The crystal structures of coexisting polar and nonpolar phases in $A$-site ordered ${\mathrm{NdBaMn}}_2{\mathrm{O}}_6$ were investigated by a combination of convergent-beam electron diffraction (CBED) and density functional theory (DFT) calculations. A symmetry analysis of the CBED patterns from several segments of $A$-site ordered ${\mathrm{NdBaMn}}_2{\mathrm{O}}_6$ at room temperature ($\text{RT}\approx 293$ K) revealed the presence of two crystallographic distinct phases with polar $C2mm$ (No. 38) and nonpolar $Cmmm$ (No. 65) space groups. The combined method was used successfully to determine the crystal structures of the coexisting phases with the same chemical composition and similar lattice parameters. DFT calculations revealed that the comparable total energies of the two phases could be the origin of their coexistence.

New ${\mathrm{BiS}}_2$-based layered compound ${\mathrm{Eu}}_3{\mathrm{Bi}}_2{\mathrm{S}}_4{\mathrm{F}}_4$ has recently been reported to exhibit superconductivity with a transition temperature of 1.5 K using polycrystalline samples. Here we report measurements of electrical resistivity $\rho$, magnetization $M$, and specific heat $C$ of ${\mathrm{Eu}}_3{\mathrm{Bi}}_2{\mathrm{S}}_4{\mathrm{F}}_4$ single crystals. At the antiferromagnetic transition temperature of 2.17 K, $C\left(T\right)$ shows a sharp $\lambda$-type peak, suggesting the existence of strong Eu magnetic moment fluctuations due to the quasi-two-dimensional nature of the Eu-ion lattice in ${\mathrm{Eu}}_3{\mathrm{F}}_4$ block layers. The average Eu-ion valence of $+2.19$, determined from the Schottky-type $4f$-electron contribution to $C\left(T\right)$ and $M(T,H)$, indicates the self-doped electron density in ${\mathrm{BiS}}_2$ layers due to the Eu mixed valence to be 0.285 electrons per Bi site. It has been revealed, however, that $\rho \left(T\right)$ shows an anomalous nonmetallic behavior with $\sim 1/T^3$ dependence without any sign of superconductivity, in marked contrast to a metallic behavior in the polycrystals. This behavior suggests an unconventional electron-localization mechanism working in the single crystals, possibly caused by ${\mathrm{Eu}}^{2+}/{\mathrm{Eu}}^{3+}$ distribution (or fluctuations) and/or lattice instabilities inherent to ${\mathrm{BiS}}_2$ layers.

External-field driven energy-level discretization, such as Landau quantization or Stark localization, is one of the most intriguing phenomena in quantum systems. We investigate the emergence of the Wannier-Stark ladder coming from the particle-hole continuum and the Stark shifts of the exciton levels in one-dimensional Mott insulators under the dc electric field. The discretized peak structure in the optical-conductivity spectra newly appears by applying the dc electric field, and the positions of these peaks can be reproduced from the energy levels of a simple effective model in the strong-coupling regime. Our results not only suggest that Mott insulators can serve as a viable platform for Stark discretization, but also pave the way for investigations of dynamical properties in correlated many-body systems under a dc electric field.

Periodic modulation of the Hamiltonian offers a powerful method to engineer Floquet band properties and create new gaps capable of hosting Floquet edge modes. However, there have been limited studies exploring the performance of Floquet edge modes in terms of dynamic control and interference properties. Here, we experimentally implement an array of staggered coupled plasmonic waveguides operating at microwave frequencies based on the Floquet Su-Schrieffer-Heeger model. Our observations reveal the coexistence of Floquet zero and $\pi$ modes within a specific periodic range of the quasienergy spectrum. Through near-field experiments, we observe a subharmonic response of the electric field propagation in the microwave, which is confirmed by the interference of eigenfields associated with the two topological end modes. Our work not only provides an approach for studying time-dependent Floquet Hamiltonians, but also opens the door to exploring period-doubling nonequilibrium topological phases.

Recently, solid crystalline ${\mathrm{GeSb}}_2{\mathrm{Te}}_4$ was found to host topological surface states and low lattice thermal conductivity. In this work, we conducted comprehensive investigations on the electronic structures and lattice dynamics of ${\mathrm{GeSb}}_2{\mathrm{Te}}_4$. By combining synchrotron-based and spin-resolved angle-resolved photoemission spectroscopy and first-principles calculations, we observed distinctive topological surface states with significant spin polarization near the Fermi level. Furthermore, our temperature-dependent Raman scattering measurements revealed weak changes in linewidths of low-energy optical modes, indicating the weak temperature-dependent phonon anharmonicity of those modes. This finding is further supported by our lattice thermal conductivity calculation, which explains the minor variation in lattice thermal conduction in the high-temperature region. Our study sheds light on the exotic electronic and lattice dynamics of ${\mathrm{GeSb}}_2{\mathrm{Te}}_4$, potentially leading to new insights into its technological applications.

Simulating collision cascades and radiation damage poses a long-standing challenge for existing interatomic potentials, both in terms of accuracy and efficiency. Machine-learning-based interatomic potentials have shown sufficiently high accuracy for radiation damage simulations, but most existing ones are still not efficient enough to model high-energy collision cascades with sufficiently large space and timescales. To this end, we here extend the highly efficient neuroevolution potential (NEP) framework by combining it with the Ziegler-Biersack-Littmark (ZBL) screened nuclear repulsion potential, obtaining a NEP-ZBL framework. We train a NEP-ZBL model for tungsten and demonstrate its accuracy in terms of the elastic properties, melting point, and various energetics of defects that are relevant for radiation damage. We then perform large-scale molecular dynamics simulations with the NEP-ZBL model with up to 8.1 million atoms and 240 ps (using a single 40-GB A100 GPU) to study the difference of primary radiation damage in both bulk and thin-foil tungsten. While our findings for bulk tungsten are consistent with existing results simulated by embedded atom method models, the radiation damage differs significantly in foils and shows that larger and more vacancy clusters as well as smaller and fewer interstitial clusters are produced due to the presence of a free surface.

In Mott insulators with a half-filled $t_{2g}$ shell the Hund's rule coupling induces a spin-3/2 orbital-singlet ground state. The spin-orbit interaction is not expected to qualitatively impact low-energy degrees of freedom in such systems. Indeed, $d^3$ cubic double perovskites (DP) of heavy transition metals are believed to exhibit conventional collinear magnetic orders. However, their inelastic neutron scattering spectra feature large gaps of unclear origin. Here we derive first-principles low-energy Hamiltonians for the cubic DP ${\mathrm{Ba}}_2\mathrm{Y}{B}^{\prime }{\mathrm{O}}_6$ ($B^{\prime }=\mathrm{Os}$, Ru) and show that they include significant multipolar—dipole-octupolar—intersite exchange terms. These terms break continuous symmetry of the spin-3/2 Hamiltonian opening an excitation gap. The calculated gap magnitudes are in good agreement with experiment. The dipole-octupolar intersite exchange is induced due to excited states of the $t_{2g}^3$ manifold that are admixed by the spin-orbit interaction into the spin-3/2 ground state.

Nanoscale $\mathrm{BaTi}{\mathrm{O}}_3$ particles $(\approx 10$ nm) prepared by ball-milling a mixture of oleic acid and heptane have been reported to have an electric polarization several times larger than that for bulk $\mathrm{BaTi}{\mathrm{O}}_3$. In this work, detailed local, intermediate, and long-range structural studies are combined with spectroscopic measurements to develop a model structure of these materials. The x-ray spectroscopic measurements reveal large Ti off-centering as the key factor producing the large spontaneous polarization in the nanoparticles. Temperature-dependent lattice parameter changes manifest sharpening of the structural phase transitions in these $\mathrm{BaTi}{\mathrm{O}}_3$ nanoparticles compared to the pure nanoparticle systems. Sharp crystalline-type peaks in the barium oleate Raman spectra suggest that this component in the composite core-shell matrix, a product of mechanochemical synthesis, stabilizes an enhanced polar structural phase of the $\mathrm{BaTi}{\mathrm{O}}_3$ core nanoparticles.

We show for generic quantum non-Hermitian tight-binding models that relaxation timescales of local observables are not controlled by the localization length ${\xi }_{\mathrm{loc}}$ associated with the non-Hermitian skin effect, contrary to popular belief. Instead, interference between eigenvectors effectively makes the extreme localization of modes largely irrelevant to relaxation; this is ultimately a consequence of causality and locality. Focusing on the paradigmatic Hatano-Nelson model, we demonstrate that there exists instead a much larger length scale ${\xi }_{\mathrm{prop}}$ which controls the rate of decay towards the steady state. Further, varying ${\xi }_{\mathrm{prop}}$ can lead to anomalously large relaxation times that scale with system size, or to the expected behavior where the dissipative gap correctly predicts the rate of decay. Our work highlights an important aspect of the non-Hermitian skin effect: the exceptional sensitivity to boundary conditions here necessarily takes a finite amount of time to manifest itself.

Using element-specific measurements of the ultrafast demagnetization of Ru/${\mathrm{Fe}}_{65}{\mathrm{Co}}_{35}$ hetero-structures, we show that Ru can exhibit a significant magnetic contrast (3% asymmetry) resulting from ultrafast spin currents emanating from the demagnetization process of the FeCo layer. We use this magnetic contrast to investigate how superdiffusive spin currents are affected by the doping of heavy elements in the FeCo layer. We find that the spin currents are strongly suppressed, and that the recovery process in Ru slows down by Re doping. This is in accordance with a change in interface reflectivity of spin currents as found by the superdiffusive spin transport model.

We investigate the pseudo-Hermitian properties of a $\text{ferromagnet}\left|\text{normal}\text{−}\text{metal}\right|\text{ferromagnet}$ heterojunction, where the two ferromagnets are subjected to two independently tuned magnetic fields and dissipatively coupled with one another through the ultrathin normal-metal film. The three-dimensional parameter space constituted by functions of the Gilbert damping, the enhanced damping, and the effective magnetic fields enables us to obtain the two-dimensional pseudo-Hermitian regions with different phases and an exceptional line. We find that the coalescence of two spectra occurs in one of the pseudo-Hermitian phases. Furthermore, the spin-wave excitation is highly enhanced on the exceptional line via a small circularly polarized microwave driving and the spin transfer torque induced negative Gilbert damping. Particularly, the intensity of spin-wave excitations in two subsystems could be imbalanced and adjustable to show a certain ratio through anti-parity-time (anti-$\mathcal{P}T)$ symmetry breaking. Through anti-$\mathcal{P}T$ symmetry breaking, additional degrees of freedom will be generated. The method for imbalanced excitation of particles at exceptional points is also applicable to other dissipatively coupled physical systems.

In this work, we synthesized single crystals of ${\mathrm{La}}_3\mathrm{Mg}{\mathrm{Bi}}_5$ and systematically investigated their structure, magnetism, and transport properties. ${\mathrm{La}}_3\mathrm{Mg}{\mathrm{Bi}}_5$ belongs to the $A_{3}M{X}_5$ family (A = Ca-Ba or rare earth, M = transition materials or Mg, X = chalcogen or pnictogen) and has a quasi-one-dimensional structure. Our magnetic and magnetoresistance measurements show the presence of de Haas–van Alphen (dHvA) and Shubnikov–de Haas (SdH) oscillations. Analysis of these quantum oscillation spectra reveals two major oscillation frequencies ($F_{\alpha }$, $F_{\beta }$) with corresponding light effective mass and the nearly π Berry phase for $F_{\alpha }$, suggesting the nontrivial band topology in ${\mathrm{La}}_3\mathrm{Mg}{\mathrm{Bi}}_5$, which is further supported by our first-principles calculations. Besides, ${\mathrm{La}}_3\mathrm{Mg}{\mathrm{Bi}}_5$ manifests a large nonsaturating magnetoresistance and a nonlinear Hall effect at low temperatures. Our results show that ${\mathrm{La}}_3\mathrm{Mg}{\mathrm{Bi}}_5$ is a topological nontrivial material and a member of the $A_{3}M{X}_5$ family exhibiting topological properties, providing a platform for further investigation of topological fermionic properties in this family.

The adoption of an accurate kinetic energy density functional (KEDF) to characterize the noninteracting kinetic energy within the framework of orbital-free density functional theory (OFDFT) is challenging. We propose a form of the nonlocal KEDF with a real-space truncation cutoff that satisfies the uniform electron gas limit and design KEDFs for simple metals and silicon. These KEDFs are obtained by minimizing a residual function, which contains the differences in the total energy and charge density of several representative systems with respect to the Kohn-Sham DFT results. By systematically testing different cutoffs of these KEDFs, we find that the cutoff plays a crucial role in determining the properties of metallic Al and semiconductor Si systems. We conclude that the KEDF with a sufficiently long cutoff performs even better than some representative non-local KEDFs in some aspects, which sheds light on optimizing the KEDFs in OFDFT to achieve better accuracy.

We investigate the Casimir forces in a planar two-body system with each backed by a plate of perfect electric conductor. We show that for the equilibrium Casimir forces, the spectra of the Casmir pressure are symmetric in the wave-vector space for reciprocal systems, and become asymmetric when reciprocity is broken. Therefore, there is a distinct signature of reciprocity in the spectra of equilibrium Casimir forces. The same signature also holds for the lateral force, which is nonzero only in the nonequilibrium case. On the other hand, for the pressure in the nonequilibrium scenario, such an asymmetry can arise for both reciprocal and nonreciprocal cases. We also elucidate a relation on the Casmir force that is connected to Newton's third law. We show that Newton's third law holds for every frequency and wave vector, as long as no exchange of photons occurs between the two-body system and the environment. Newton's third law is distinct from the reciprocity constraints. We illustrate the theoretical results with numerical calculations described by anisotropic permittivity tensors that are either symmetric and hence reciprocal, or asymmetric and hence nonreciprocal.

Using numerically exact diagonalization, we study the correlated Haldane-Hubbard model in the presence of dissipation. Such dissipation can be modeled at short times by the dynamics governed by an effective non-Hermitian Hamiltonian, of which we present a full characterization. If the dissipation corresponds to a two-body loss, the repulsive interaction of the effective Hamiltonian acquires an imaginary component. A competition between the formation of a charge-ordered Mott insulator state and a topological insulator ensues, but with the non-Hermitian contribution aiding in stabilizing the topologically nontrivial regime, delaying the onset of the formation of a local order parameter. Lastly, we analyze the robustness of the ordered phase by following the full dissipative many-body real-time dynamics. An exponentially fast melting of the charge order occurs, whose characteristic rate is roughly independent of the interaction strength, for the case of one-body dissipation.

We theoretically study the quantum transport in a three-dimensional spin-1 chiral fermion system in the presence of Coulomb impurities based on the self-consistent Born approximation. We find that the flat-band states anomalously enhance the screening effect, and the electrical conductivity is increased in the low-energy region. It is also found that reducing the screening length leads to an increase in the forward-scattering contribution and, thus, an increase in the vertex correction in the high-energy region.

We study the emergence of spontaneous skyrmion lattices in an Fe monolayer in fcc and hcp stacking on the Ir(111) surface using density functional theory (DFT). For fcc-Fe/Ir(111) we find the well-known square nanoskyrmion lattice. However, for hcp-Fe/Ir(111) the hexagonal skyrmion lattice previously proposed based on experiments is energetically unfavorable with respect to a hexagonal multi-$Q$ state with nearly collinear magnetic moments. By mapping our DFT calculations to an atomistic spin model we demonstrate that the interplay of pairwise exchange, higher-order exchange, and Dzyaloshinskii-Moriya interaction is decisive for the symmetry and collinearity of the obtained spin lattice.

Ergodic many-body systems are expected to reach thermal equilibrium. Here, we demonstrate that, surprisingly, high-energy electrons, which are injected into a quantum Hall edge mode with finite-range interactions, stabilize at a far-from-thermalized state over a long timescale. To detect this nonequilibrium state, one positions an energy-resolved detector downstream of the injection point. So far, nonequilibrium distributions in integrable systems were either found not to display relaxation at all, or generically relax to near-thermal asymptotic states. In stark contrast, the here-obtained many-body state comprises fast-decaying transient components, followed by a nearly frozen distribution with a peak near the injection energy.

${\mathrm{YbFeO}}_3$ is exceptional among the rare-earth orthoferrites for having the lowest spin-reorientation transition (SRT) temperature, $T_{\mathrm{SRT}}\simeq 8\mathrm{K}$, which makes it particularly appealing to examine the interplay between noncollinear magnetism of the Fe sublattice and quasi-one-dimensional XXZ effective $S=1/2$ chains of ${\mathrm{Yb}}^{3+}$ moments. Our paper focuses on the magnetic dynamics of ${\mathrm{YbFeO}}_3$ using inelastic neutron scattering (INS), at temperatures below and above the SRT, under an applied hydrostatic pressure of 2 GPa, and in magnetic fields up to 4 T. The low-energy zero-field excitation spectrum at ambient pressure and temperatures below the SRT is dominated by a gapped magnon mode of the Yb subsystem at 0.84 meV with a dispersion only in the $\left[00L\right]$ direction. Above $T_{\mathrm{SRT}}$, a continuum appears on top of the magnon mode because of temperature population of the magnon band, and the gap decreases to around 0.4 meV. The INS spectra in the magnetic field, both above and below $T_{\mathrm{SRT}}$, are characterized by two well-separated gapped modes. The SRT is clearly visible at low fields $B<1\mathrm{T}$, but it gradually disappears at higher magnetic fields. The hydrostatic pressure of $p=2\mathrm{GPa}$ effectively reduces the transition width, $\mathrm{\Delta }{T}_{\mathrm{SRT}}$, and keeps the SRT at higher fields up to $B\simeq 3\mathrm{T}$. We discuss the effect of the applied pressure in the frame of the modified mean-field theory and show that in the vicinity of the $T_{\mathrm{SRT}}$ the pressure tunes the fourth-order anisotropy constant that effectively reduces the $\mathrm{\Delta }{T}_{\mathrm{SRT}}$.