The discovery of a charge density wave in FeGe extends the discussion of the nature of charge order in kagome metals to a magnetic compound. Motivated by this observation, we combine density functional theory (DFT) and functional-renormalization-group calculations to study interaction-induced Fermi-surface instabilities of the magnetic state of FeGe. We argue that the leading intra-band contribution to electronic correlations are approximately two-dimensional (2D) and come from Van Hove points at the projected $M$ points. By varying parameters around DFT values, we determine a phase diagram for the quasi-2D scenario as function of on-site and nearest-neighbor interactions. We discuss universal aspects in the electronic mechanisms for the resulting phases, as well as the role of SU(2) symmetry breaking. We find FeGe to be in a regime of strong competition between $p$-wave charge density wave, $f$-wave pairing, and $d$-wave spin Pomeranchuk instabilities. This interplay can be influenced in favor of superconducting pairing for slightly increased nearest-neighbor interaction, suggesting a potential to induce superconductivity in FeGe.

The nature of the phonon triplet in the region of OH-stretching modes in hydrogen-bonded materials is often explained by the interplay of OH-stretching modes and combinations and overtones of OH-bending modes. To elucidate both contributions in ${\mathrm{KH}}_2{\mathrm{PO}}_4$ (KDP), we compare the pressure dependence of the OH-bending and stretching modes from ab initio calculations and inelastic neutron scattering (INS) measurements. The ab initio calculations predict a hardening of OH-bending modes and a softening of OH-stretching modes with pressure. At the same time, INS measurements in the region of OH-stretching modes indicate a hardening of the phonon triplet together with the bending modes. This means that this triplet in INS measurements is mainly due to combinations and overtones of OH-bending modes, while the intensity of OH-stretching modes appears to be relatively low. This conclusion may also apply to other hydrogen-bonded materials.

We investigate the dielectric response as a function of temperature and applied strain of the prototypical perovskite ${\mathrm{SrTiO}}_3$ using ab initio molecular dynamics to calculate the complex, dynamical, temperature-dependent lattice dielectric function. The results show excellent agreement with experiment. On the application of tensile uniaxial strain, the static susceptibility shows divergence indicative of a ferroelectric transition. We calculate the required critical strain $(3.8\%)$ and estimate the critical exponents of the second-order phase transition. We also see the emergence of the antiferrodistortive phase transition at $\sim 110\mathrm{K}$. The method can be readily applied to study the dielectric properties and phase transitions in other ferroelectric materials at finite temperatures.

The metastable A15 β phase of tungsten exhibits the largest spin Hall angle among all transition metals and is of interest for applications in spintronic devices. However, β-W is known to support very high stresses in such applications, which may affect properties and reliability. To understand these stresses, the thermomechanical properties must be known. The coefficient of thermal expansion (CTE) and elastic constants of β-W thin films containing 0–11.1 at. % nitrogen were determined within the ${24}^{\circ }-100{}^{\circ }\mathrm{C}$ temperature range using three methods: substrate curvature measurements of thermoelastic curves for films deposited on $\mathrm{Si}\langle 100\rangle$ and fused silica substrates, ab initio molecular dynamics (AIMD) simulations, and classical molecular dynamics (MD) simulations using a Spectral Neighbor Analysis Potential fitted from AIMD results using machine learning. Single crystal elastic constants obtained from the simulations were used along with the measured textures of the films to calculate biaxial elastic moduli. Results from all three approaches were consistent. The results of the same methods applied to the well-known BCC α phase of W were within 3% of reference values, and those for β-W are expected to have a similar accuracy. It was found that the CTE of β-W increases while the biaxial modulus decreases with increasing nitrogen content. The average CTE and biaxial modulus of pure β-W within ${24}^{\circ }-100{}^{\circ }\mathrm{C}$ were determined to be 4.931 $\times {10}^{-6}{/}^{\circ }\mathrm{C}$ and 503 GPa, respectively, which are 10% higher and 11% lower than those of α-W.

Exploiting non-Hermitian wave-matter interactions in time-modulated media to enable the dynamic control of electromagnetic waves requires advanced theoretical tools. In this article we bridge concepts from photonic quasinormal modes (QNMs) and time-varying metamaterials providing the foundation for designing dynamic optical devices with prescribed scattering properties. Establishing the QNM framework for slabs with time-periodic permittivity, and solving the associated nonlinear eigenvalue problem, allows us to derive the QNM expansion capturing the resonant features of the system. This reduced-order model enables highly efficient computation of scattered fields while revealing insight into how modulation couples to resonant modes, creating tailored gain-loss engineering. Our approach is validated through numerical experiments on time-modulated systems, and we design strategies to engineer tailored excitations selectively amplifying or suppressing specific modal contributions.

We present a numerical study of three-layer graphene heterostructures in which the layers are twisted by the magic angle ($\sim 1.1^{\circ }$) or by $\sim {30}^{\circ }$ to form a graphene quasicrystal. The heterostacks are described using realistic structural relaxations and tight-binding Hamiltonians, and their transport properties are computed for both pristine and disordered systems containing up to $\sim 8$ million atoms. Owing to the weak interlayer coupling, we resolve the hybridization between magic-angle flat bands and quasicrystalline states, which are modified in distinct ways across low- and high-energy windows, revealing a different hybrid electronic regime to explore.

A slab of time-reversal-symmetric Weyl semimetal serves as a realistic material platform for studying Anderson localization (AL) in two-dimensional (2D) systems, as it belongs to the symplectic symmetry class, which is the only 2D system that hosts an AL transition. In the present work, we theoretically explore the features of AL in such a slab induced by conventional white-noise disorder and random surface adsorption. By means of the scaling analysis of the localization length, we find that the surface adsorption can facilitate the AL. Specifically, a higher surface adsorption concentration results in a lower critical disorder strength of white noise. However, the bond energy of surface adsorption hardly influences the AL in the slab, since surface adsorption can equivalently be treated as random surface vacancies at a given concentration. More interestingly, when the slab with surface adsorption enters the localization regime after the AL transition, we observe anomalous near-degeneracy by statistics of the adjacent energy-level spacings of the slab. Through numerical analysis, we find that such an anomaly associated with the random surface adsorption can be justified in terms of weak level repulsion between exotic and intrinsic atoms on the slab surface in the strong localization regime.

Spin-liquid and spin-glass states represent two distinct phases of disordered quantum spin systems. These states are, in principle, distinguished by quantum-entangled fluctuations and spin freezing, but identifying each state and characterizing the transition between them remains challenging. Here, we systematically explore the relationship between the spin-liquid and spin-glass states using a model with all-to-all random interactions among $p$ spins, which interpolates between the Ising-like one-component, XY-like two-component, and isotropic three-component cases. By analyzing the system-size $N$ dependence of the Edwards-Anderson order parameter and the density of states, we identify the transition from the spin liquid to the spin glass for various values of $p$. We show that the phase diagrams for different $p$ can be unified through a scaling with $N/p^2$, revealing that increasing anisotropy in the interactions systematically suppresses the spin-liquid phase and extends the spin-glass regime. Furthermore, we examine the competition between multiple-spin interactions and anisotropy under an external magnetic field in the isotropic case, and find that the spin-liquid phase transitions into the spin-glass phase before entering a quantum paramagnetic phase. Our findings provide insights into quantum disordered phases and the transitions between them.

We introduce and implement a reformulation of the strong-disorder renormalization group method in real space, well suited to study bond-disordered antiferromagnetic power-law coupled quantum spin chains. We apply it to a chain of randomly placed spins coupled by power-law long-range interaction with power $\alpha$. First, keeping only interactions between adjacent spins, we derive the Master equation and confirm that it is solved by the infinite randomness fixed-point distribution. Then, we derive the master equation for power-law long-range interaction between all spins for any anisotropy $\gamma$ ranging from the XX limit, $\gamma =0$, to the isotropic Heisenberg limit, $\gamma =1,$ which corresponds to a tight-binding chain of disordered long-range interacting fermions with long-range hopping. We thereby show that the distribution function of couplings $J$ smaller than renormalization scale $\mathrm{\Omega }$ flows to the strong-disorder fixed-point distribution of finite width $2\alpha .$ We find only small corrections to that distribution, which depend on power exponent $\alpha$ and coupling anisotropy $\gamma .$ As a consequence, the low-temperature magnetic susceptibility diverges with an anomalous power law. The distribution of singlet lengths $l$ is found to decay as $l^{-2}$. The entanglement entropy of a subsystem of length $n$ increases in the ground state logarithmically for all $\alpha$ and $\gamma$. After a global quantum quench, the entanglement entropy increases with time logarithmically as $S\left(t\right)\sim ln\left(t\right)/\left(2\alpha \right)$.

The deflection of waves by combining the effects of time modulation with anisotropy has been recently proposed in the context of electromagnetism. In this work, we characterize this phenomenon, called temporal aiming, for water waves using a time-varying metabathymetry. This metabathymetry is composed of thin vertical plates that are periodically arranged at the fluid bottom and which act as an effective anisotropic medium for the surface wave in the long-wavelength approximation. When this plate array is vertically lifted at the fluid bottom at a given time, the medium switches from isotropic to anisotropic, causing a wave packet to scatter in time and deflect from its initial trajectory. Following a simple modeling, we obtain the scattering coefficients of the two waves generated due to the sudden medium change as well as the angle of deviation with respect to the incident angle. We then numerically evaluate this scattering problem with simulations of the full two-dimensional effective anisotropic wave equation, with a time-dependent anisotropy tensor. Finally, we provide experimental evidence of the temporal aiming, using space-time resolved measurement techniques, demonstrating the trajectory shift of a wave packet and measuring its angle of deviation.

Understanding the lattice dynamics in $p$-type PbTe-based materials is vital to their phonon engineering and to further lowering thermal conductivity in this prominent thermoelectric family. Here we report a systematic study of the high-pressure behavior of ${\mathrm{Pb}}_{0.99}{\mathrm{Na}}_{0.01}\mathrm{Te}$ using Raman scattering and x-ray diffraction measurements together with first-principles calculations. Our calculations reveal the important contributions of the optical modes to the heat transfer. The evolution of both the phonon vibrational and structural properties with pressure is established within the initial phase, which has a cubic NaCl structure. The experimentally determined average Grüneisen parameter of about 4 in this phase is larger than those of most other high-performance thermoelectric materials, demonstrating the giant anharmonicity in such a rock-salt lattice. We show that the primary contributor to this strong anharmonicity originates from the transverse optical mode, which possesses the highest mode-Grüneisen parameter due to its sensitive dependence on the lattice change. Our findings suggest that managing the highly volume-dependent phonons in thermoelectrics is the key to further tailoring their heat conduction.

We investigate the temperature-dependent free electron conduction, terahertz phonon modes, and electron-phonon interactions, in the ternary topological insulator ${\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_2{\mathrm{Te}}_3$. Terahertz time-domain spectroscopy and Raman spectroscopy on three $n$-type ${\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_2{\mathrm{Te}}_3$ samples of various thicknesses were performed. Strong phonon activity and electron–phonon coupling were experimentally revealed and found to be sensitive to temperature changes and subtle structural variations between the samples. The experimental results, well described by Drude-Lorentz-Fano model, were compared to density functional theory calculations, yielding the phonon mode assignment. Further, an additional broadened background absorption was observed and attributed to the morphology of the conductive top surface of the samples.

Quantum many-body scars (QMBSs) offer a mechanism for weak ergodicity breaking, enabling nonthermal dynamics to persist in a chaotic many-body system. While most studies of QMBS focus on anomalous eigenstate properties or long-lived revivals of local observables, their potential for quantum information processing remains largely unexplored. In this work, we demonstrate that perfect quantum state transfer can be achieved in a strongly interacting, quantum chaotic spin chain by exploiting a sparse set of QMBS eigenstates embedded within an otherwise thermal spectrum. These results show that QMBSs in chaotic many-body systems may be harnessed for information transport tasks typically associated with integrable models.

Decoherence effects for entangled triplet exciton pairs in organic molecular crystals are analyzed for the case when excitons can hop between inequivalent lattice sites. The fluorescence quantum beats caused by quantum interference upon triplet-triplet recombination into an emissive singlet state are predicted as a function of hopping time and magnetic field based on a Monte Carlo analysis. Depending on exciton hopping rates, it is possible to have global decoherence and complete suppression of fluorescence quantum beats in the limit of zero magnetic field, and to have quantum beats that decay at different rates depending on magnetic field strength.

Accurate prediction of lattice thermal conductivity $\left({\kappa }_l\right)$ in strongly anharmonic materials requires renormalized interatomic force constants (IFCs) and appropriate incorporation of diagonal and off-diagonal contributions and higher-order scattering. We investigate CuCl, a highly anharmonic system with a simple zincblende structure and ultralow ${\kappa }_l$. Our calculations, including IFC renormalization and four-phonon scattering, show excellent agreement with the experiment, underscoring the critical role of both effects in the accurate estimation of ${\kappa }_l$. Furthermore, the unusual pressure dependence of ${\kappa }_l$ is explored using a rigorously validated machine-learned force field, with the predicted values showing good agreement with the experimentally observed trend of monotonic decrease. This behavior is primarily driven by a significant increase in four-phonon scattering and a reduction in the group velocity of transverse acoustic modes. Overall, this study establishes a robust framework for modeling thermal transport in strongly anharmonic materials.

The topological properties of non-Hermitian systems are quite different from those of their Hermitian counterparts and include defective edge states, breakdown of the bulk-boundary correspondence, and the topological structure of the point gap. Therefore, in non-Hermitian systems, we may use an alternative approach to tune their topological properties. In this work, we present an approach for tuning topological states by inhomogeneous dissipation (or gain/loss). As the dissipation increases, the system evolves from periodic boundary conditions (PBCs) to open boundary conditions (OBCs) at the critical point. A system under closed boundary conditions (CBCs) is torn into two subsystems under OBCs with increasing dissipation. These conclusions can be demonstrated numerically and analytically. During this process, the global phase diagrams are hybrids of those under PBCs and OBCs, and the topological regions are both dissipation tuneable and analytically solvable. Remarkably, various phases and phase transitions appear, and each phase transition corresponds to unique behavioral signatures in the generalized Brillouin zone (GBZ), thus revealing rich GBZ configurations. Additionally, our results can be straightforwardly applied to other non-Hermitian topological states (including higher-dimensional systems) because of their low-energy effective Hamiltonians. Therefore, our work helps researchers better understand non-Hermitian topological systems.

Single-shot all-optical helicity-independent switching processes are investigated using advanced micromagnetic modeling in a ferrimagnetic thin film embedded in a multilayer stack. Building on recent experimental findings, our multiscale simulations realistically account for heat transport in the stack, focusing on the influence of a metallic copper underlayer with varying thickness. We analyze how this thermal transport affects the final magnetic state of the ferrimagnet as a function of both the laser pulse duration and fluence. Our results reproduce the experimentally observed switching behaviors and elucidate the physical mechanisms that govern the emergence of three distinct final magnetic states. In particular, we demonstrate how these states are critically influenced by the thickness of the underlying copper layer.

In this paper, we successfully grow layered single-crystal ${\mathrm{K}}_{0.79}{\mathrm{Co}}_2{\mathrm{S}}_2$ and systematically investigate its crystal structure, magnetic properties, electrical transport, specific heat, and even magnetic critical behavior. Single-crystal x-ray diffraction measurements indicate that ${\mathrm{K}}_{0.79}{\mathrm{Co}}_2{\mathrm{S}}_2$ crystallizes into a tetragonal structure with space group I4/mmm (No. 139) and lattice parameters of $a=b=3.711\left(4\right)\AA$ and $c=13.140\left(2\right)\AA$. Physical properties measurements demonstrate that ${\mathrm{K}}_{0.79}{\mathrm{Co}}_2{\mathrm{S}}_2$ displays a metallic behavior with a ferromagnetic phase transition at $T_C\approx 113\mathrm{K}$, which was confirmed by the anomaly in the vicinity of $T_C$ that occurs in electrical transport and specific heat curves. The calculated Rhodes-Wolfarth ratio $(\approx 1.56)$ reveals the itinerant ferromagnetic characteristic of ${\mathrm{K}}_{0.79}{\mathrm{Co}}_2{\mathrm{S}}_2$. A set of critical exponents was generated to be $\beta =0.3006\left(4\right)$ with $T_C\approx 113.3\left(6\right)\mathrm{K}$ and $\gamma =1.1802\left(4\right)$ with $T_C\approx 113.2\left(5\right)\mathrm{K}$ based on the modified Arrott plot and $\delta =5.18\left(7\right)$ is yielded using a critical isotherm analysis at $T_C$, revealing a three-dimensional critical behavior. The magnetic interaction is determined to be a three-dimensional long-range coupling with the magnetic exchange distance decaying as $J\left(r\right)\sim r^{-4.91}$. Furthermore, a maximum of the magnetic entropy change $-\mathrm{\Delta }{S}_M^{max}$ for the applied field parallel to the $ab$ plane is estimated to be $\approx 2.51$ $\mathrm{J}{\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ around $T_C$.