}$ for graphene, silicene, and germanene decreases monotonically with decreasing Debye temperature, unexpected higher $\ensuremath{\kappa}$ is observed in stanene. By enforcing all the invariance conditions in 2D materials and including Ge $3d$ and Sn $4d$ electrons as valence electrons for germanene and stanene, respectively, the lattice dynamics in these materials are accurately described. A large acoustic-optical gap and the bunching of the acoustic-phonon branches significantly reduce phonon scattering in stanene, leading to higher thermal conductivity than germanene. The vibrational origin of the acoustic-optical gap can be attributed to the buckled structure. Interestingly, a buckled system has two competing influences…

A magnetic skyrmionium is a nontopological soliton, which has a doughnut-like out-of-plane spin texture in thin films, and can be phenomenologically viewed as a coalition of two topological magnetic skyrmions with opposite topological numbers. Due to its zero topological number ($Q=0$) and doughnut-like structure, the skyrmionium has its distinctive characteristics as compared to the skyrmion with $Q=\ifmmode\pm\else\textpm\fi{}1$. Here we systematically study the generation, manipulation, and motion of a skyrmionium in ultrathin magnetic nanostructures by applying a magnetic field or a spin-polarized current. It is found that the skyrmionium moves faster than the skyrmion when they are driven by the out-of-plane current, and their velocity difference is proportional to the driving force. However, the skyrmionium and skyrmion exhibit an identical current-velocity relation when they are driven by the in-plane current. It is also found that a moving skyrmionium is less deformed in the current-in-plane geometry compared with the skyrmionium in the current-perpendicular-to-plane…

Mode-dependent phonon and electron transport properties in Al, Ag, and Au are predicted using density functional theory and lattice dynamics calculations. The predicted thermal conductivities, electrical conductivities, electron-phonon coupling coefficients, and electron-phonon mass enhancement parameters are in agreement with experimental measurements. At a temperature of 100 K, the phonon contribution to the total thermal conductivity of Al is 5% in bulk and increases to 15% for a 50 nm thick film. In all three metals, phonons with mean free paths between 1 and 10 nm are the dominate contributors to the thermal conductivity at a temperature of 300 K, while the relevant electron mean free paths are 10--100 nm. Despite similar atomic masses, the phonon thermal conductivity of Al is an order of magnitude smaller than that of silicon due to a larger three-phonon phase space and stronger anharmonicity. These results will impact the interpretation of thermoreflectance experiments that can resolve carrier-level contributions to thermal conductivity.

We report charged exciton (trion) formation dynamics in doped monolayer transition metal dichalcogenides, specifically molybdenum diselenide $(\mathrm{MoS}{\mathrm{e}}_{2})$, using resonant two-color pump-probe spectroscopy. When resonantly pumping the exciton transition, trions are generated on a picosecond time scale through exciton-electron interaction. As the pump energy is tuned from the high energy to low energy side of the inhomogeneously broadened exciton resonance, the trion formation time increases by $\ensuremath{\sim}50%$. This feature can be explained by the existence of both localized and delocalized excitons in a disordered potential and suggests the existence of an exciton mobility edge in transition metal dichalcogenides.

In metallic condensed matter systems two different types of Weyl fermions can in principle emerge, with either a vanishing (type-I) or with a finite (type-II) density of states at the Weyl node energy. So far only ${\mathrm{WTe}}_{2}$ and ${\mathrm{MoTe}}_{2}$ were predicted to be type-II Weyl semimetals. Here we identify ${\mathrm{TaIrTe}}_{4}$ as a third member of this family of topological semimetals. ${\mathrm{TaIrTe}}_{4}$ has the attractive feature that it hosts only four well-separated Weyl points, the minimum imposed by symmetry. Moreover, the resulting topological surface states---Fermi arcs connecting Weyl nodes of opposite chirality---extend to about 1/3 of the surface Brillouin zone. This large momentum-space separation is very favorable for detecting the Fermi arcs spectroscopically and in transport experiments.

Based on first-principles calculations and an effective Hamiltonian analysis, we systematically investigate the electronic and topological properties of alkaline-earth compounds $A{X}_{2}$ $(A=\text{Ca}$, Sr, Ba; $X=\text{Si}$, Ge, Sn). Taking ${\mathrm{BaSn}}_{2}$ as an example, we find that when spin-orbit coupling is ignored, these materials are three-dimensional topological nodal-line semimetals characterized by a snakelike nodal loop in three-dimensional momentum space. Drumheadlike surface states emerge either inside or outside the loop circle on the (001) surface depending on surface termination, while complicated double-drumhead-like surface states appear on the (010) surface. When spin-orbit coupling is included, the nodal line is gapped and the system becomes a topological insulator with ${\mathbb{Z}}_{2}$ topological invariants (1;001). Since spin-orbit coupling effects are weak in light elements, the nodal-line semimetal phase is expected to be achievable in some alkaline-earth germanides and silicides.

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

Unlike the local density approximation (LDA) and the generalized gradient approximation (GGA), calculations with meta-generalized gradient approximations (meta-GGA) are usually done according to the generalized Kohn-Sham (gKS) formalism. The exchange-correlation potential of the gKS equation is nonmultiplicative, which prevents systematic comparison of meta-GGA band structures to those of the LDA and the GGA. We implement the optimized effective potential (OEP) of the meta-GGA for periodic systems, which allows us to carry out meta-GGA calculations in the same KS manner as for the LDA and the GGA. We apply the OEP to several meta-GGAs, including the new SCAN functional [Phys. Rev. Lett. 115, 036402 (2015)]. We find that the KS gaps and KS band structures of meta-GGAs are close to those of GGAs. They are smaller than the more realistic gKS gaps of meta-GGAs, but probably close to the less-realistic gaps in the band structure of the exact KS potential, as can be seen by comparing with the gaps of the EXX+RPA OEP potential. The well-known grid sensitivity of meta-GGAs is much more…

Single-layer transition metal dichalcogenides exhibit a variety of atomic structures and associated exotic electronic and magnetic properties. Density-functional calculations using the LDA+$U$ approximation show that single-layer ${\mathrm{VS}}_{2}$ is a strongly correlated material, where the stability, phonon spectra, and magnetic moments of the octahedral ($1T$) and the trigonal prismatic ($2H$) structures significantly depend on the effective Hubbard $U$ parameter, ${U}_{\mathrm{eff}}$. Comparison with the HSE06 hybrid density functional used as a benchmark indicates that ${U}_{\mathrm{eff}}=2.5$ eV, which consistently shows that the $2H$ structure is more stable than the $1T$ structure and a ferromagnetic semiconductor. The magnetic moments are localized on the V atoms and coupled ferromagnetically due to the superexchange interactions mediated by the S atoms. Calculations of the magnetic anisotropy show an easy plane for the magnetic moment. Assuming a classical $\mathit{XY}$ model with nearest neighbor coupling, we determine the critical temperature, ${T}_{\mathrm{c}}$, for…

We consider topological phases in periodically driven (Floquet) systems exhibiting many-body localization, protected by a symmetry $G$. We argue for a general correspondence between such phases and topological phases of undriven systems protected by symmetry $\mathbb{Z}\ensuremath{\rtimes}G$ where the additional $\mathbb{Z}$ accounts for the discrete time-translation symmetry. Thus, for example, the bosonic phases in $d$ spatial dimensions without intrinsic topological order [symmetry-protected topological (SPT) phases] are classified by the cohomology group ${H}^{d+1}[\mathbb{Z}\ensuremath{\rtimes}G,\mathrm{U}(1)]$. For unitary symmetries, we interpret the additional resulting Floquet phases in terms of the lower-dimensional SPT phases that are pumped to the boundary during one time step. These results also imply the existence of novel symmetry-enriched topological (SET) orders protected solely by the periodicity of the drive.

Within the framework of density functional theory and Hedin's $GW$ approximation for single-particle excitations, we present quasiparticle band structures and densities of states for two gallium oxide polymorphs: rhombohedral $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ and monoclinic $\ensuremath{\beta}\ensuremath{-}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$. The gap problem is attacked. In addition, their electron effective mass tensors are given. Solving the Bethe-Salpeter equation we also calculate excitonic optical spectra of the two polymorphs. The treatment of excitonic effects allows for a trustable prediction of optical properties from the band gap to the ultraviolet region. In addition, for few other polymorphs we also discuss the frequency-dependent dielectric tensor within the independent-particle approximation (random phase approximation) and densities of states on density functional level. We demonstrate that apart from subtle details, the overall densities of states and optical spectra, in particular the isotropically averaged spectra, are rather…

Spin transport at interfaces between nonmagnets and ferromagnets plays an important role in spintronic devices. Lately, there is an increasing suspicion that spin-orbit coupling, which couples the spin and momentum of carriers, might contribute significantly to this process. Unfortunately, the existing description of spin transport at such interfaces, magnetoelectronic circuit theory, is not valid when spin-orbit coupling is present at the interface. This paper presents a generalization of magnetoelectronic circuit theory to interfaces with spin-orbit coupling. Like the original theory, this generalization describes spin transport in terms of drops in spin and charge accumulations across the interface, but also includes responses to in-plane electric fields and offsets in spin accumulations. The most important result is a description of the way in-plane electric fields generate spin accumulations, spin currents, and torques at the interface. The effects described by this generalized circuit theory impact the interpretation of experiments involving spin-orbit torques, spin pumping,…

WTe${}_{2}$ is well known for its manifestation of anomalously large magnetoresistance. It has recently attracted much attention because it is theoretically predicted to be the first material candidate that may realize the ``type-II'' Weyl state. This work reports combined experimental and theoretical investigations on the electronic structure of WTe${}_{2}$. Taking advantage of the latest-generation laser-based angle-resolved photoemission (ARPES) system with superior instrumental resolution, a complete picture of the electronic structure of WTe${}_{2}$ is revealed. The existence of a surface state that connects the bulk electron and hole pockets is identified. High-temperature ARPES measurements make it possible to reveal electronic states above the Fermi level where the Weyl points are predicted to be located. The observed connection of the surface state with the bulk bands, its momentum evolution, and its momentum and energy locations, are all in good agreement with the calculated band structures. These results provide key information to understand the anomalous transport…

We perform ab initio calculations of the coupling between electrons and small-momentum polar-optical phonons in monolayer transition-metal dichalcogenides of the $2H$ type: ${\mathrm{MoS}}_{2},\phantom{\rule{0.28em}{0ex}}{\mathrm{MoSe}}_{2},\phantom{\rule{0.28em}{0ex}}{\mathrm{MoTe}}_{2},\phantom{\rule{0.28em}{0ex}}{\mathrm{WS}}_{2}$, and ${\mathrm{WSe}}_{2}$. The polar-optical coupling with longitudinal optical phonons, or Fr\"ohlich interaction, is fundamentally affected by the dimensionality of the system. In a plane-wave framework with periodic boundary conditions, the Fr\"ohlich interaction is affected by the spurious interaction between the two-dimensional (2D) material and its periodic images. To overcome this difficulty, we perform density functional perturbation theory calculations with a truncated Coulomb interaction in the direction perpendicular to the plane of the 2D material. We show that the two-dimensional Fr\"ohlich interaction is much stronger than assumed in previous ab initio studies. We provide analytical models depending on the effective charges and dielectric…

We present results on the radiative lifetimes of excitons and trions in a monolayer of metal dichalcogenide ${\mathrm{MoS}}_{2}$. The small exciton radius and the large exciton optical oscillator strength result in radiative lifetimes in the 0.18--0.30 ps range for excitons that have small in-plane momenta and couple to radiation. Average lifetimes of thermally distributed excitons depend linearly on the exciton temperature and can be in the few picoseconds range at small temperatures and more than a nanosecond near room temperature. Localized excitons exhibit lifetimes in the same range and the lifetime increases as the localization length decreases. The radiative lifetimes of trions are in the hundreds of picosecond range and increase with the increase in the trion momentum. Average lifetimes of thermally distributed trions increase with the trion temperature as the trions acquire thermal energy and larger momenta. We expect our theoretical results to be applicable to most other 2D transition metal dichalcogenides.

The many-body localization transition (MBLT) between ergodic and many-body localized phases in disordered interacting systems is a subject of much recent interest. The statistics of eigenenergies is known to be a powerful probe of crossovers between ergodic and integrable systems in simpler examples of quantum chaos. We consider the evolution of the spectral statistics across the MBLT, starting with mapping to a Brownian motion process that analytically relates the spectral properties to the statistics of matrix elements. We demonstrate that the flow from Wigner-Dyson to Poisson statistics is a two-stage process. First, a fractal enhancement of matrix elements upon approaching the MBLT from the delocalized side produces an effective power-law interaction between energy levels, and leads to a plasma model for level statistics. At the second stage, the gas of eigenvalues has local interactions and the level statistics belongs to a semi-Poisson universality class. We verify our findings numerically on the XXZ spin chain. We provide a microscopic understanding of the level statistics…

Here, the authors demonstrate a new first-principles computational method for performing predictive calculations of the optical absorption spectra of solids at finite temperature, including both zero-point fluctuations and phonon-assisted optical transitions. The comparison between calculations and measurements for the case of silicon, diamond, and gallium arsenide shows that this new method is not only efficient, but also very accurate, and is capable of yielding absorption coefficients in agreement with experiment over several orders of magnitude.

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

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

Extensive studies have demonstrated the promising capability of the organic-inorganic hybrid halide perovskite ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbI}}_{3}$ in solar cells with a high power conversion efficiency exceeding 20%. However, the intrinsic as well as extrinsic instabilities of this material remain the major challenge to the commercialization of perovskite-based solar cells. Mixing halides is expected to resolve this problem. Here, we investigate the effect of chemical substitution in the position of the halogen atom on the structural, electronic, and optical properties of mixed halide perovskites ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}\mathrm{Pb}{({\mathrm{I}}_{1\ensuremath{-}x}{\mathrm{Br}}_{x})}_{3}$ with a pseudocubic phase using the virtual crystal approximation method within density functional theory. With an increase of Br content $x$ from 0.0 to 1.0, the lattice constant decreases in proportion to $x$ with the function of $a(x)=6.420\ensuremath{-}0.333x$ (\AA{}), while the band gap and the exciton binding energy increase with the quadratic function of…

Two-dimensional (2D) materials have attracted considerable interest due to their remarkable properties and potential applications for nanoelectronics, electrodes, energy storage devices, among others. However, many well-studied 2D materials lack appreciable conductivity and tunable mechanical strength, limiting their applications in flexible devices. Newly developed MXenes open up the opportunity to design novel flexible conductive electronic materials. Here, using density functional theory (DFT), we investigate systematically the effects of several functional groups on the stabilization, mechanical properties, and electronic structures of a representative MXene. It is found that oxygen possesses the largest adsorption energy as compared to other functional groups, indicating its good thermodynamic stabilization. In comparison with bare and other functionalized titanium carbides, the oxygen functionalized one exhibits the most superior ideal strength; however, the premature softening of the long-wave phonon modes might limit the intrinsic strength for…

The canonical Su-Schrieffer-Heeger (SSH) array is one of the basic geometries that have spurred significant interest in topological band-gap modes. Here, we show that the judicious inclusion of third-order Kerr nonlinearities in SSH arrays opens rich physics in topological insulators, including the possibility of supporting self-induced topological transitions, as a function of the applied intensity. We highlight the emergence of a class of topological solutions in nonlinear SSH arrays localized at the array edges and with unusual properties. As opposed to their linear counterparts, these nonlinear states decay to a plateau of nonzero amplitude inside the array, highlighting the local nature of topologically nontrivial band gaps in nonlinear systems. We study the conditions under which these states can be excited and their temporal dynamics as a function of the applied excitation, paving the way to interesting directions in the physics of topological edge states with robust propagation properties based on nonlinear interactions in suitably designed periodic arrays.

Among a variety of layered materials used as building blocks in van der Waals heterostructures, hexagonal boron nitride (hBN) appears as an ideal platform for hosting optically active defects owing to its large band gap ($\ensuremath{\sim}6 \mathrm{eV}$). Here we study the optical response of a high-purity hBN crystal under green laser illumination. By means of photon correlation measurements, we identify individual defects emitting a highly photostable fluorescence under ambient conditions. A detailed analysis of the photophysical properties reveals a high quantum efficiency of the radiative transition, leading to a single photon source with very high brightness ($\ensuremath{\sim}4\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0.28em}{0ex}}\text{counts} {\mathrm{s}}^{\ensuremath{-}1}$). These results illustrate how the wide range of applications offered by hBN could be further extended to photonic-based quantum information science and metrology.

Neutron scattering from single crystals has been used to determine the magnetic structure and magnon dynamics of ${\text{FePS}}_{3}$, an $S=2$ Ising-like quasi-two-dimensional antiferromagnet with a honeycomb lattice. The magnetic structure has been confirmed to have a magnetic propagation vector of ${\mathbf{k}}_{M}=\left[01\frac{1}{2}\right]$ and the moments are collinear with the normal to the $ab$ planes. The magnon data could be modeled using a Heisenberg Hamiltonian with a single-ion anisotropy. Magnetic interactions up to the third in-plane nearest neighbor needed to be included for a suitable fit. The best fit parameters for the in-plane exchange interactions were ${J}_{1}=1.46, {J}_{2}=\ensuremath{-}0.04$, and ${J}_{3}=\ensuremath{-}0.96$ meV. The single-ion anisotropy is large, $\mathrm{\ensuremath{\Delta}}=2.66$ meV, explaining the Ising-like behavior of the magnetism in the compound. The interlayer exchange is very small, ${J}^{\ensuremath{'}}=\ensuremath{-}0.0073$ meV, proving that ${\text{FePS}}_{3}$ is a very good approximation to a two-dimensional magnet.

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

The linear energy dispersion of graphene electrons leads to a strongly nonlinear electromagnetic response of this material. We develop a general quantum theory of the third-order nonlinear local dynamic conductivity of graphene ${\ensuremath{\sigma}}_{\ensuremath{\alpha}\ensuremath{\beta}\ensuremath{\gamma}\ensuremath{\delta}}({\ensuremath{\omega}}_{1},{\ensuremath{\omega}}_{2},{\ensuremath{\omega}}_{3})$, which describes its nonlinear response to a uniform electromagnetic field. The derived analytical formulas describe a large number of different nonlinear phenomena such as the third-harmonic generation, the four-wave mixing, the saturable absorption, the second-harmonic generation stimulated by a dc electric current, etc., which may be used in different terahertz and optoelectronic devices.

Strain engineering is one of the most promising and effective routes toward continuously tuning the electronic and optic properties of materials, while thermal properties are generally believed to be insensitive to mechanical strain. In this paper, the strain-dependent thermal conductivity of monolayer silicene under uniform biaxial tension is computed by solving the phonon Boltzmann transport equation with interatomic force constants extracted from first-principles calculations. Unlike the commonly believed understanding that thermal conductivity only slightly decreases with increased tensile strain for bulk materials, it is found that the thermal conductivity of silicene can increase dramatically with strain. Depending on the size, the maximum thermal conductivity of strained silicene can be a few times higher than that of the unstrained case. Such an unusual strain dependence is mainly attributed to the dramatic enhancement in the acoustic phonon lifetime. Such enhancement plausibly originates from the flattening of the buckling of the silicene structure upon stretching, which is…

The ternary intermetallic compounds ${\mathrm{Gd}}_{2}{\mathrm{NiSi}}_{3}$ and ${\mathrm{Er}}_{2}{\mathrm{NiSi}}_{3}$ are synthesized in chemically single phase, which are characterized using dc magnetization, ac magnetic susceptibility, heat capacity, and neutron diffraction studies. Neutron diffraction and heat capacity studies confirm that long-range magnetic ordering coexists with the frustrated glassy magnetic components for both compounds. The static and dynamical features of dc magnetization and frequency-dependent ac susceptibility data reveal that ${\mathrm{Gd}}_{2}{\mathrm{NiSi}}_{3}$ is a canonical spin-glass system, while ${\mathrm{Er}}_{2}{\mathrm{NiSi}}_{3}$ is a reentrant spin cluster-glass system. The spin freezing temperature merges with the long-range antiferromagnetic ordering temperature at 16.4 K for ${\mathrm{Gd}}_{2}{\mathrm{NiSi}}_{3}$. ${\mathrm{Er}}_{2}{\mathrm{NiSi}}_{3}$ undergoes antiferromagnetic ordering at 5.4 K, which is slightly above the spin freezing temperature at 3 K. The detailed studies of nonequilibrium dynamical behavior, viz., the memory…

The authors present a new method for variationally optimizing projected entangled-pair states (PEPS) in the thermodynamic limit based on the global energy functional and novel tensor contraction strategies. It is successfully benchmarked using Ising and Heisenberg Hamiltonians. The method is completely independent from previous PEPS implementations and improves on the variational energies and order parameters as compared to other state-of-the-art PEPS simulations. The algorithm presented here should prove valuable in simulating ground-state properties of strongly correlated quantum spin systems, can be used with other variational ansatzes, and can be applied to tensor network states in general.

Metal-semiconductor contacts are a pillar of modern semiconductor technology. Historically, their microscopic understanding has been hampered by the inability of traditional analytical and numerical methods to fully capture the complex physics governing their operating principles. Here we introduce an atomistic approach based on density functional theory and nonequilibrium Green's function, which includes all the relevant ingredients required to model realistic metal-semiconductor interfaces and allows for a direct comparison between theory and experiments via $I\ensuremath{-}{V}_{\mathrm{bias}}$ curve simulations. We apply this method to characterize an Ag/Si interface relevant for photovoltaic applications and study the rectifying-to-Ohmic transition as a function of the semiconductor doping. We also demonstrate that the standard ``activation energy'' method for the analysis of $I\ensuremath{-}{V}_{\mathrm{bias}}$ data might be inaccurate for nonideal interfaces as it neglects electron tunneling, and that finite-size atomistic models have problems in describing these interfaces in…

We explore the adsorption of ${\mathrm{MoS}}_{2}$ on a range of metal substrates by means of first-principles density functional theory calculations. Including van der Waals forces in the density functional is essential to capture the interaction between ${\mathrm{MoS}}_{2}$ and a metal surface, and obtain reliable interface potential steps and Schottky barriers. Special care is taken to construct interface structures that have a mismatch between the ${\mathrm{MoS}}_{2}$ and the metal lattices of $<1%$. ${\mathrm{MoS}}_{2}$ is chemisorbed on the early transition metal Ti, which leads to a strong perturbation of its (electronic) structure and a pinning of the Fermi level 0.54 eV below the ${\mathrm{MoS}}_{2}$ conduction band due to interface states. ${\mathrm{MoS}}_{2}$ is physisorbed on Au, where the bonding hardly perturbs the electronic structure. The bonding of ${\mathrm{MoS}}_{2}$ on other metals lies between these two extreme cases, with interface interactions for the late $3d$ transition metals Co, Ni, Cu and the simple metal Mg that are somewhat stronger than for the late…

We present numerical studies of fermion and boson models with random all-to-all interactions (the Sachdev-Ye-Kitaev models). The high-temperature expansion and exact diagonalization of the $N$-site fermion model are used to compute the entropy density: our results are consistent with the numerical solution of $N=\ensuremath{\infty}$ saddle-point equations, and the presence of a nonzero entropy density in the limit of vanishing temperature. The exact-diagonalization results for the fermion Green's function also appear to converge well to the $N=\ensuremath{\infty}$ solution. For the hard-core boson model, the exact-diagonalization study indicates spin-glass order. Some results on the entanglement entropy and the out-of-time-order correlators are also presented.

The search for metallic boron allotropes has attracted great attention in the past decades and recent theoretical works predict the existence of metallicity in monolayer boron. Here, we synthesize the ${\ensuremath{\beta}}_{12}$-sheet monolayer boron on a Ag(111) surface and confirm the presence of metallic boron-derived bands using angle-resolved photoemission spectroscopy. The Fermi surface is composed of one electron pocket at the $\overline{S}$ point and a pair of hole pockets near the $\overline{X}$ point, which is supported by the first-principles calculations. The metallic boron allotrope in ${\ensuremath{\beta}}_{12}$ sheet opens the way to novel physics and chemistry in material science.

We perform systematic investigation on the geometric, energetic, and electronic properties of group IV-VI binary monolayers $(\mathit{XY})$, which are the counterparts of phosphorene, by employing density functional theory based electronic structure calculations. For this purpose, we choose the binary systems $XY$ consisting of equal numbers of group IV $(X$ = C, Si, Ge, Sn) and group VI elements ($Y$ = O, S, Se, Te) in three geometrical configurations, the puckered, buckled and planar structures. The results of binding energy calculations show that all the binary systems studied are energetically stable. It is observed that, the puckered structure, similar to that of phosphorene, is the energetically most stable geometric configuration. Moreover, the binding energies of buckled configuration are very close to those of the puckered configuration. Our results of electronic band structure predict that puckered SiO and CSe are direct band semiconductors with gaps of 1.449 and 0.905 eV, respectively. Band structure of CSe closely resembles that of phosphorene. Remaining group IV-VI…

By using first-principles calculations, we propose that WC-type ZrTe is a new type of topological semimetal (TSM). It has six pairs of chiral Weyl nodes in its first Brillouin zone, but it is distinguished from other existing TSMs by having an additional two paris of massless fermions with triply degenerate nodal points as proposed in the isostructural compounds TaN and NbN. The mirror symmetry, threefold rotational symmetry, and time-reversal symmetry require all of the Weyl nodes to have the same velocity vectors and locate at the same energy level. The Fermi arcs on different surfaces are shown, which may be measured by future experiments. It demonstrates that the ``material universe'' can support more intriguing particles simultaneously.

The authors present here several advances towards a comprehensive understanding of WTe${}_{2}$. Using microfocus laser ARPES, they resolve for the first time the distinct electronic structure of both inequivalent top and bottom (001) surfaces of WTe${}_{2}$. Moreover, they demonstrate for the first time the presence of large surface state Fermi arcs on both surfaces. Using surface electronic structure calculations, they further demonstrate that these Fermi arcs are topologically trivial and that their existence is independent of the presence of type-II Weyl points in the bulk band structure. Contrary to common belief, the observation of surface state Fermi arcs is thus not suitable to robustly identify a type-II Weyl semimetal. Finally, the authors use the observation of Fermi arcs and distinct top and bottom surfaces to clarify the controversial bulk electronic structure of WTe${}_{2}$. They show that the bulk Fermi surface is formed by three-dimensional electron and hole pockets with areas that are found to be in good agreement with transport experiments, with the exception of…

Noncentrosymmetric transition-metal monopnictides, including TaAs, TaP, NbAs, and NbP, are emergent topological Weyl semimetals (WSMs) hosting exotic relativistic Weyl fermions. In this Rapid Communication, we elucidate the physical origin of the unprecedented charge carrier mobility of NbP, which can reach $1\ifmmode\times\else\texttimes\fi{}{10}^{7}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{2}{\phantom{\rule{0.16em}{0ex}}\mathrm{V}}^{\ensuremath{-}1}{\phantom{\rule{0.16em}{0ex}}\mathrm{s}}^{\ensuremath{-}1}$ at 1.5 K. Angle- and temperature-dependent quantum oscillations, supported by density function theory calculations, reveal that NbP has the coexistence of $p$- and $n$-type WSM pockets in the ${k}_{z}=1.16\ensuremath{\pi}/c$ plane (W1-WSM) and in the ${k}_{z}=0$ plane near the high symmetry points $\mathrm{\ensuremath{\Sigma}}$ (W2-WSM), respectively. Uniquely, each W2-WSM pocket forms a large dumbbell-shaped Fermi surface enclosing two neighboring Weyl nodes with the opposite chirality. The magnetotransport in NbP is dominated by these highly anisotropic W2-WSM pockets, in…

The mechanical properties of monolayer GaS and GaSe crystals are investigated in terms of their elastic constants: in-plane stiffness (C), Poisson ratio $(\ensuremath{\nu})$, and ultimate strength $({\ensuremath{\sigma}}_{U})$ by means of first-principles calculations. The calculated elastic constants are compared with those of graphene and monolayer ${\mathrm{MoS}}_{2}$. Our results indicate that monolayer GaS is a stiffer material than monolayer GaSe crystals due to the more ionic character of the Ga-S bonds than the Ga-Se bonds. Although their Poisson ratio values are very close to each other, 0.26 and 0.25 for GaS and GaSe, respectively, monolayer GaS is a stronger material than monolayer GaSe due to its slightly higher ${\ensuremath{\sigma}}_{U}$ value. However, GaS and GaSe crystals are found to be more ductile and flexible materials than graphene and ${\mathrm{MoS}}_{2}$. We have also analyzed the band-gap response of GaS and GaSe monolayers to biaxial tensile strain and predicted a semiconductor-metal crossover after $17%$ and $14%$ applied strain, respectively, for…

We derive electronic structure models for weakly interacting bilayers such as graphene-graphene and graphene--hexagonal boron nitride, based on density functional theory calculations followed by Wannier transformation of electronic states. These transferable interlayer coupling models can be applied to investigate the physics of bilayers with arbitrary translations and twists. The functional form, in addition to the dependence on the distance, includes the angular dependence that results from higher angular momentum components in the Wannier ${p}_{z}$ orbitals. We demonstrate the capabilities of the method by applying it to a rotated graphene bilayer, which produces the analytically predicted renormalization of the Fermi velocity, Van Hove singularities in the density of states, and moir\'e pattern of the electronic localization at small twist angles. We further extend the theory to obtain the effective couplings by integrating out neighboring layers. This approach is instrumental for the design of van der Walls heterostructures with desirable electronic features and transport…

The trends in electronic band structure are studied in the cubic $AB{X}_{3}$ halide perovskites for $A=\text{Cs}$; $B=\text{Pb}$, Sn, Ge, Si; and $X=\text{I}$, Br, Cl. The gaps are found to decrease from Pb to Sn and from Ge to Si, but increase from Sn to Ge. The trend is explained in terms of the atom $s$ levels of the group-IV element and the atomic sizes which changes the amount of hybridization with $X\text{\ensuremath{-}}p$ and hence the valence bandwidth. Along the same series spin-orbit coupling also decreases and this tends to increase the gap because of the smaller splitting of the conduction band minimum. Both effects compensate each other to a certain degree. The trend with halogens is to reduce the gap from Cl to I, i.e., with decreasing electronegativity. The role of the tolerance factor in avoiding octahedron rotations and octahedron edge sharing is discussed. The Ge containing compounds have tolerance factor $t>1$ and hence do not show the series of octahedral rotation distortions and the existence of edge-sharing octahedral phases known for Pb and Sn-based compounds,…

In polar semiconductors and oxides, the long-range nature of the electron-phonon $(e\text{\ensuremath{-}}\mathrm{ph})$ interaction is a bottleneck to compute charge transport from first principles. Here, we develop an efficient ab initio scheme to compute and converge the $e\text{\ensuremath{-}}\mathrm{ph}$ relaxation times (RTs) and electron mobility in polar materials. We apply our approach to GaAs, where by using the Boltzmann equation with state-dependent RTs, we compute mobilities in excellent agreement with experiment at $250\text{--}500\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The $e\text{\ensuremath{-}}\mathrm{ph}$ RTs and the phonon contributions to intravalley and intervalley $e\text{\ensuremath{-}}\mathrm{ph}$ scattering are also analyzed. Our work enables efficient ab initio computations of transport and carrier dynamics in polar materials.

Recent work suggests that a sharp definition of ``phase of matter'' can be given for periodically driven ``Floquet'' quantum systems exhibiting many-body localization. In this work, we propose a classification of the phases of interacting Floquet localized systems with (completely) spontaneously broken symmetries; we focus on the one-dimensional case, but our results appear to generalize to higher dimensions. We find that the different Floquet phases correspond to elements of $Z(G)$, the center of the symmetry group in question. In a previous paper [C. W. von Keyserlingk and S. L. Sondhi, preceding paper, Phys. Rev. B 93, 245145 (2016)], we offered a companion classification of unbroken, i.e., paramagnetic phases.

An average-atom ($A$-atom) embedded-atom-method potential for random multicomponent alloys at any composition is derived analytically and validated by comparing $A$-atom and true random alloys bulk and defect properties, in model Fe-Ni-Cr systems. The $A$-atom can be mixed with the individual alloying-element potentials, thus enabling computation of defect/defect interactions. Its use provides quantitative insight into the physical role of the fluctuations, and has many applications, such as in atomistic/continuum modeling of random alloys and the development of new potentials with controlled properties.

We study nonlinear magneto-optical responses of metals by a semiclassical Boltzmann equation approach. We derive general formulas for linear and second-order nonlinear optical effects in the presence of magnetic fields that include both the Berry curvature and the orbital magnetic moment. Applied to Weyl fermions, the semiclassical approach (i) captures the directional anisotropy of linear conductivity under a magnetic field as a consequence of an anisotropic ${B}^{2}$ contribution, which may explain the low-field regime of recent experiments; and (ii) predicts strong second harmonic generation proportional to $B$ that is enhanced as the Fermi energy approaches the Weyl point, leading to large nonlinear Kerr rotation. Moreover, we show that the semiclassical formula for the circular photogalvanic effect arising from the Berry curvature dipole is reproduced by a full quantum calculation using a Floquet approach.

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