We performed a combined study of the (001)-cleavage plane of lanthanum hexaboride $\left({\mathrm{LaB}}_6\right)$ using scanning tunneling microscopy and density-functional theory (DFT). Experimentally, we found a $(2\times 1)$ reconstructed surface on a local scale. The reconstruction is only short-range ordered and tends to order perpendicularly to step edges. At larger distances from surface steps, the reconstruction evolves to a labyrinthlike pattern. These findings are supported by low-energy electron diffraction experiments. Slab calculations within the framework of DFT show that the atomic structure consists of parallel lanthanum chains on top of boron octahedra. Scanning tunneling spectroscopy shows a prominent spectral feature at $-0.6\mathrm{eV}$. Using DFT, we identify this structure as a surface resonance of the $(2\times 1)$ reconstructed ${\mathrm{LaB}}_6$ (100) surface which is dominated by boron dangling bond states and lanthanum $d$ states.

We theoretically study the finite-size effects in the dynamical response of a quantum anomalous Hall insulator in the disk geometry. Semianalytic and numerical results are obtained for the wave functions and energies of the disk within a continuum Dirac Hamiltonian description subject to a topological infinite mass boundary condition. Using the Kubo formula, we obtain the frequency-dependent longitudinal and Hall conductivities and find that optical transitions between edge states contribute dominantly to the real part of the dynamic Hall conductivity for frequency values both within and beyond the bulk band gap. We also find that the topological infinite mass boundary condition changes the low-frequency Hall conductivity to $e^2/h$ in a finite-size system from the well-known value $e^2/2h$ in an extended system. The magneto-optical Faraday rotation is then studied as a function of frequency for the setup of a quantum anomalous Hall insulator mounted on a dielectric substrate, showing both finite-size effects of the disk and Fabry-Pérot resonances due to the substrate. Our work demonstrates the important role played by the boundary condition in the topological properties of finite-size systems through its effects on the electronic wave functions.

We study theoretically the role of compensation points in all-optical magnetization switching (AOS) in rare-earth transition-metal ferrimagnets. We use the s-d model in which localized spins are coupled with spins of itinerant electrons via the s-d exchange interaction. The main advantage of the model is that it considers the dynamics of both localized and itinerant spin population. We show that the compensation temperature, which is relevant to AOS, is determined by the net spin polarization, i.e., by the sum of the spin polarizations of localized and itinerant spins. This temperature does not coincide exactly with either the magnetization compensation temperature or the angular momentum compensation temperature.

We report an experimental study of the magnetic order and electronic structure and transport of the layered pnictide ${\mathrm{EuMnSb}}_2$, performed using neutron diffraction, angle-resolved photoemission spectroscopy (ARPES), and magnetotransport measurements. We find that the Eu and Mn sublattices display antiferromagnetic (AFM) order below $T_{\mathrm{N}}^{\mathrm{Eu}}=21\left(1\right)\mathrm{K}$ and $T_{\mathrm{N}}^{\mathrm{Mn}}=350\left(2\right)\mathrm{K}$, respectively. The former can be described by an A-type AFM structure with the Eu spins aligned along the $c$ axis (an in-plane direction), whereas the latter has a C-type AFM structure with Mn moments along the $a$ -axis (perpendicular to the layers). The ARPES spectra reveal Dirac-like linearly dispersing bands near the Fermi energy. Furthermore, our magnetotransport measurements show strongly anisotropic magnetoresistance and indicate that the Eu sublattice is intimately coupled to conduction electron states near the Dirac point.

Employing the concept of two-mode squeezed states from quantum optics, we demonstrate a revealing physical picture for the antiferromagnetic ground state and excitations. Superimposed on a Néel ordered configuration, a spin-flip restricted to one of the sublattices is called a sublattice magnon. We show that an antiferromagnetic spin-up magnon is composed of a quantum superposition of states with $n+1$ spin-up and $n$ spin-down sublattice magnons and is thus an enormous excitation despite its unit net spin. Consequently, its large sublattice spin can amplify its coupling to other excitations. Employing von Neumann entropy as a measure, we show that the antiferromagnetic eigenmodes manifest a high degree of entanglement between the two sublattices, thereby establishing antiferromagnets as reservoirs for strong quantum correlations. Based on these insights, we outline strategies for exploiting the strong quantum character of antiferromagnetic (squeezed) magnons and give an intuitive explanation for recent experimental and theoretical findings in antiferromagnetic magnon spintronics.

Machine-learning techniques have proved successful in identifying ordered phases of matter. However, it remains an open question how far they can contribute to the understanding of phases without broken symmetry, such as spin liquids. Here we demonstrate how a machine-learning approach can automatically learn the intricate phase diagram of a classical frustrated spin model. The method we employ is a support vector machine equipped with a tensorial kernel and a spectral graph analysis which admits its applicability in an effectively unsupervised context. Thanks to the interpretability of the machine we are able to infer, in closed form, both order parameter tensors of phases with broken symmetry, and the local constraints which signal an emergent gauge structure, and so characterize classical spin liquids. The method is applied to the classical XXZ model on the pyrochlore lattice where it distinguishes, among others, between a hidden biaxial spin-nematic phase and several different classical spin liquids. The results are in full agreement with a previous analysis by Taillefumier et al. [Phys. Rev. X 7, 041057 (2017)], but go further by providing a systematic hierarchy between disordered regimes, and establishing the physical relevance of the susceptibilities associated with the local constraints. Our work paves the way for the search of new orders and spin liquids in generic frustrated magnets.

Here we report the effect of structural and superconductivity properties on Ru-doped $\mathrm{CuI}{\mathrm{r}}_2\mathrm{T}{\mathrm{e}}_4$ telluride chalcogenide. X-ray diffraction results suggest that $\mathrm{CuI}{\mathrm{r}}_{2-x}\mathrm{R}{\mathrm{u}}_x\mathrm{T}{\mathrm{e}}_4$ maintains the disordered trigonal structure with space group $P\overline{3}m1$ (no. 164) for $x$ ≤ 0.3. The lattice constants, $a$ and $c$, both decrease with increasing Ru content. Temperature-dependent resistivity, magnetic susceptibility, and specific heat measurements are performed to characterize the superconducting properties systematically. Our results suggest that the optimal doping level for superconductivity in $\mathrm{CuI}{\mathrm{r}}_{2-x}\mathrm{R}{\mathrm{u}}_x\mathrm{T}{\mathrm{e}}_4$ is $x=0.05$, where $T_c$ is 2.79 K with the Sommerfeld constant γ of $11.52\mathrm{mJ}\mathrm{mo}{\mathrm{l}}^{-1}{\mathrm{K}}^{-2}$, and the specific heat anomaly at the superconducting transition, $\mathrm{\Delta }C/\gamma T_c$, is approximately 1.51, which is slightly higher than the Bardeen-Cooper-Schrieffer value of 1.43 and demonstrates bulk superconductivity in our $\mathrm{CuI}{\mathrm{r}}_{1.95}\mathrm{R}{\mathrm{u}}_{0.05}\mathrm{T}{\mathrm{e}}_4$ compound. The values of the lower $\left\{H_{c1}\left(0\right)\right\}$ and upper $\left\{H_{c2}\left(0\right)\right\}$ critical field calculated from isothermal magnetization {M(H)} and magnetotransport $\left\{\rho \right(T,H\left)\right\}$ measurements are 0.98 and 2.47 kOe, respectively, signifying that the compound is clearly a type-II superconductor. Finally, a “domelike” shape superconducting transition temperature $\left(T_c\right)$ vs $x$ content phase diagram has been established. A low substitution ($x=0.03$) of Ru for Ir leads to the disappearance of the charge density wave transition, while $T_c$ rises and reaches a maximum value of 2.79 K at $x=0.05$, followed by a decrease of $T_c$ as $x$ increases. This feature of the competition between the charge density wave we have established and the superconductivity could be caused by tuning the Fermi surface and density of states with Ru chemical doping.

In ergodic quantum systems, physical observables have a nonrelaxing component if they “overlap” with a conserved quantity. In interacting microscopic models, how to isolate the nonrelaxing component is unclear. We compute exact dynamical correlators governed by a Hamiltonian composed of two large interacting random matrices $H=A+B$. We analytically obtain the late-time value of $\langle A\left(t\right)A\left(0\right)\rangle$; this quantifies the nonrelaxing part of the observable $A$. The relaxation to this value is governed by a power law determined by the spectrum of the Hamiltonian $H$, independent of the observable $A$. For Gaussian matrices, we further compute out-of-time-ordered correlators and find that the existence of a nonrelaxing part of $A$ leads to modifications of the late-time values and exponents. Our results follow from exact resummation of a diagrammatic expansion and hyperoperator techniques.

Time crystals break the discrete time translational invariance of an external periodic drive by oscillating at an integer multiple of the driving period. In addition to this fundamental property, other aspects are often considered to be essential characteristics of a time crystal, such as the presence of disorder or interactions, robustness against small variations of system parameters, and the free choice of the initial quantum state. We study a finite-length polarized XX spin chain engineered to display a spectrum of equidistant energy levels without drive and show that it keeps a spectrum of equidistant Floquet quasienergies when subjected to a large variety of periodic driving schemes. Arbitrary multiples of the driving period can then be reached by adjusting parameters of the drive, for arbitrary initial states. This behavior is understood by mapping the XX spin chain with $N+1$ sites to a single large spin with $S=N/2$ invoking the closure of the group SU(2). Our simple model is neither intrinsically disordered nor is it an interacting many-body system (after suitable mapping), and it does not have a thermodynamic limit in the conventional sense. It does, however, show controllable discrete time translational symmetry breaking for arbitrary initial states and a degree of robustness against perturbations, thereby carrying some characteristic traits of a discrete time crystal.

We study the systematic doping evolution of nodal dispersions by in situ angle-resolved photoemission spectroscopy on the continuously doped surface of a high-temperature superconductor ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+x}$ and reveal that the nodal dispersion has three fundamentally different segments separated by two kinks, located at ∼10 meV and roughly 70 meV, respectively. These three segments have different band velocities and different doping dependence. In particular, in the underdoped region the velocity of the high-energy segment increases monotonically as the doping level decreases and can even surpass the bare band velocity. We propose that electron fractionalization is a possible cause for this anomalous nodal dispersion and may even play a key role in the understanding of exotic properties of cuprates.

The 2014–2015 prediction, discovery, and confirmation of record high-temperature superconductivity above 200 K in compressed ${\mathrm{H}}_3\mathrm{S}$, followed by the 2018 extension to superconductivity in the 250–260 K range in lanthanum hydride, mark a new era in the long-standing quest for room-temperature superconductivity: quest achieved, at the cost of supplying 1.5–2 Mbar pressure. Predictions of numerous high-temperature superconducting metal hydrides $X{\text{H}}_n$ ($X$=metal atom) have appeared but are providing limited understanding of why some transition temperatures $T_c$ are high while others are low. We make use of the small mass ratio $M_H/M_X$ to obtain an atomic decomposition of the coupling strength to reveal that although the $X$ atom provides coupling strength via ${\lambda }_X$ as commonly calculated, it is irrelevant for $T_c$ because the resulting lowering of frequency moments compensates (and sometimes overcompensates) for the increase in $\lambda$. It is important for analysis and for understanding that the $X$ atom contribution is neglected because $T_c$ depends more transparently on ${\lambda }_H.$ Five $X{\mathrm{H}}_n$ compounds, predicted in harmonic approximation to have $T_c$ in the 150–300 K range, are analyzed consistently for their relevant properties, revealing some aspects that confront conventional wisdom. A phonon frequency–critical temperature (${\omega }_2-T_c$) phase diagram is obtained that reveals a common structural phase instability boundary limiting $T_c$ at the low-pressure range of each compound. The hydrogen scattering matrix elements are obtained and found to differ strongly over the hydrides. A quantity directly proportional to $T_c$ in these hydrides is identified, indicating that (in common notation) $N_H\left(0\right)I_H^2/{\omega }_H={\eta }_H/{\omega }_H$ is the parameter combination to be maximized in hydrides.

Even after more than 50 years of development, a major issue in silicon-based technology is the understanding of the $\mathrm{Si}/{\mathrm{SiO}}_2$ interface and its defects, particularly the unsaturated silicon dangling bonds which have to be passivated by hydrogen during fabrication. Although it is well known that hydrogen dissociation from an initially passivated interfacial Si dangling bond results in an electrically active defect, there is still no consensus on the actual microscopic Si–H bond-breaking mechanism, despite a significant research effort. The most thorough theoretical study in the field was published by Tuttle and Van de Walle 20 years ago. Although it was then suggested that the hydrogen dissociates most likely into a bond-center site, no clean argument for bond rupture could be given at that time. In order to take a fresh look at this highly important problem, we employ the latest $ab$ initio methods available, including the method of well-tempered metadynamics and nudged-elastic-band calculations based on density functional theory (DFT). This allows us to study the interactions of a Si–H bond with its realistic environment in a three-dimensional Si/$a–{\mathrm{SiO}}_2$ interface in considerable detail. Using classical force fields and well-tempered metadynamics in conjunction with DFT, we provide new insights into the dissociation kinetics. We find that one of the previously suggested dissociation paths only leads into a neutral, metastable state which would not facilitate bond dissociation. By sampling the configuration space in greater detail than ever before, we propose a trajectory whereby the H first moves towards an adjacent Si and in a second step relaxes into a configuration between the next-nearest Si–Si bond. The final statistical analysis on a large number of defects on this amorphous interface yields potential energy surfaces and barriers which are in excellent agreement with experimental values.

The phonon Casimir effect is the long-range interaction between impurities in condensed matter systems, mediated by vacuum fluctuations of the phonon field. For pairs of impurities, this interaction has been shown to follow a quasi-power-law at zero-temperature and evolve into an exponentially decaying form as the temperature is increased. This work introduces an approach to deal with systems of more than two impurities, both at zero and finite temperatures.

We put forward the use of transformation optics to map surface waves that exist as one-dimensional modes supported by anisotropic structures into bound states in two-dimensional geometries. Specifically, we show the conformal mapping of Dyakonov waves existing in infinite planar surfaces separating birefringent media into bound modes supported by a cylindrical structure made of suitable metamaterials. In contrast with the original Dyakonov waves, the resulting fiber-like modes are highly dispersive, may exist as fundamental as well as higher-order states, feature helical wave fronts, and exhibit a lower and upper frequency cutoff. The program we put forward can be applied to all wave phenomena currently known to occur only in planar geometries in different types of anisotropic media.

Various properties of Pb(111) nanofilms, prototypical quantum films, have been studied extensively. However, key ab initio-level energy barriers for Pb adatom diffusion on stepped Pb(111) nanofilm surfaces are still not available. Using first-principles density functional theory, we calculate these barriers for films with thicknesses of few monolayers (ML). We find that two-atom exchange is always much more favorable than direct hopping to cross A- or B-type steps. Ehrlich-Schwoebel (ES) barriers for downward transport to a higher-coordination step-edge site depend strongly on the film thickness. For such transport from 2- to 1-ML terraces, or from 4- to 3-ML terraces, there is no an ES barrier, but large ES barriers of more than 100 meV are found from 3- to 2-ML terraces. We also obtain the barriers for diffusion along the step edges and find that these step-edge barriers are significantly larger than terrace diffusion barriers. In addition, we analyze energetics for diffusion on the top flat surface of a nanofilm supported on a vicinal surface, and thus having underlying buried or ghost steps. We quantify the tilted potential energy surface in both ghost A- and B-step regions separating 2- and 3-ML (as well as 3- and 4-ML) terraces. Consequences are discussed for the growth kinetics of supported Pb nanofilms, where the support does not strongly affect electronic states within the film.

We present a theoretical study of nonlocal magnetoelectric effects in diffusive hybrid structures with an intrinsic linear-in-momentum spin-orbit coupling (SOC) which is assumed to be spatially inhomogeneous. Our analysis is based on the SU(2)-covariant drift-diffusion equations from which we derive boundary conditions at hybrid interfaces for SOC of any kind. Within this formulation, the spin current is covariantly conserved when the spin relaxation is only due to the intrinsic SOC. This conservation leads to the absence of spin Hall (SH) currents in homogeneous systems. If, however, extrinsic sources of spin relaxation (ESR), such as magnetic impurities and/or a random SOC at nonmagnetic impurities, are present the spin is no longer covariantly conserved, and SH currents appear. We apply our model to describe nonlocal transport in a two-dimensional system with an interface separating two regions: one normal region without intrinsic SOC and one with a Rashba SOC. We first explore the inverse spin-galvanic effect, i.e., a spin polarization induced by an electric field. We demonstrate how the spatial behavior of such spin density depends on both the direction of the electric field and the strength of the ESR rate. We also study the spin-to-charge conversion, and compute the charge current and the distribution of electrochemical potential in the whole system when a spin current is injected into the normal region. In systems with an inhomogeneous SOC varying in one spatial direction, we find an interesting nonlocal reciprocity between the spin density induced by a charge current at a given point in space, and the spatially integrated current induced by a spin density injected at the same point.

Nonstoichiometric ${\mathrm{Sr}}_{1-y}{\mathrm{Mn}}_{1-z}{\mathrm{Sb}}_2(y,z<0.1)$ is known to exhibit a coexistence of magnetic order and the nontrivial semimetallic behavior. In this paper, we report the magnetism and its strong coupling to the semimetallic behavior, by a combined use of inelastic neutron scattering (INS) and density functional theory (DFT). A phase separation consisting of a majority antiferromagentic phase and a minority ferromagnetic phase is proposed. We found a relatively large spin excitation gap $\approx 8.5\mathrm{meV}$ at 5 K, and the interlayer magnetic exchange constant only 2.8% of the dominant intralayer magnetic interaction, evidencing a quasi-2D magnetism in ${\mathrm{Sr}}_{1-y}{\mathrm{Mn}}_{1-z}{\mathrm{Sb}}_2$. Using DFT, we find a strong influence of magnetic orders on the electronic band structure and the Dirac dispersions near the Fermi level along the Y-S direction in the presence of a ferromagnetic order. Furthermore, we demonstrate that the size of the ferromagnetic ordered moment is an effective strategy to tune Dirac/Weyl dispersions near the Fermi level. Our study unveils novel interplay between the magnetic order, ordered moment, and electronic band topology in ${\mathrm{Sr}}_{1-y}{\mathrm{Mn}}_{1-z}{\mathrm{Sb}}_2$ and opens pathways to control the relativistic band structure.

Of all stoichiometric filled-skutterudite superconductors, ${\mathrm{LaRu}}_4{\mathrm{As}}_{12}$ has the highest critical field and temperature. Here we report on a detailed Fermi-surface investigation of ${\mathrm{LaRu}}_4{\mathrm{As}}_{12}$ by means of de Haas–van Alphen measurements and density-functional-theory calculations. We find evidence for a nearly spherical and a multiply connected Fermi-surface sheet. The different effective masses and mass enhancements for the two sheets support two-band superconductivity, which was inferred from previous specific-heat measurements. Furthermore, quantum oscillations persist as well in the superconducting phase. We use two models to describe the additional damping, yielding energy gaps differing by a factor of 5.

We predict from density functional theory based electronic structure calculations that a monolayer made up of carbon and arsenic atoms with a chemical composition $\left({\mathrm{CAs}}_3\right)$ forms an energetically and dynamically stable system in two geometrical arrangements, namely, buckled and puckered configurations. The results of electronic structure calculations predict that the puckered monolayer is a metal, whereas the buckled monolayer is a semimetal. Interestingly, the electronic band structure of the buckled configuration possesses a linear dispersion and a Dirac cone at the Fermi level around the high-symmety $K$ point in the reciprocal lattice. Thus, at low-energy excitation (up to 105 meV), the charge carriers in this system behave as massless Dirac fermions. Detailed analysis of partial density of state indicates that the $2p_z$ orbital of C atoms contributes significantly to the states which form the linear dispersion and hence the Dirac cone around the Fermi level. This suggests the existence of a strong correlation between the linear dispersion (Dirac cone) and ${sp}^2$-like hybridization of the orbitals of C atom. Thus the electronic properties of ${\mathrm{CAs}}_3$ monolayer are similar to those of graphene and other group-IV based monolayers like, silicene and germanene. In addition, we have also investigated the influence of mechanical strain on the properties of ${\mathrm{CAs}}_3$ monolayer. Our results indicate that the monolayer possesses linear dispersion in the electronic band structure for a wide range of mechanical strain from $-12$% to 20%, though the position of Dirac point may not lie exactly at the Fermi level. Finally, we wish to point out that ${\mathrm{CAs}}_3$ monolayer belongs to the class of Dirac materials where the behavior of particles, at low-energy excitations, is characterized by the Dirac-like Hamiltonian rather than the Schrodinger Hamiltonian.