High-pressure single-crystal x-ray diffraction experiments reveal that the superconducting kagome metal ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ transforms from hexagonal ($P6/mmm$) to monoclinic ($C2/m$) symmetry above 10 GPa if nonhydrostatic pressure conditions are created in a diamond anvil cell with silicon oil as the pressure-transmitting medium. This is contrary to the behavior of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ under quasihydrostatic conditions in neon, with the hexagonal symmetry retained up to at least 20 GPa. Monoclinic distortion leaves the kagome planes almost unchanged, but deforms honeycomb nets of the Sb atoms. While the onset of the distortion almost coincides with the reentrance of superconductivity, our ab initio density-functional calculations reveal only minor changes in the electronic structure compared to the quasihydrostatic case. In particular, Fermi surface reconstruction driven by the formation of interlayer Sb-Sb bonds is observed in both monoclinic and hexagonal ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ structures at high pressures and comes out as the likely cause for the reentrant behavior.

To date, a number of valley materials have been discovered with spin-valley or valley-layer couplings. It is highly desirable to realize the interplay of more electronic degrees of freedom in a valley material. Based on the first-principles calculations, we demonstrate the valley and spin degeneracy liftings in the band structure of a TiSiCO monolayer, with the help of the selective expression of the layer pseudospin via atomic adsorption. The introduction of the transition-metal adatoms provides an effective electric field and magnetic proximity effect, giving rise to considerable valley and spin splittings. The larger valley splitting and the largest spin splitting for the band-edge states have magnitudes of up to 190 meV and 148 meV, respectively. The valley and spin splittings can be further tuned by applying an external electric field. According to the modified band structure, various interlayer excitons with different combinations of spins and electric dipoles are selectively created under the optical field of appropriate frequencies. The tunable spin and valley splittings in atom-adsorbed TiSiCO monolayer offer opportunities for exploring the interactions between spin, valley, and layer pseudospins and designing advanced optoelectronic devices.

The application of gate voltages in heavy metal/ferromagnet/oxide multilayer stacks has been identified as one possible candidate to manipulate their anisotropy at will. However, this method has proven to show a wide variety of behaviors in terms of reversibility, depending on the nature of the metal/oxide interface and its degree of oxidation. In order to shed light on the microscopic mechanism governing the complex magneto-ionic behavior in $\text{Ta/CoFeB/}{\text{HfO}}_2$, we perform ab initio simulations on various setups comprising Fe/O and ${\text{Fe/HfO}}_2$ interfaces with different oxygen atom interfacial geometries. After the determination of the more stable interfacial configurations, we calculate the magnetocrystalline anisotropy energy on the different unit cell configurations and formulate a possible mechanism that well describes the recent experimental observations in $\text{Ta/CoFeB/}{\text{HfO}}_2$.

We theoretically consider the effect of vertex correction on spin pumping from a ferromagnetic insulator (FI) into a two-dimensional electron gas (2DEG) in which the Rashba and Dresselhaus spin-orbit interactions coexist. The Gilbert damping in the FI is enhanced by elastic spin-flipping or magnon absorption. We show that the Gilbert damping due to elastic spin-flipping is strongly enhanced by the vertex correction when the ratio of the two spin-orbit interactions is near a special value at which the spin relaxation time diverges while that due to magnon absorption shows only small modification. We also show that the shift in the resonant frequency due to elastic spin-flipping is strongly enhanced in a similar way as the Gilbert damping.

Magnetophononics, the modulation of magnetic interactions by driving infrared-active lattice excitations, is emerging as a key mechanism for the ultrafast dynamical control of both semiclassical and quantum spin systems by coherent light. We demonstrate that, in a quantum magnet with strong spin-phonon coupling, resonances between the driven phonon and the spin excitation frequencies exhibit an intrinsic self-blocking effect, whereby only a fraction of the available laser power is absorbed by the phonon. Using the quantum master equations governing the nonequilibrium steady states of the coupled spin-lattice system, we show how self-blocking arises from the self-consistent alteration of the resonance frequencies. We link this to the appearance of mutually repelling collective spin-phonon states, which in the regime of strong hybridization become composites of a phonon and two triplons. We then identify the mechanism and optimal phonon frequencies by which to control a global nonequilibrium renormalization of the lattice-driven spin excitation spectrum and demonstrate that this effect should be observable in ultrafast THz experiments on a number of known quantum magnetic materials.

The structure of charge density wave states in ${A\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A$ = K, Rb, Cs) kagome superconductors remains elusive, with three possible $2a\times 2a\times 2c$ candidates: trihexagonal, star-of-David, and their mixture. In this study, we conducted a systematic first-principles investigation of the nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) spectra for the $2a\times 2a\times 2c{\mathrm{CsV}}_3{\mathrm{Sb}}_5$ structures. By comparing our simulations with experimental data, we have concluded that the NQR spectrum indicates the trihexagonal structure as the proper structure for ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ after its charge density wave phase transition. The NMR calculation results obtained from the trihexagonal structure are also consistent with the experimental data.

Quantum coherence is the key to new quantum technologies, although the details of decoherence processes and times are still not well understood in solids. We demonstrate how to measure the decoherence time of electronic excited states in $n$-type gallium arsenide ($n$-GaAs) crystals using a relative-phase locked double pulse excitation method and quantitatively evaluate with a simple model including temperature dependence. The interference patterns of both electronic and phonon coherence are observed in $n$-GaAs using ultrafast quantum path interferometry with $\pi /4$ polarized femtosecond pulses, via the amplitude of longitudinal optical phonons as a function of pump-pump delay. The electronic coherence shows uplifting and splitting in the collapse and revival of the electronic interference, which is sensitive to temperature, and these features are well reproduced using a simple quantum mechanical model. The decoherence time is determined using quantum calculations, from the splitting and uplifting in the interference shape, to be $27.8\pm 0.5, 23.0\pm 0.3$, and $12.9\pm 0.3$ fs at 10, 90, and 290 K, respectively. The temperature dependence of the decoherence time is well reproduced by the density of electrons in the conduction band with the group velocity of the photoinduced electrons.

We present a data-driven approach to efficiently approximate nonlinear transient dynamics in solid-state systems. Our proposed machine-learning model combines a dimensionality reduction stage with a nonlinear vector autoregression scheme. We report an outstanding time-series forecasting performance combined with an easy-to-deploy model and an inexpensive training routine. Our results are of great relevance as they have the potential to massively accelerate multiphysics simulation software and thereby guide the future development of solid-state-based technologies.

The electrical control of magnetization compensation temperature ($T_{\mathrm{com}}$) is investigated in ferrimagnetic GdFeCo alloy deposited on ferroelectric $\mathrm{Pb}\left({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}\right){\mathrm{O}}_3\text{−}30\%\mathrm{PbTi}{\mathrm{O}}_3$ (PMN-PT). It is found that the applied electric field can cause a 19 K shift in $T_{\mathrm{com}}$. $T_{\mathrm{com}}$ under different applied electric fields across the PMN-PT substrate suggests that such electrical control is induced by piezostrain at the ferroelectric-ferrimagnetic interface. Spin-polarized first-principles calculations show that the change of exchange coupling strength in GdFeCo are caused by the lattice strains. Nonvolatile switching of resistance and direct bipolar control of exchange bias are achieved when an electric field is applied to appropriately designed multilayers. Our findings pave the way for electric field control of $T_{\mathrm{com}}$ in ferrimagnetic GdFeCo alloy and have implications for the development of energy efficient and nonvolatile ferrimagnetic magnetic random access memory.

We present heat capacity, electrical, and thermal transport measurements of kagome metals $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ $(A=\mathrm{K},\mathrm{Rb},\mathrm{Cs})$. In all three compounds, the development of short-range charge fluctuations above the charge density wave (CDW) transition temperature $T_{\mathrm{CDW}}$ strongly scatters phonons via the electron-phonon coupling, leading to the glasslike phonon heat transport, i.e., the phonon thermal conductivity decreases weakly upon cooling. Once the long-range charge order sets in below $T_{\mathrm{CDW}}$, short-range charge fluctuations are quenched, and the typical Umklapp scattering dominated phonon heat transport is recovered. The charge-fluctuations-induced glasslike phonon thermal conductivity implies sizable electron-phonon coupling in $A{\mathrm{V}}_3{\mathrm{Sb}}_5$.

We have developed a symmetry-adapted modeling procedure for molecules and crystals. By using the completeness of multipoles to express spatial and time-reversal parity-specific anisotropic distributions, we can generate systematically the complete symmetry-adapted multipole basis set to describe any of electronic degrees of freedom in isolated cluster systems and periodic crystals. The symmetry-adapted modeling is then achieved by expressing the Hamiltonian in terms of the linear combination of these bases belonging to the identity irreducible representation, and the model parameters (linear coefficients) in the Hamiltonian can be determined so as to reproduce the electronic structures given by the density-functional computation. We demonstrate our method for the modeling of graphene and emphasize usefulness of the symmetry-adapted basis to analyze and predict physical phenomena and spontaneous symmetry breaking in a phase transition. The present method is complementary to de facto standard Wannier tight-binding modeling, and it provides us with a fundamental basis to develop a symmetry-based analysis for materials science.

The quantum anomalous Hall effect was first observed experimentally by doping the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ materials family with chromium, where 5% doping induces an exchange field of around 0.1 eV. In ultrathin films, a topological phase transition from a normal insulator to a Chern insulator can be induced with an exchange field proportional to the hybridization gap. Subsequent transitions to states with higher Chern numbers require an exchange field larger than the (bulk) band gap, but are prohibited in practice by the detrimental effects of higher doping levels. Here, we show that threshold doping for these phase transitions in thin films is controllable by strain. As a consequence, higher Chern states can be reached with experimentally feasible doping, sufficiently dilute for the topological insulator to remain structurally stable. Such a facilitated realization of higher Chern insulators opens prospects for multichannel quantum computing, higher-capacity circuit interconnects, and energy-efficient electronic devices at elevated temperatures.

A broadly used strategy to go beyond the well-known local-density approximation of density functional theory relies on the choice of a so-called exchange-correlation (xc) enhancement factor $F_{\mathrm{xc}}$, defined as the enhancement of a realistic xc energy density over its local exchange-only counterpart. To date, this density functional, $F_{\mathrm{xc}}$, has been constructed by following either semiempirical strategies or nonempirical schemes that impose the fulfillment of exact constraints. Here, we follow a totally different route, which is based on an attempt to construct a universal exchange enhancement factor $F_x$ from the exact exchange energy density of a given family of electron density profiles and which we implement on the basis of jellium-slab exact-exchange self-consistent calculations. We find that such an enhancement factor can, indeed, be built which obeys, within our sample of electron-density profiles, most exact constraints and thus represents a benchmark towards the construction of a universal exchange enhancement factor suitable for all electron densities. We provide, in particular, an analytical parametrization of our ab initio calculations at the level of the generalized gradient approximation.

The recent synthesis of a new polymorph of two-dimensional (2D) germanium monochalcogenides, namely, $\gamma$-GeSe with a four-atomic-layer-thick hexagonal lattice, has received considerable attention due to its novel properties and potential applications. This exciting advancement paves the path for extensive experimental and theoretical investigations on the family of $\gamma$-MX crystals in which $M$ and $X$ are elements of group IV and VI, respectively. In this regard, herein we conduct first-principles-based calculations to explore the structural, vibrational, mechanical, electronic, and piezoelectric properties of $\gamma \text{−}\mathrm{Ge}X$ and Janus $\gamma \text{−}{\mathrm{Ge}}_2{XX}^{\prime }$ ($X/X^{\prime }$ : S, Se, and Te) monolayers. We performed a detailed analysis of the suggested systems' dynamical, thermal, and mechanical stability through phonon-band-dispersion calculations, ab initio molecular dynamics (AIMD) simulations, and elastic tensor analyses, respectively, and all six possible nanosheets are found to be stable. The computed Raman spectra of the monolayers reveal that, different from binary systems, the formation of Janus monolayers results in the appearance of additional Raman active modes. The mechanical response of the proposed crystals is examined by calculating in-plane stiffness $\left(Y_{2D}\right)$ and the Poisson's ratio $\left(\nu \right)$ within the elastic regime, and the obtained results ascertain their flexibility. It is found that similar to their binary counterparts, Janus monolayers are indirect-band-gap semiconductors, and their valence-band maxima show a Mexican hat dispersion along the high-symmetry points of the Brillouin zone. Additionally, it is demonstrated that the construction of Janus crystals enhances the piezoelectric coefficients of $\gamma \text{−}\mathrm{Ge}X$ monolayers, both in the in-plane and out-of-plane directions. Our findings not only provide a comprehensive insight into physical and electronic properties of $\gamma \text{−}\mathrm{Ge}X$ and $\gamma \text{−}{\mathrm{Ge}}_2{XX}^{\prime }$ monolayers but also reveal their promising features for various nanoelectronic and nanoelectrochemical applications.

We analyze the relation between quantum chaotic behavior and phase transitions of the Yukawa–Sachdev-Ye-Kitaev model as a function of filling and temperature, which describes random Yukawa interactions between $N$ complex fermions and $M$ bosons in zero spatial dimensions for both the non-Fermi liquid and insulating states at finite temperature and chemical potential. We solve the ladder equations for the out-of-time-order correlator (OTOC) for both bosons and fermions. Despite the appearance of the chemical potential in the Hamiltonian, which explicitly introduces an additional energy scale, the OTOCs for the fermions and bosons in the non-Fermi liquid state turn out to be unaffected, and the Lyapunov exponents that diagnose chaos remain maximal. As the chemical potential increases, the system is known to experience a first-order transition from a critical phase to a gapped insulating phase. We postulate that the boundary of the region in parameter space where each phase is (meta)stable coincides with the curve on which the Lyapunov exponent is maximal. By calculating the exponent in the insulating phase and comparing to numerical results on the boundaries of stability, we show that this is plausible.

We propose a scheme to perform tensor network based finite-size scaling analysis for two-dimensional classical models. In the tensor network representation of the partition function, we use higher-order tensor renormalization group (HOTRG) method to coarse grain the weight tensor. The renormalized tensor is then used to construct the approximated transfer matrix of an infinite strip of finite width. By diagonalizing the transfer matrix we obtain the correlation length, the magnetization, and the energy density, which are used in finite-size scaling analysis to determine the critical temperature and the critical exponents. As a benchmark we study the two-dimensional classical Ising model. We show that the critical temperature and the critical exponents can be accurately determined. With HOTRG bond dimension $D=70$, the absolute errors of the critical temperature $T_c$ and the critical exponent $\nu$, $\beta$ are at the order of ${10}^{-7},{10}^{-5}$, ${10}^{-4}$, respectively. Furthermore, the results can be systematically improved by increasing the bond dimension of the HOTRG method. Finally, we study the length scale induced by the finite cutoff in bond dimension and elucidate its physical meaning in this context.

Recent experiments have demonstrated interesting physics in a family of two-dimensional composition-tunable materials ${\mathrm{Nb}}_{2n+1}{\mathrm{Si}}_n{\mathrm{Te}}_{4n+2}$. Here we show that, owing to their intrinsic low symmetry, metallic nature, tunable composition, and ambient stability, these materials offer a good platform for studying the Berry curvature dipole (BCD) and nonlinear Hall effect. Using first-principles calculations, we find that the BCD exhibits pronounced peaks in monolayer ${\mathrm{Nb}}_3{\mathrm{SiTe}}_6$ (the $n=1$ case). Its magnitude decreases monotonically with $n$ and completely vanishes in the $n\to \infty$ limit. This variation manifests a special hidden dimensional crossover of the low-energy electronic states in this system. The resulting nonlinear Hall response from the BCD in these materials is discussed. Our work reveals pronounced geometric quantities and nonlinear transport physics in ${\mathrm{Nb}}_{2n+1}{\mathrm{Si}}_n{\mathrm{Te}}_{4n+2}$ family materials, which should be readily detected in experiment.

Using first-principles density functional theory (DFT) methods and size-converged supercell models, we analyze the electronic and atomic structure of magnetic $3d$ transition metal dopants in cubic gallium nitride (c-GaN). All stable defect charge states for Fermi levels across the full experimental gap are computed using a method that correctly resolves the boundary condition problem (without a jellium approximation) and eliminates finite-size errors. The resulting computed defect levels are not impacted by the DFT band-gap problem, they span a width consistent with the experimental gap rather than being limited to the single-particle DFT gap. All defects with electronically degenerate (half-metal) $T_d$ ground states are found to have significant distortions, relaxing to $D_{2d}$ structures driven by the Jahn-Teller instability. This leads to insulating ground states for all substitutional $3d$ dopants, refuting claims in the literature that $+U$ or hybrid functional methods are required to avoid artificial half-metal results. Interpreting the $d^n$ atomic occupations within a crystal-field model and exchange splittings, we identify a systematic trend across the $3d$ transition metal series. Approaches to estimate excited-state energies as observed in photoluminescence from defect centers are assessed, ranging from a Koopmans-type single-particle energy interpretation to relaxed total energy differences in fully self-consistent DFT. The single-particle interpretations are found to be qualitatively predictive and the calculations are consistent with the limited available experimental data across the $3d$ dopant series. These results provide a baseline understanding to guide future studies and a conceptual framework within which to interpret new results.