We present a comprehensive investigation of the structural, electrical transport, and magnetic properties of ${\mathrm{FeSe}}_x$ ($x$ = 1.14, 1.18, 1.23, 1.28, and 1.32) to unravel the mechanism of the metal-insulator transition observed in these systems. For this, we systematically evaluated the structural parameters of ${\mathrm{FeSe}}_{\mathit{x}}$ as a function of Se concentration and temperature. We observe increased lattice constants and cell volume with increased Se concentration. On the other hand, the temperature-dependent x-ray diffraction studies suggest unusual lattice change around the metal-insulator (MI) transition temperature of the respective compositions. This remarkable observation suggests that the MI transition originates in the anomalous lattice effect in these systems. Additionally, our density of states (DOS) calculations on ${\mathrm{FeSe}}_{1.14}$ qualitatively explain the MI transition, as the low-temperature (50 K) structure DOS suggests a metallic nature and the high-temperature (300 K) structure DOS shows a gap near the Fermi level.

Localization of quasiparticles in disordered and amorphous environments is one of the most intriguing phenomena of condensed matter physics. Thus far, temperature-induced inelastic scattering and magnetic field-induced time-reversal symmetry breaking are the widely known methods to manipulate electron localization in materials. Here, we show conclusive experimental evidence of the photoinduced manipulation of localized electron transport in a prototype-disordered system, chromium nitride (CrN). We inspect this phenomenon by establishing a strong correlation between the temperature-dependent photoconductivity response and the electronic state of CrN. In metallic CrN, the photoresponse is consistently negative, while in semiconducting films, it is positive. However, CrN films exhibiting localized electronic transport at low temperatures exhibit a positive-to-negative photoresponse transition with increasing light intensity. This behavior signifies a localized to metal-like delocalized electronic transition, which we explained through the light-induced breakdown of electron phase coherence responsible for the electron localization and the recharging of traps upon high-intensity photoirradiation mechanisms. Our results illustrate the optical manipulation of electron localization and hold great potential for achieving on-demand localization control for adaptive device applications.

Scanning tunneling microscope data from a dinuclear Co(II) complex adsorbed on Au(111) are analyzed using density functional theory calculations. We find that the interaction with the substrate substantially changes the geometry of the nonplanar molecule. Its electronic states, however, remain fairly similar to those calculated for a gas-phase molecule. The calculations reproduce intriguing contrasts observed in experimental maps of the differential conductance $dI/dV$ and reveal the relative importance of geometric and electronic factors that impinge on the image contrasts. For a meaningful comparison, it is important that the calculations closely mimic the experimental mode of measurement.

Quantum batteries (QBs) exploit principles of quantum mechanics to accelerate the charging process and aim to achieve optimal energy storage. However, analytical results for investigating these problems remain lacking due to the challenges associated with nonequilibrium dynamics. In this work we analytically investigate a central-spin QB model in which $N_b$ spin-$\frac{1}{2}$ battery cells interact with $N_c$ spin-$\frac{1}{2}$ charger units, using $m$ initially excited charger units as a resource. By employing the invariant subspace method and the shifted Holstein-Primakoff transformation, we identify four scenarios in which optimal energy storage can be achieved: (i) $N_b\ll m\ll N_c$, (ii) $m\ll N_b\ll N_c$, (iii) $m\ll N_c\ll N_b$, and (iv) $N_b\ll m=k{N}_c [k\in (0,1\left)\right]$. In these cases, optimal storage is ensured by the SU(2) symmetry emerging from the charging dynamics. The first three cases map the central-spin QB to different Tavis-Cummings (TC) QBs, while the fourth corresponds to the non-TC limit. We analytically determine the charging time and demonstrate that in the fully charging cases (i) and (iv), the collective charging exhibits an $N_b$-fold enhancement in speedup compared to the parallel charging scheme. Additionally, we numerically observe a unified charging behavior when $m=N_c$, showing that asymptotically optimal energy storage is possible when $N_b=m=N_c$. In this case, we find a collective charging enhancement scaling as $N_b^{0.8264}$. The origin of the collective charging advantage in central-spin quantum batteries is also analyzed through the quantum speed limit and a multipartite entanglement witness. Our results highlight the crucial role of dynamically emergent SU(2) symmetry in providing an analytical understanding of nonequilibrium charging dynamics in QBs.

We theoretically propose chiral magnon in ferromagnetic chiral crystals. We show that the crystal chirality is imprinted in orbital angular momentum of magnons which exhibits the opposite signs for opposite chiralities of the crystal. We also show that a finite magnon orbital angular momentum can be induced by a temperature gradient which is a magnonic analog of the Edelstein effect.

Altermagnets are collinear compensated magnets whose magnetic symmetries at zero spin-orbit coupling break spin degeneracy leading to spin-split electronic and magnonic bands that reflect an underlying multipolar order. When there is an approximate $U\left(1\right)$ symmetry the magnons in altermagnets are split into equal and opposite chiral pairs. We show that in altermagnets polarized neutrons provide a means to detect the population of time-reversed domains and allow direct measurement of the magnon chirality anisotropy in momentum space—the central signature of the altermagnetic phase. We demonstrate this response to polarized neutrons in two candidate materials ${\mathrm{MnF}}_2$ and MnTe and show that the presence of these chiralities is stable to small perturbations that break spin-rotation symmetry. This provides a magnonic analog of spin-polarized ARPES that has been used to discern altermagnetism in the electronic band structures of various candidate materials.

An intense light field can dress both Bloch electrons inside crystals and photo-emitted free electrons in the vacuum, dubbed Floquet and Volkov states, respectively. These quantum states can further interfere coherently, modulating light-field dressed states. Here we report experimental evidence of the Floquet-Volkov interference in a semiconductor—black phosphorus. A highly asymmetric modulation of the spectral weight is observed for the Floquet-Volkov states, and such an asymmetry can be further controlled by rotating the pump polarization. Our work reveals the quantum interference between different light-field dressed electronic states, providing insights for material engineering on the ultrafast timescale.

Flat band in twisted graphene bilayer has garnered widespread attention, and whether flat bands can be realized in carbon monolayers is an interesting topic worth exploring in condensed matter physics. In this work, we demonstrate that based on the theory of compact localized states, a series of two-dimensional carbon allotropes with flat bands can be achieved. Two of them, named 191-8-66-C-r567x-1 and 191-10-90-C-r567x-1, are evaluated to be dynamically stable carbon phases with isolated or weakly overlapped flat bands at the Fermi level. The maximum Fermi velocities of the flat-band electrons are evaluated to be $1\times {10}^4$ and $0.786\times {10}^4$ m/s, respectively, both of which are lower than the Fermi velocity of the flat-band electrons in magic-angle graphene ($4\times {10}^4$ m/s). Furthermore, 191-8-66-C-r567x-1 has been predicted to be a flat-band related magnetic half metal with a magnetic moment of 1.854 µB per cell, while 191-10-90-C-r567x-1 is a flat-band related magnetic normal metal with a magnetic moment of 1.663 µB per cell. These results not only show that flat bands can be constructed in carbon monolayers, but also indicate the potential for achieving metal-free magnetic materials with light elements based on flat-band theory.

Recently, the full version of the eigenstate thermalization hypothesis (ETH) has been systematized using free probability. In this paper, we present a detailed discussion of the free-cumulants approach to many-body dynamics within the microcanonical ensemble. Differences between the latter and canonical averages are known to manifest in the time-dependent fluctuations of extensive operators. Thus, the microcanonical ensemble is essential to extend the application of free probability to the broad class of extensive observables. We numerically demonstrate the validity of our approach in a nonintegrable spin-chain Hamiltonian for extensive observables at finite-energy density. Our results confirm the full ETH properties, specifically the suppression of crossing contributions and the factorization of noncrossing ones, thus demonstrating that the microcanonical free cumulants encode ETH smooth correlations for both local and extensive observables.

$R{\mathrm{Mn}}_6{\mathrm{Sn}}_6$ ($R=\mathrm{Y}, \mathrm{Gd}–\mathrm{Lu}$) kagome metals are promising materials hosting flat electronic bands and Dirac points that interact with magnetism. The coupling between the two magnetic $R$ and Mn sublattices can drive complex magnetic states with potential consequences for spin and charge transport and other topological properties. Here, we use a detailed magnetic Hamiltonian to calculate and predict the magnetic phase diagrams for $R{\mathrm{Mn}}_6{\mathrm{Sn}}_6$ kagome metals within the mean-field approximation. These calculations reveal a variety of collinear, noncollinear, and noncoplanar phases that arise from competition between various interlayer magnetic exchange interactions and magnetic anisotropies of the $R$ and Mn ions. We enumerate these phases and their magnetic space groups for future analysis of their impact on topological and trivial bands near the Fermi surface.

In this paper, we investigate the underlying mechanisms of the universal and intrinsic negative piezoelectricity in perovskite thin film ferroelectrics by using our developed thermodynamic theory of single domain (310) oriented films. Perovskite ferroelectric $\mathrm{Pb}\left(\mathrm{Z}{\mathrm{r}}_{1–x}\mathrm{T}{\mathrm{i}}_x\right){\mathrm{O}}_3$ (PZT) is analyzed in detail as a typical example, but the theory can be applied to any other perovskite piezoelectrics. Consistent with recent experimental work [O. Paull et al., Nat. Mater. 21, 74 (2022)], our theory also indicates that there exists a strong coupling between $P_2$ (along the [1–30] direction) and $P_3$ (along the [310] direction) allowed by the 2/$m$ point group symmetry of the paraelectric state. From the calculated composition–misfit strain and misfit strain–misfit strain phase diagrams of $\mathrm{Pb}\left(\mathrm{Z}{\mathrm{r}}_{1–x}\mathrm{T}{\mathrm{i}}_x\right){\mathrm{O}}_3$, it can be seen that isomorphic transitions of 1 symmetry between three triclinic states ($a_1{a}_{2}c, a_{1}c$, and $a_1{a}_2$) take place near morphotropic phase boundary composition and under both isotropic tensile strain and anisotropic tensile strain. The negative piezoelectric coefficient $d_{33}$ appears in the quasi-$a_1{a}_2$ phase under tensile strain, which stems from the very approximate energies between $a_1{a}_{2}c$ and $a_1{a}_2$ under tensile strain due to the strong coupling between $P_2$ and $P_3$ caused by symmetry breaking. Moreover, this coupling leads to negative partial differentiation of longitudinal strain on longitudinal polarization in the quasi-$a_1{a}_2$ phase, which also gives rise to negative $d_{33}$. We also demonstrate the longitudinal negative piezoelectricity in (210) and (410) PZT thin films. The results are consistent with those of (310) films due to their same symmetry. This paper will deepen understanding about the anomalous negative piezoelectricity of perovskite ferroelectric materials, and provide strategies for discovering materials with novel electromechanical properties.

The skyrmion lattice (SkL) in MnSi was studied using small-angle neutron scattering and under the influence of a radial electric current in a Corbino geometry. In response to the applied current, the SkL undergoes an angular reorientation with respect to the MnSi crystal lattice. The reorientation is nonmonotonic with increasing current, with the SkL rotating first in one direction and then the other. The SkL reorientation was studied at different sample locations and found to depend on the local current density as inferred from a finite-element analysis. The nonmonotonic response indicates the presence of two competing effects on the SkL, most likely due to the presence of both radial electric and thermal currents. Such a scenario is supported by micromagnetic simulations, which show how these effects can act constructively or destructively to drive the SkL rotation, depending on the direction of the electric current. In addition, the simulations also suggest how the direction of the skyrmion flow may affect the SkL orientation.

A reentrant behavior of temperature-dependent magnetic ac susceptibility [or excess susceptibility (ES)] at lower temperature is observed in a composite made of superconductor ${\text{La}}_{1.85}{\text{Sr}}_{0.15}{\text{CuO}}_4$ (LCu) and an insulating paramagnetic salt ${\text{Gd}}_2{\text{O}}_3$ (GdO). The ES exhibits an exponential characteristic that varies with temperature ($exp[-\frac{T_0}{T}]$), $T_0$ is characteristics temperature. The characteristic temperature $T_0$ decreases as the effective interface diminishes and the amplitude of the dc magnetic field increases. The creation of ferromagnetic dimers between ${\mathrm{Gd}}^{+3}$ ions in GdO is observed as a result of vortex-dipole interaction, which causes the observation of this unusual ES at temperatures much lower than the superconducting onset temperature $T_S^{\text{onset}}$. This vortex-mediated ferromagnetic interaction between unpaired ${\mathrm{Gd}}^{+3}$ spins, with its characteristic temperature dependence on superconducting strength, is very much novel in nature and may be interpreted as a possible manifestation of Yu-Shiba-Rusinov–type states, reminiscent of the behavior observed with magnetic impurities embedded within a superconductor.

Trigonal ${\mathrm{PtBi}}_2$ is one of the attractive van-der-Waals materials because of the enhancement of its superconducting transition temperature $T_{\mathrm{c}}$ by doping chalcogen elements such as Se and Te. Recently, it has been reported that $T_{\mathrm{c}}$ of $\mathrm{Pt}{\left({\mathrm{Bi}}_{1-x}{\mathrm{Se}}_x\right)}_2$ is enhanced by a factor of four, compared to the pristine ${\mathrm{PtBi}}_2$, together with the polar-nonpolar structural phase transition. Thus, it is desirable to study electrical transport properties of this new superconducting compound. Here, we have performed magnetotransport measurements on $\mathrm{Pt}{\left({\mathrm{Bi}}_{1-x}{\mathrm{Se}}_x\right)}_2$ ($x=0.06$ and 0.08) thin-film devices and observed a peculiar magnetoresistance, where a finite hysteresis appears when the superconducting state is broken. By measuring the magnetoresistance systematically, we have attributed this behavior to the two-level critical state where fluxons pinned in $\mathrm{Pt}{\left({\mathrm{Bi}}_{1-x}{\mathrm{Se}}_x\right)}_2$ play an important role.

We examine the charge and spin properties of an effective single-band model representing a moiré superlattice of the ${\mathrm{WSe}}_2/{\mathrm{WS}}_2$ heterobilayer. We focus on the $2/3$ electron filling, which corresponds to the formation of a generalized Wigner crystal, as evidenced experimentally. Our approach is based on the extended Hubbard model on a triangular lattice, with a noninteracting part effectively describing a spin-split band caused by Ising-type spin-orbit coupling. We investigate the system in the regime of strong on-site Coulomb repulsion and the ground state of the Hamiltonian is obtained using the density matrix renormalization group formulated within the matrix product state approach. According to our analysis, on the basis of the density-density correlation functions resolved in momentum space, a transition from the metallic to the insulating state appears with increasing intersite electron-electron interactions. This transition is identified as concomitant with the emergence of a generalized Wigner crystal that realizes the honeycomb lattice pattern. We investigate the magnetic properties of such a Wigner crystal state and find that increasing intersite repulsion results in out-of-plane antiferromagnetic correlations and in-plane canted spin-spin ferromagnetic correlations. The latter are shown to become antiferromagnetic when the ground state is subjected to a transform, which simultaneously converts the Hamiltonian into an SU(2)-invariant form.

We present an experimental study that offers first insights into the unoccupied electronic states of a pristine $(\sqrt{2}\times \sqrt{2})\text{R}45{}^{\circ }$ reconstructed ${\mathrm{Fe}}_3{\mathrm{O}}_4$(100) surface. We initially demonstrate the successful sample preparation by analyzing the surface periodicity and chemical composition by low-energy electron diffraction and Auger electron spectroscopy, respectively. Using angle-resolved inverse photoemission, we study the unoccupied electronic states. We observe three nondispersive emissions, one dominant feature around $2\mathrm{eV}$ above the Fermi level and two additional ones at higher energies. For the low-lying triple-split feature, we propose two possible interpretations based on former theoretical studies. We either ascribe it to $3d$-related bandgroups originating from tetrahedrally and octahedrally coordinated bulk iron atoms or attribute it to a mixture of $3d$ states originating from octahedrally coordinated iron atoms within the bulk and the reconstructed surface layer. The high-lying features are ascribed to bulk bands based on the experimentally observed characteristics.

The quasistatic transverse conductivity of clean Fermi liquids at long wavelengths displays a remarkably universal behavior: it is determined solely by the radius of curvature of the Fermi surface and does not depend on details such as the quasiparticle mass or their interactions. Here we demonstrate that Berry phases do not alter such universality by directly computing the transverse conductivity of two-dimensional electronic systems with Dirac dispersions, such as those appearing in graphene and its chiral multilayer variants. Interestingly, however, such universality ceases to hold at wave vectors comparable to the Fermi radius, where Dirac fermions display a vividly distinct transverse conductivity relative to parabolic Fermions, with a rich wave vector dependence that includes divergences, oscillations, and zeros. We discuss how this can be probed by measuring the $T_1$ relaxation time of spin qubits, such as NV centers or nuclear spins, placed near such 2D systems.