Crystal structure, electronic state of ${\mathrm{Ir}}^{4+}$, and magnetic properties of the antifluorite compound ${\mathrm{K}}_2{\mathrm{IrBr}}_6$ are studied using high-resolution synchrotron x-ray diffraction, resonant inelastic x-ray scattering (RIXS), thermodynamic and transport measurements, and ab initio calculations. The crystal symmetry is reduced from cubic at room temperature to tetragonal below 170 K and eventually to monoclinic below 122 K. These changes are tracked by the evolution of the noncubic crystal-field splitting $\mathrm{\Delta }$ measured by RIXS. Nonmonotonic changes in $\mathrm{\Delta }$ are ascribed to the competing effects of the tilt, rotation, and deformation of the ${\mathrm{IrBr}}_6$ octahedra as well as tetragonal strain on the electronic levels of ${\mathrm{Ir}}^{4+}$. The Néel temperature of $T_N=11.9$ K exceeds that of the isostructural ${\mathrm{K}}_2{\mathrm{IrCl}}_6$, and the magnitude of frustration on the fcc spin lattice decreases. We argue that the replacement of Cl by Br weakens electronic correlations and enhances magnetic couplings.

Silicon carbide with optically and magnetically active point defects offers unique opportunities for quantum technology applications. Since interaction with these defects commonly happens through optical excitation and deexcitation, a complete understanding of their light-matter interaction in general and optical signatures in particular is crucial. Here, we employ quantum mechanical density functional theory calculations to investigate the photoluminescence line shapes of selected, experimentally observed color centers (including single vacancies, double vacancies, and vacancy-impurity pairs) in 4H-SiC. The analysis of zero-phonon lines as well as Huang-Rhys and Debye-Waller factors is accompanied by a detailed study of the underlying lattice vibrations. We show that the defect line shapes are governed by strong coupling to bulk phonons at lower energies and localized vibrational modes at higher energies. Generally, good agreement with the available experimental data is obtained, and thus we expect our theoretical work to be beneficial for the identification of defect signatures in the photoluminescence spectra and thereby advance the research in quantum photonics and quantum information processing.

In this paper, using low-temperature scanning tunneling microscopy, we focus on understanding the native defects in pristine $1T\text{−}\mathrm{Ti}{\mathrm{Se}}_2$ at the atomic scale. We probe how they perturb charge density waves (CDWs) and lead to local CDW modulated region formation. These defects influence the correlation length of CDWs. We establish a connection between suppression of CDWs and Ti intercalation, and show how this supports the exciton condensation model of CDW formation in $1T\text{−}\mathrm{Ti}{\mathrm{Se}}_2$.

We present a method for calculating the full counting statistics of a nonequilibrium quantum system based on the propagator noncrossing approximation (NCA). This numerically inexpensive method can provide higher-order cumulants for extended parameter regimes, rendering it attractive for a wide variety of purposes. We compare NCA results to Born-Markov quantum master equations (QME) results to show that they can access different physics, and to numerically exact inchworm quantum Monte Carlo data to assess their validity. As a demonstration of its power, the NCA method is employed to study the impact of correlations on higher-order cumulants in the nonequilibrium Anderson impurity model. The four lowest-order cumulants are examined, allowing us to establish that correlation effects have a profound influence on the underlying transport distributions. Higher-order cumulants are therefore demonstrated to be a proxy for the presence of Kondo correlations in a way that cannot be captured by simple QME methods.

Environmentally benign $\left(1\text{−}x\right)\mathrm{Ba}\left({\mathrm{Ti}}_{0.8}{\mathrm{Zr}}_{0.2}\right){\mathrm{O}}_3\text{−}x\left({\mathrm{Ba}}_{0.7}{\mathrm{Ca}}_{0.3}\right){\mathrm{TiO}}_3$ (BZT-BCT) ceramics are promising materials due to their remarkable high piezoresponse [Liu and Ren, Phys. Rev. Lett. 103, 257602 (2009)]. In this Letter, by focusing on local and average structure in combination with macroscopic electromechanical and dielectric measurements we demonstrate the structure property relationship in the tetragonal BZT-BCT ceramic. During high-temperature cubic to tetragonal phase transformation, polar nanoregions are manifested through the spontaneous volume ferroelectrostriction at temperatures below $\sim 477$ K. Temperature-dependent local structural investigations across the Zr $K$ edge extended x-ray absorption fine-structure spectroscopy reveal an anomalous collaboration between the ${\mathrm{ZrO}}_6$ and ${\mathrm{TiO}}_6$ octahedra. These octahedra compromise their individuality during polarization development. The presence of domains of submicron size embedded inside the macroscopic ferroelectric regions below $T_m$, as well as their hierarchical arrangement, is observed by piezoresponse force microscopy. Effects of the existence of the structural/polar heterogeneities below $T_m$ are observed also when polarizabilities of the poled and unpoled samples are compared; the poled sample is found to be more susceptible to the electric field. In addition, by using electric field dependent x-ray diffraction studies we also show that this ceramic under field exhibits a reduction of tetragonal distortion, which is consistent with earlier reports.

Optically generated nonequilibrium phonon distribution is used for exploring the origin of a nonlocal adiabatic response in an interacting Anderson insulator. Exposing the system to weak infrared radiation is shown to effectively suppress a long-range effect observed in field-effect experiments while producing little heating and barely changing the system conductance. These effects are shown to be consistent with the quantum nature of the effect and therefore are peculiar to disordered systems that are quantum coherent.

Exact conservation of the angular momentum is worked out for an elastic medium with spins. The intrinsic anharmonicity of the elastic theory is shown to be crucial for conserving the total momentum. As a result, any spin-lattice dynamics inevitably involves multiphonon processes and interaction between phonons. This makes transitions between spin states in a solid fundamentally different from transitions between atomic states in a vacuum governed by linear electrodynamics. Consequences for using solid-state spins as qubits are discussed.

Protected edge modes are the cornerstone of topological states of matter. The simplest example is provided by the integer quantum Hall state at Landau level filling unity, which should feature a single chiral mode carrying electronic excitations. In the presence of a smooth confining potential it was hitherto believed that this picture may only be partially modified by the appearance of additional counterpropagating integer-charge modes. Here, we demonstrate the breakdown of this paradigm: The system favors the formation of edge modes supporting fractional excitations. This accounts for previous observations, and leads to additional predictions amenable to experimental tests.

Using first-principles calculations, the energetic stability of two-dimensional (2D) binary compounds $XY$ is investigated, where $X$ and $Y$ indicate the metallic element from Li to Pb in the periodic table. Here, 1081 compounds in the buckled honeycomb (BHC), buckled square, B2, $\mathrm{L}{1}_0$, and ${\mathrm{B}}_h$ structures are studied. For the compounds that have negative formation energy in the BHC structure or the compounds that can have the ${\mathrm{B}}_h$ structure, the phonon dispersions of the 2D structures are also calculated. We demonstrate that (i) a negative formation energy is neither a sufficient nor necessary condition for yielding the dynamical stability of 2D compounds; and (ii) if a compound in the ${\mathrm{B}}_h$ structure has been synthesized experimentally, that in the BHC structure is dynamically stable.

Despite the importance of $\left({\mathrm{Na}}_{1/2}{\mathrm{Bi}}_{1/2}\right)\mathrm{Ti}{\mathrm{O}}_3$ as an end member in lead-free ferroelectrics and as an oxide ion conductor, the relaxor/ferroelectric nature remains unclear. In order to understand the relaxor-like behavior, frequency-dependent macroscopic mechanical measurements of polycrystalline $\left({\mathrm{Na}}_{1/2}{\mathrm{Bi}}_{1/2}\right)\mathrm{Ti}{\mathrm{O}}_3$ were performed as a function of poling state, revealing the role of a potential field-induced long-range ferroelectric order on the nonlinear hysteretic stress-strain behavior. The mechanical measurements showed an increase in remanent strain and decrease in coercive stress with electrical poling, consistent with previous studies of relaxors. Electrical poling and mechanical texturing were found to influence the frequency dispersion of the relative permittivity, highlighting the potentially relaxor-like response. Further, the relative permittivity showed a directional dependence with respect to the previously applied electrical and mechanical fields. These data are discussed in conjunction with ex situ stress- and electric-field-dependent piezoresponse force microscopy measurements that revealed a clear ferroelectric domain switching through the application of a sufficiently high electric field, but no change of the domain configuration for uniaxial compressive stresses up to −750 MPa. The in situ stress-dependent crystal structure, which was characterized using synchrotron x-ray diffraction, however, indicates stress-induced ferroelastic domain switching as the primary hysteretic process.

In the warm dense regime, where the electron temperature is increased to the same order of the Fermi temperature, the dynamical stability of elemental metals depends on its electronic band structure as well as its crystal structure. It has been known that phonon hardening occurs due to an enhanced internal pressure caused by electron excitations as in close-packed simple metals, whereas phonon softening occurs at a specific point in the Brillouin zone as in body-centered cubic metals. Here, we investigate the dynamical stability of binary ordered alloys (Au and Cu) in the ${\mathrm{L}1}_0$ and ${\mathrm{L}1}_2$ structures. By performing first-principles calculations on phonon dispersions, we demonstrate that warm dense Au-Cu systems show phonon hardening behavior except the lowest frequency phonon mode at point $R$ of AuCu in the ${\mathrm{L}1}_0$ structure. We show that when a phonon mode is stabilized by long-range interatomic interactions at ambient condition, such a phonon will be destabilized by the short-range nature of the warm dense matters.

High-pressure equation of state and isentropic sound speed data for fluid silicon to pressures of 2100 GPa (21 Mbar) are reported. Principal Hugoniot measurements were performed using impedance matching techniques with α-quartz as the reference. Sound speeds were determined by time correlating imposed shock-velocity perturbations in both the sample (Si) and reference material (α-quartz). A change in shock velocity versus particle velocity ($u_s–u_p$) slope on the fluid silicon principal Hugoniot is observed at 200 GPa. Density functional theory based quantum molecular dynamics simulations suggest that both an increase in ionic coordination and a 50% increase in average ionization are coincident with this experimentally observed change in slope.

Due to studies in nonequilibrium (periodically driven) topological matter, it is now understood that some topological invariants used to classify equilibrium states of matter do not suffice to describe their nonequilibrium counterparts. Indeed, in Floquet systems the additional gap arising from the periodicity of the quasienergy Brillouin zone often leads to unique topological phenomena without equilibrium analogs. In the context of Floquet Weyl semimetal, Weyl points may be induced at both quasienergy zero and $\pi /T$ ($T$ being the driving period) and these two types of Weyl points can be very close to each other in the momentum space. Because of their momentum-space proximity, the chirality of each individual Weyl point may become hard to characterize in both theory and experiments, thus making it challenging to determine the system's overall topology. In this work, inspired by the construction of dynamical winding numbers in Floquet Chern insulators, we propose a dynamical invariant capable of characterizing and distinguishing between Weyl points at different quasienergy values, thus advancing one step further in the topological characterization of Floquet Weyl semimetals. To demonstrate the usefulness of such a dynamical topological invariant, we consider a variant of the periodically kicked Harper model (the very first model in studies of Floquet topological phases) that exhibits many Weyl points, with the number of Weyl points rising unlimitedly with the strength of some system parameters. Furthermore, we investigate the two-terminal transport signature associated with the Weyl points. Theoretical findings of this work pave the way for experimentally probing the rich topological band structures of some seemingly simple Floquet semimetal systems.

Magnetization dynamics driven by the current-induced spin torque is conventionally determined by the classical Landau-Lifshitz-Gilbert-Slonczewski equation in which the spin (magnetization) fluctuation at finite temperature is modeled by a white-noise random field. We propose a quantum approach for current driven magnetization switching that explicitly includes the spin fluctuation by the quantum statistics of magnon excitations. We find that the spin fluctuation substantially reduces the critical spin torque at high temperatures. Since the spin fluctuations are fundamentally stronger in lower-dimensional systems, this reduction is stronger in two-dimensional (2D) than in three-dimensional magnets. The result implies that the 2D magnets may have an advantage in terms of electrically manipulating magnetization states for spintronic applications.

High-resolution Zeeman spectroscopy of electronic-nuclear hyperfine levels of ${}^5{\mathrm{I}}_8\to {}^5{\mathrm{I}}_7$ transitions in ${\mathrm{Ho}}^{3+}$:${\mathrm{Y}}_2{\mathrm{SiO}}_5$ is reported. Crystal-field parameters determined for the two $C_1$ symmetry sites in ${\mathrm{Er}}^{3+}$:${\mathrm{Y}}_2{\mathrm{SiO}}_5$ are successfully used to model the Zeeman-hyperfine data, including the prediction of avoided crossings between hyperfine levels under the influence of an external magnetic field. The two six- and seven-coordinate substitutional sites may be distinguished by comparing the spectra with crystal-field calculations.

Compositional patterning (CP) in binary alloys during energetic particle irradiation is studied using a kinetic model that considers two competing kinetic processes, a thermally activated one promoting macroscopic phase separation (MPS) of the concentration field $c\left(\mathbf{r},t\right),$ and a forced one resulting in finite-range random atomic mixing. The forced mixing is modeled by a Gaussian relocation distribution with a characteristic distance $R$. A series of approximate kinetic models are introduced by expanding the mixing function into a series of $n$ terms, thus replacing the nonlocal evaluations of the concentration field $c\left({\mathbf{r}}^{\prime }-\mathbf{r},t\right)$ by local derivatives of $c\left(\mathbf{r},t\right)$. This approach makes it possible to obtain exact effective potentials and build steady-state diagrams for each order-n model. Phase-field (PF) simulations using these order-n models reveal that near the onset of patterning, phase evolution is accurately described using an order-3 model, which changes smoothly from an extended Cahn-Hilliard free energy in the MPS regime to a one-mode Swift-Hohenberg functional in the CP regime. Deeper into the patterning regime, higher-order models are required to achieve convergence, yielding squarelike concentration profiles characteristic of a strong segregation regime. These higher-order effective free energies are analogous to multimodal Swift-Hohenberg functionals. An alternative definition for the effective interfacial energy is proposed in the CP regime, since the interfacial area is no longer an excess quantity in that regime, precluding the use of the standard thermodynamic definition of interfacial energy.

In a recent Letter [Phys. Rev. Lett. 125, 180604 (2020)], we introduced a closed-form analytic expression for the average bipartite von Neumann entanglement entropy of many-body eigenstates of random quadratic Hamiltonians, namely, of Hamiltonians whose single-particle eigenstates have random coefficients in the position basis. A paradigmatic Hamiltonian for which the expression is valid is the quadratic Sachdev-Ye-Kitaev (SYK2) model in its Dirac fermion formulation. Here we show that the applicability of our result is much broader. Most prominently, it is also relevant for local Hamiltonians such as the three-dimensional (3D) Anderson model at weak disorder. Moreover, it describes the average entanglement entropy in Hamiltonians without particle-number conservation, such as the SYK2 model in the Majorana fermion formulation and the 3D Anderson model with additional terms that break particle-number conservation. We extend our analysis to the average bipartite second Rényi entanglement entropy of eigenstates of the same quadratic Hamiltonians, which is derived analytically and tested numerically. We conjecture that our results for the entanglement entropies of many-body eigenstates apply to quadratic Hamiltonians whose single-particle eigenstates exhibit quantum chaos, which we refer to as quantum-chaotic quadratic Hamiltonians.

In the presence of a magnetic field $\mathbf{B}$, the negative transverse magnetoconductivity (TMC) for $\mathbf{E}\perp \mathbf{B}$, with $\mathbf{E}$ being the applied electric field, and anomalous positive longitudinal magnetoconductivity (LMC) for $\mathbf{E}\parallel \mathbf{B}$ of a three-dimensional (3D) Weyl semimetal (WSM) both exhibit periodic-in-$1/B$ quantum oscillations. We develop a semiclassical theory of the magnetoconductivities of a 3D WSM, taking into account the effect of the Landau level broadening and finite temperatures. We find that if one fixes the direction of $\mathbf{B}$ and switches the direction of $\mathbf{E}$ to be perpendicular and parallel to $\mathbf{B}$, there exists an interesting peak-valley correspondence, or, say, a relative $\pi$ oscillation phase shift, between the oscillating TMC and LMC. This can serve as unambiguous measurable evidence manifesting the distinct underlying physical mechanisms of the TMC and LMC in WSMs, which has so far not been reported in theories or experiments.

Using soft x-ray absorption spectroscopy we were able to determine unambiguously the charge and spin states of the transition metal ions in stoichiometric $\mathrm{Y}{\mathrm{Cu}}_3{\mathrm{Co}}_4{\mathrm{O}}_{12}$ and $\mathrm{Ca}{\mathrm{Cu}}_3{\mathrm{Co}}_4{\mathrm{O}}_{12}$. The trivalent and low-spin nature of both the Cu and Co ions in $\mathrm{Y}{\mathrm{Cu}}_3{\mathrm{Co}}_4{\mathrm{O}}_{12}$ makes this correlated system to be effectively a nonmagnetic band semiconductor. The substitution of Y by Ca produces formally tetravalent Co ions but the doped holes are primarily on the oxygen ligands. Concerning the spin degrees of freedom, the trivalent Co ions in $\mathrm{Y}{\mathrm{Cu}}_3{\mathrm{Co}}_4{\mathrm{O}}_{12}$ remain low spin upon the Y-Ca substitution, very much unlike the ${\mathrm{La}}_{1\text{−}x}{\mathrm{Sr}}_x\mathrm{Co}{\mathrm{O}}_3$ system. The tetravalent Co ions in $\mathrm{Ca}{\mathrm{Cu}}_3{\mathrm{Co}}_4{\mathrm{O}}_{12}$ are interestingly also in the low-spin state, which then explains the good electrical conductivity of $\mathrm{Ca}{\mathrm{Cu}}_3{\mathrm{Co}}_4{\mathrm{O}}_{12}$ since charge exchange between neighboring ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}$ ions will not be hampered by the spin-blockade mechanism that otherwise would be in effect if the ${\mathrm{Co}}^{4+}$ and ${\mathrm{Co}}^{3+}$ spin quantum numbers were to differ by more than one-half. We infer that the stability of the Co low-spin state is related to the very short Co-O bond lengths.

We develop a theory of gapped domain wall between topologically ordered systems in two spatial dimensions. We find a new type of superselection sector—referred to as the parton sector—that subdivides the known superselection sectors localized on gapped domain walls. Moreover, we introduce and study the properties of composite superselection sectors that are made out of the parton sectors. We explain a systematic method to define these sectors, their fusion spaces, and their fusion rules, by deriving nontrivial identities relating their quantum dimensions and fusion multiplicities. We propose a set of axioms regarding the ground-state entanglement entropy of systems that can host gapped domain walls, generalizing the bulk axioms proposed in B. Shi, K. Kato, and I. H. Kim, Ann. Phys. 418, 168164 (2020). Similar to our analysis in the bulk, we derive our main results by examining the self-consistency relations of an object called information convex set. As an application, we define an analog of topological entanglement entropy for gapped domain walls and derive its exact expression.