First-principles-based atomistic simulations are used to reveal ferroelectric phases and phase transitions induced in a semiconductor ferroelectric, ${\mathrm{CsGeBr}}_3$, by external loads: hydrostatic pressure, uniaxial and biaxial stresses, and misfit strain. Hydrostatic pressure was found to suppress the Curie point at the rate $-0.45T_C\left(0\right)$ K/GPa, where $T_C\left(0\right)$ is the zero pressure Curie temperature. Stresses and misfit strains were found to induce additional ferroelectric phase transitions and phases not available under normal conditions. We find that tensile load significantly enhances both the Curie temperature and spontaneous polarization, while compressive load has the opposite effect but with the difference that the Curie temperature is only slightly suppressed. The isothermal dependencies of polarization on pressure and stresses are highly nonlinear, which could result in large nonlinear piezoelectric responses. The phase diagrams reveal the diversity of the phases accessible through mechanical load, which include tetragonal, orthorhombic, and monoclinic symmetries in addition to the rhombohedral and cubic ones realizable under normal conditions. We believe that this work reveals the potential of Ge-based halide perovskites for applications in energy converting devices, which is especially significant in the current pursuit of environmental friendly lead-free technologies.

We investigate phonon lifetimes in $\mathrm{V}{\mathrm{O}}_2$ single crystals. We do so in the metallic state above the metal-insulator transition (MIT), where strong structural fluctuations are known to take place. By combining inelastic x-ray scattering and Raman spectroscopy, we track the temperature dependence of several acoustic and optical phonon modes up to 1000 K. Contrary to what is commonly observed, we find that phonon lifetimes either stay constant or slightly increase with temperature. Our results show that pretransitional fluctuations in the metallic state give rise to strong electron-phonon scattering that onsets hundreds of degrees above the transition and increases as the MIT is approached. Notably, this effect is not limited to specific points of reciprocal space that could be associated with the structural transition.

Composite topological phases with intriguing topology like Möbius strips emerge in sublattice symmetric non-Hermitian systems due to spontaneous breaking of time-reversal symmetry at some parameter regime. While these phases have been characterized by nonadiabatic complex geometric phases of multiple participating complex bands, the physical properties of these phases remain largely unknown. We explore the dynamical response of these phases by studying Loschmidt echo from an initial state of the Hermitian Su-Schrieffer-Heeger (SSH) model, which is evolved by a non-Hermitian SSH Hamiltonian after a sudden quench in parameters. Topology-changing quenches display nonanalytical temporal behavior of return rates (logarithm of the Loschmidt echo) for the non-Hermitian SSH Hamiltonian in the trivial, Möbius, and topological phase. Moreover, the dynamical topological order parameter appears only at one side of the Brillouin zone for the Möbius phase case in contrast to both sides of the Brillouin zone for quench by the trivial and topological phase of the non-Hermitian SSH model. The last feature is a dynamical signature of different symmetry constraints on the real and imaginary parts of the complex bands in the Möbius phase.

Spin-torque oscillators (STOs) are dissipative magnetic systems that provide a natural platform for exploring non-Hermitian phenomena. We theoretically study a two-dimensional (2D) array of STOs and show that its dynamics can be mapped to a 2D, non-Hermitian Su-Schrieffer-Heeger (SSH) model. We calculate the energy spectrum and identify the one-dimensional (1D) edge states of our model, corresponding to auto-oscillation of STOs on the boundary of the system while the bulk oscillators do not activate. We show that tuning the Gilbert damping, injected spin current, and coupling between STOs allows for exploring the edge-state properties under different parameter regimes. Furthermore, this system admits 1D edge states with nonuniform probability density, and we explore their properties in systems of different sizes. Additional symmetry analysis indicates that these states are not topologically protected but are nevertheless confined to the edge of the system, as the bulk is protected by $\mathcal{P}\mathcal{T}\text{symmetry}$. These results indicate that 2D arrays of STOs may be useful to explore novel edge-state behavior in dissipative systems.

This paper introduces an approach to approximating exchange interactions in FePt, characterizing their effect on the Curie temperature and magnetic anisotropy, properties crucial for heat-assisted magnetic recording. The proposed model employs the Ruderman-Kittel-Kasuya-Yosida (RKKY) function, offering a different perspective on magnetization processes within finite FePt grains. The RKKY function is derived as a specific instance of a fractal curve, such as the Jacobi elliptical function. The study showcases a holographic implementation of these exchange interactions, translatable into an atomistic spin dynamics model, yielding valid outcomes for finite-size scaling laws. The primary goal is to formulate an approximate spin Hamiltonian with fewer neighbors, enhancing the efficiency of simulations for the FePt which is a candidate for heat-assisted magnetic recording media. Additionally, the research delves into how the number of interactions per bond impacts magnetization evolution across different temperatures and system sizes. Our findings suggest that this model is more computationally efficient for Monte Carlo simulations than the comprehensive density-functional-theory-based spin Hamiltonian proposed by Mryasov et al.

In $\delta \text{−}{\mathrm{Ag}}_{2/3}{\mathrm{V}}_2{\mathrm{O}}_5$ with charge degrees of freedom in V, it is known that the charge ordering state and physical properties of V that appear at low temperatures depend strongly on the ordering state of Ag. In this study, we focus on the Ag ions in the interlayer and study the structure using synchrotron radiation powder diffraction depending on temperature. We find that when the sample is slowly cooled from room temperature and ordering occurs at the Ag sites, ${\mathrm{V}}^{4+}/{\mathrm{V}}^{5+}$ charge ordering of V and subsequent ${\mathrm{V}}^{4+}\text{−}{\mathrm{V}}^{4+}$ structural dimers are produced. Although quenching the sample from room temperature suppresses the ordering of Ag, annealing at around 160 K promotes partial ordering of Ag and allows a metastable phase to be realized. This metastable phase is maintained even when the temperature is lowered again, producing a remarkable change in low-temperature properties. These results indicate that the ordered state of Ag, which is the key to control the charge-ordered state and physical properties, can be controlled by low-temperature annealing. The results of this study may provide a methodology for the realization of metastable states in a wide range of material groups of vanadium compounds, in which competition among various charge-ordered states underlies the physical properties.

The structural origin of the first sharp diffraction peak (FSDP) in amorphous silica is studied by analyzing chemical and radial ordering of silicon (Si) and oxygen (O) atoms in binary amorphous networks. The study shows that the chemical order involving Si–O and O–O pairs play a major role in the formation of the FSDP in amorphous silica. This is supplemented by small contributions arising from the relatively weak Si–Si correlations in the Fourier space. A shell-by-shell analysis of the radial correlations between Si–Si, Si–O, and O–O atoms in the network reveals that the position and the intensity of the FSDP are largely determined by atomic pair correlations originating from the first two/three radial shells on a length scale of about 5–8 Å, whereas the fine structure of the intensity curve in the vicinity of the FSDP is perturbatively modified by atomic correlations arising from the radial shells beyond 8 Å. The study leads to a simple mathematical relationship between the position of the radial peaks $\left(r_k\right)$ in the partial pair-correlation functions and the diffraction peaks $\left(Q_k\right)$ that can be used to obtain approximate positions of the FSDP and the principal peak. The results are complemented by numerical calculations and an accurate semi-analytical expression for the diffraction intensity obtained from the partial pair-correlation functions of amorphous silica for a given radial shell.

We employ density functional theory based phonon transport methods to provide a rigorous understanding of the nature of thermal transport in coherent short-period AlN/GaN superlattices (SLs), with period lengths up to three unit cells of each, and compare these with properties of their bulk constituents. Increasing the period length leads to phonon band folding with frequency gaps and thus smaller phonon velocities and more phonon scattering than in bulk, both of which reduce lattice thermal conductivity ($\kappa$). Contrary to expectations, we find that velocity variations among larger-period AlN/GaN SLs play only a minor role in cross-plane $\kappa$ reductions, while variations in intrinsic phonon scattering are strongly correlated with their transport behaviors. This work provides insights into the microscopic behaviors of technologically relevant nanostructured materials, which are likely applicable to a wider range of SL systems and other nanostructures.

Dynamical many-body freezing occurs in periodic transverse field-driven integrable quantum spin systems. Under freezing conditions, quantum dynamics causes practically infinite hysteresis in the drive response, maintaining its starting value. We find similar resonant freezing in the Lipkin-Meshkov-Glick (LMG) model. In the LMG, the freezing conditions in the driving field suppresses the heating postulated by the eigenstate thermalization hypothesis (ETH) by inducing dynamical many-body localization, or DMBL. This is in contrast to many-body localization (MBL), which requires disorder to suppress ETH. DMBL has been validated by the inverse participation ratio (IPR) of the quasistationary Floquet modes. Similarly to the TFIM, the LMG exhibits high-frequency localization only at freezing points. IPR localization in the LMG deteriorates with an inverse system size law at lower frequencies, which indicates heating to infinite temperature. Furthermore, adiabatically increasing frequency and amplitude from low values raises the Floquet state IPR in the LMG from nearly zero to unity, indicating a phase crossover. This occurrence enables a future technique to construct an MBL engine in clean systems that can be cycled by adjusting drive parameters only.

In the field of symmetry-protected topological phases, a common wisdom is that the symmetries may fix the nontrivial topological classifications, but they alone cannot determine whether a system is topologically nontrivial. Here, we show that this is no longer true in cases where symmetries are projectively represented. Particularly, the Zak phase, a topological invariant of a one-dimensional system, can be entirely determined by the projective symmetry algebra (PSA). To demonstrate this remarkable effect, we propose a minimal model, termed the flux Su-Schrieffer-Heeger (f-SSH) model, where the bond dimerization in the original SSH model is replaced by a flux dimerization. We present experimental realization of the f-SSH model in an electric-circuit array, and our predictions are directly confirmed by electric measurement. Our work refreshes the understanding of the relationship between symmetry and topology, opens up avenues for exploring novel PSA determined topological phases, and suggests flux dimerization as an approach for designing topological crystals.

We analyze optical conductivity of a clean two-dimensional electron system in a Fermi liquid regime near a $T=0$ Ising-nematic quantum critical point (QCP) and extrapolate the results to a QCP. We employ direct perturbation theory up to the two-loop order to elucidate how the Fermi surface's geometry (convex vs concave) and fermionic dispersion (parabolic vs nonparabolic) affect the scaling of the optical conductivity $\sigma \left(\omega \right)$ with frequency $\omega$ and correlation length $\xi$. We find that for a convex Fermi surface the leading terms in the optical conductivity cancel out, leaving a subleading contribution $\sigma \left(\omega \right)\propto {\omega }^2{\xi }^4\mathcal{L}$, where $\mathcal{L}=\mathrm{const}$ for a parabolic dispersion and $\mathcal{L}\propto ln\omega {\xi }^3$ in a generic case. For a concave Fermi surface, the leading terms do not cancel, and $\sigma \left(\omega \right)\propto {\xi }^2$. We extrapolate these results to a QCP and obtain $\sigma \left(\omega \right)\propto {\omega }^{2/3}$ for a convex Fermi surface and $\sigma \left(\omega \right)\propto 1/{\omega }^{2/3}$ for a concave Fermi surface.

In this work, the realization of nonreciprocal transport in Weyl semimetals is proposed by manipulating surface states. Our method, distinct from traditional techniques focusing on bulk band asymmetry, involves surface asymmetries characterized by an asymmetric mass term. Utilizing the Boltzmann transport equation, we unveil second-order conductivity responses, leading to nonreciprocal transport primarily governed by Fermi arc surface states when the Fermi surface is proximal to the Weyl point. Our method deviates from conventional techniques focused on bulk band asymmetry, shedding light on distinctive transport properties from engineering surface states in Weyl semimetals. Importantly, our findings provide an observable signal for surface state engineering, enhancing the understanding of nonlinear transport phenomena in these topological semimetals.

We theoretically consider bilayers of two-dimensional (2D) electron gases as in semiconductor quantum wells, and investigate possible spontaneous symmetry breaking transitions at low carrier densities driven by interlayer Coulomb interactions. We use a self-consistent technique implementing mean-field truncations of the interacting four-fermion terms, and find a U(1) layer symmetry breaking transition at low carrier densities where the individual layer identities are lost leading to an effective pseudospin XY ferromagnet in the 2D plane. Our results validate earlier theoretical studies using simpler restricted Hartree-Fock techniques, and establish the pseudospin XY ferromagnet as a possible low-density symmetry broken phase of 2D bilayers.

Two-dimensional magnetic materials are at the forefront of the next generation of spintronic devices. The possibility to interface them with other van der Waals materials such as transition metal dichalcogenides has opened new possibilities for the observation of new and existing physical phenomena. Here, we present a proof-of-concept valleytronic device based on ${\mathrm{CrBr}}_3$-encapsulated ${\mathrm{WSe}}_2$ showing an unprecedented valley splitting of $\sim 100\mathrm{meV}$ under the compressive strain of the ${\mathrm{WSe}}_2$, able to be tuned by the relative magnetization of the encapsulating layers. Multiscale transport simulations performed on this device show a spin-valley current with a polarization higher than $80\%$ than is maintained in a range of $\sim 0.3\mathrm{V}$ gate voltage in a field-effect transistor configuration. The impact of the stacking configuration on the valley splitting is also evaluated.

We investigated hole (Cd)- and electron (Sn)-doped ${\mathrm{CeCoIn}}_5$ [$\mathrm{CeCo}{\left({\mathrm{In}}_{1-x}{T}_x\right)}_5$ ($T=\mathrm{Cd}$ or Sn)] using infrared spectroscopy. Doping-dependent hybridization-gap distribution functions were obtained from the optical conductivity spectra based on the periodic Anderson model formalism. The hybridization-gap distribution exhibits two components: in-plane and out-of-plane hybridization gaps. The doping-dependent evolution of the two gaps indicated that the out-of-plane gap was more sensitive to doping. Furthermore, the magnetic optical resistivity exhibited a doping-dependent evolution of the $f$-electron amplitude. The two dopant types exhibited different physical properties depending on the level of doping. The Sn dopant increases the $f$-electron amplitude, whereas the Cd dopant does not affect the $f$-electron amplitude. Doping-dependent effective mass is peaked at pure (or undoped) ${\mathrm{CeCoIn}}_5$. Our spectroscopic results may help understand the doping-dependent electronic evolution of one of the canonical heavy-fermion systems, ${\mathrm{CeCoIn}}_5$.

We show how distinct phases of matter can be generated by performing random single-qubit measurements on a subsystem of toric code. Using a parton construction, such measurements map to random Gaussian tensor networks, and in particular, random Pauli measurements map to a classical loop model in which watermelon correlators precisely determine measurement-induced entanglement. Measuring all but a 1d boundary of qubits realizes hybrid circuits involving unitary gates and projective measurements in $1+1$ dimensions. We find that varying the probabilities of different Pauli measurements can drive transitions in the unmeasured boundary between phases with different orders and entanglement scaling, corresponding to short- and long-loop phases in the classical model. Furthermore, by utilizing single-site boundary unitaries conditioned on the bulk measurement outcomes, we generate mixed-state ordered phases and transitions that can be experimentally diagnosed via linear observables. This demonstrates how parton constructions provide a natural framework for measurement-based quantum computing setups to produce and manipulate phases of matter.

Epsilon-near-zero (ENZ) materials have attracted significant attention in the far- and near fields of thermal radiation in recent years because of their unique optical characteristics. However, it is not considered an optimal carrier for near-field radiative heat transfer (NFRHT) due to the excessively low frustrated mode. In this paper, we address this drawback with a metamaterial composed of artificial hypothetical nonpolar ENZ dielectric-filled Si gratings. The behavior of NFRHT with ENZ has been investigated based on fluctuational electrodynamic and rigorous coupled-wave analysis. An artificial mode named Meta-NP ENZ mode is presented to reveal the significant enhancement of NFRHT, which can be demonstrated by the electric field intensity enhancement. Furthermore, we find that the increasing imaginary part may lead to an anomalous amplification of heat flux despite causing a recession in the Meta-NP ENZ mode, and this mode remains robust with respect to the plasma frequency shift of grating material. Our findings demonstrate that the ENZ dielectric exhibits outstanding performances similar to those observed in far-field radiation, surpassing the limitations of both Si gratings and nonpolar ENZ dielectric in NFRHT.

The response of traditional two-dimensional (2D) materials to electromagnetic fields is typically homogeneous and local, as the wavelength of the electromagnetic field is much longer than the lattice constant of 2D materials. In contrast, plasmons in moiré flatband systems enable the coupling of electromagnetic fields and electrons at extremely small length scales. In this paper, we demonstrate that the plasmons in magic-angle twisted trilayer graphene (TTG) exhibit significant inhomogeneity and nonlocality, along with violation of in-plane wave-vector conservation. The exceptional optical scenarios arising from inhomogeneous response can be characterized by the 2D distribution patterns of the plasmonic electric fields. Our findings provide additional means for detecting the electronic states of materials by near-field optical experiments, as well as an innovative approach for advancing the development of optoelectronic devices.