We revisit the connection between entanglement entropy and quantum metric in topological lattice systems, and provide an elegant and concise proof of this connection. In gapped two-dimensional lattice models with well-defined tight-binding Hamiltonians, we show that the entanglement entropy is intimately related to the quantum metric of electronic states, and bounded by the Chern number of (Slater-determinant) interacting topological insulators. Our results hold promising applicability to the recently discovered twisted transition metal dichalcogenides, characterized by flat topological bands at low twist angles, where these bounds can be applied at integer filling, and new pathways to enhance entanglement measures by engineering heterostructures with higher Chern numbers.

The behavior of higher-order boundary states in non-Hermitian systems is elusive, thus finding the mechanism behind these states is both essential and significant. Here, we uncover a novel mechanism that induces anomalous higher-order boundary states. The mechanism originates from the sensitivity of the non-normal boundary Hamiltonian to intrinsic perturbations, which refer here to the influence of the bulk on the topological boundaries. Based on the mechanism, we reveal a new kind of phase transition, i.e., the transition between hybrid skin-topological states and scale-free topological boundary states. We also find that scale-free topological boundary states exhibit size-dependent spectra, influencing the existence of higher-order topological boundary states. Unlike conventional hybrid skin-topological states or the higher-order non-Hermitian skin effect, the above two kinds of anomalous higher-order boundary states exhibit size-dependent characteristics. Our work opens a new horizon for the control of higher-order boundary states and topological properties of non-Hermitian systems.

Perovskite ferroelectrics are widely used in modern science and technology due to their exceptional properties. Phase transitions in perovskite are accompanied by changes in lattice dynamics characteristics. These changes can be used to identify phase transitions and also serve as important factors responsible for the performance variations caused by phase transitions. Accordingly, understanding the evolution of lattice dynamics as a result of phase transitions is essential for establishing the relationship between the phase structures and properties of perovskites. Potassium tantalate niobate ($\text{K}{\mathrm{Ta}}_{1-x}{\mathrm{Nb}}_x{\mathrm{O}}_3$, abbreviated as KTN) is a perovskite ferroelectric with the advantage of a tunable phase structure. In this study, we investigate the evolution of the KTN crystal lattice dynamics during phase transitions by combining density-functional-theory calculations and experimental Raman spectroscopy. The changes in symmetry and intrinsic vibrational displacement due to phase transitions are explored and further demonstrated through vibrations of the intermediate structures between transitions. Additionally, ab initio molecular dynamics calculations are employed to investigate the structural characteristics and order-disorder behavior of paraelectric KTN, which is responsible for the Raman intensity. Our results provide standards for determining symmetry or complex multiphase structure via Raman spectroscopy. Moreover, the findings are crucial foundations for establishing the phase-structure-property relationship, which will hopefully guide research on the origins and optimization of the performance of KTN crystals and other perovskite systems.

We performed first-principles calculations at finite temperatures to investigate the temperature dependence of the magnetic properties, such as exchange stiffness constants and Curie temperatures, of ${\text{Co}}_2\text{Mn}Z$ ($Z$ = Si, Al) assuming $L{2}_1$ and $B2$ structures. In $L{2}_1$ structures, we confirmed a relatively high Curie temperature for ${\text{Co}}_2\text{MnAl}$, comparable with that of ${\text{Co}}_2\text{MnSi}$; however, its exchange stiffness constant and single-site magnetic excitation energy at zero temperature are much smaller than those of ${\text{Co}}_2\text{MnSi}$. This might indicate that the Curie temperature of itinerant magnets cannot be determined by the exchange interaction at zero temperature. We also investigated the temperature dependence of the exchange stiffness constants of both alloys, and we found robustness in the temperature dependence of the exchange stiffness constant for ${\text{Co}}_2\text{MnAl}$, assuming an $L{2}_1$ structure. This might lead to a high Curie temperature, contrary to its small exchange stiffness constant. Meanwhile, for the results of the $B2$ structures, the exchange stiffness constants monotonically decrease with increasing temperatures for both alloys. In addition, we found that the behavior of the temperature dependences of the exchange stiffness constant can be controlled by the electron filling for both structures. Finally, we examined the temperature dependence of the electronic structure and how spin disorder alters the electronic structure. We also confirmed that the spin polarization at the chemical potential effectively increases and half metallicity is effectively induced with increasing temperature due to the altered electronic structure induced by the spin disorder of ${\text{Co}}_2\text{MnAl}$ with the $L{2}_1$ structure. Our results might indicate that renormalization of the electronic structure due to spin disorder at finite temperature influences the robustness of the exchange interactions and half-metallic properties of ${\text{Co}}_2\text{MnAl}$ with $L{2}_1$ structure.

The unique optical properties of the nitrogen-vacancy (NV) center in diamond have established it as a workhorse for quantum science applications. Both charge and spin dynamics, driven by laser excitation, are critically mediated by the singlet state, yet several key mechanisms remain elusive. In this work, we provide experimental evidence of a novel charge conversion pathway, a transition from the neutral charge state (${\mathrm{NV}}^0$) to the singlet manifold of the negative charge state (${\mathrm{NV}}^{-}$) under high laser power. Furthermore, we directly confirm the two-photon ionization nature of the singlet state using near-infrared excitation and present an extensive discussion of the ionization mechanisms of ${\mathrm{NV}}^{-}$. These findings offer deeper insights into the singlet-mediated charge-spin interplay, providing a foundation for potential advancements in spin readout technologies of NV centers.

Degeneracy points of parameter-dependent Hermitian matrices play a fundamental role in quantum physics, as illustrated by the concept of the Berry phase in quantum dynamics, the Weyl semimetals in condensed-matter physics, and the robust ground-state degeneracies in topologically ordered quantum systems. Here, we construct simple ball-and-spring mechanical systems, whose eigenfrequency degeneracies mimic the behavior of degeneracy points of electronic band structures. These classical-mechanical arrangements can be viewed as “de-quantized” versions of Weyl Josephson circuits, i.e., superconducting nanostructures proposed recently to mimic band-structure effects of Weyl semimetals. In the mechanical setups we study, we identify degeneracy patterns beyond simple Weyl points, including the chirality flip effect and a quadratic degeneracy point. Our theoretical work is a step toward simple and illustrative table-top experiments exploring topological and differential geometrical aspects of physics.

This study utilizes first-principles computational methods to comprehensively analyze the impact of A-site doping on the proton conduction properties of ${\mathrm{BaHfO}}_3$. The goal is to offer theoretical support for the advancement of electrolyte materials for solid oxide fuel cells. Our research has uncovered that ${\mathrm{BaHfO}}_3$ demonstrates promising potential for proton conduction, with a low proton migration barrier of 0.28 eV, suggesting efficient proton conduction can be achieved at lower temperatures. Through A-site doping, particularly with low-valence-state ions and the introduction of Ba vacancies, we can effectively decrease the formation energy of oxygen vacancies ($E_{\text{vac}}$), leading to an increase in proton concentration. Additionally, our study reveals that the primary mechanism for proton migration in ${\mathrm{BaHfO}}_3$ is the Grotthuss mechanism rather than the vehicle mechanism. Examination of the changes in lattice parameters during proton migration indicates that while doping or vacancy control strategies do not alter the mode of ${\mathrm{H}}^{+}$ migration, they do influence the migration pathway and barrier. These findings provide valuable insights into optimizing the proton conduction properties of ${\mathrm{BaHfO}}_3$ through A-site doping and lay a solid theoretical foundation for the development of novel, highly efficient solid oxide fuel cell electrolyte materials.

Native ring structures within amorphous networks play a critical role in determining structural and optical properties, in part due to their ability to host dopants such as rare-earth ions in silicate systems. In this work, we demonstrate that the universal features of structural anisotropy in amorphous networks can be efficiently simulated using a model based on stochastically deformed, edge-sharing $N$-member native ring structures. This model isolates and characterizes the structural anisotropy generated during the annealing-quenching process that is independent of any constituent-specific interactions. We refer to this computational framework as indistinguishable simulated folding (ISF), a stochastic process that mimics a simulated annealing-quenching procedure. Formulated as a Markov process, ISF is governed by two physically meaningful parameters: the number of Markov steps, representing the mean duration of each ring-folding event, and the stochastic deformation magnitude, which quantifies thermally induced structural changes per event. Furthermore, we show that the logarithm of any positive-valued anisotropy measure generated by ISF is a skewed random variable, reflecting the growing entropy production rate during the Markov evolution. ISF provides both a conceptual framework for understanding the universal stochastic origin of structural anisotropy in amorphous networks and a practical tool for simulating constituent-independent features, without requiring full-scale molecular dynamics simulations.

It is widely accepted that the localized states in disordered systems must exhibit exponential localization with a pure point spectrum. However, we establish that this belief may be incorrect. In this work, we identify a fundamental localized quantum state in a mobility edge phase. Owing to the balancing effect of this state, the mean inverse participation ratio (MIPR) of this phase neither shows a strong dependence on system size as seen in the extended and critical phases nor undergoes further localization with increasing disorder as observed in the traditional localized phase. Instead, its MIPR is entirely independent of both system size and disorder strength, consistently manifesting as a fixed value. Further, the wave function of this state is distinctly different from those of extended, critical, and traditional localized states, exhibiting a unique fixed-length continuous spectrum nature. Our results may demonstrate the existence of a different fundamental quantum state and introduce an alternative avenue for insights into the nature of quantum states in disordered systems.

We experimentally investigate the magnetization angle dependence $M\left(\alpha \right)$ for single crys tals of MnTe altermagnetic candidate. In high magnetic fields, experimental $M\left(\alpha \right)$ curves mostly reflect standard antiferromagnetic spin-flop processes, which are allowed below the Néel vector reorientation field. In low magnetic fields and at low temperatures, spontaneous magnetization appears as a sharp $M\left(T\right)$ magnetization jump around 81 K. In this regime, $M\left(\alpha \right)$ dependence is quite unusual: the easy magnetization axis is $\pi /2$ rotated either by increasing the field above 1 kOe or the temperature above 81 K. The observed behavior cannot be expected for antiferromagnetics, e.g., it differs strongly from the well-known weak ferromagnetism. Thus, it requires to take into account the formation of the altermagnetic ground state for MnTe altermagnetic candidate. Despite MnTe is expected to have $g$-wave order parameter, $M\left(\alpha \right)$ magnetization symmetry confirms the prevailing population of one from three easy axes, as it has been shown previously by temperature-dependent angle-resolved photo-emission spectroscopy.

The development of high-intensity ultrashort laser pulses unlocks the potential of pump-probe spectroscopy in subfemtosecond timescale. Notably, subcycle pump pulses can generate electronic states unreachable by conventional multicycle pulses, leading to a phenomenon that we refer to as subcycle-pulse engineering. In this study, we employ the time-dependent density-matrix renormalization group method to unveil the transient absorption spectra of superconducting and metallic states in nearly half-filled one-dimensional and two-dimensional Hubbard models excited by an ultrashort half-cycle pulse, which can induce a current with inversion-symmetry breaking. In a superconducting state realized in attractive on-site interactions, we find the transient activation of absorptions at energies corresponding to the amplitude modes of superconducting and charge-density-wave states. On the other hand, in a metallic state realized in the two-dimensional model with repulsive on-site interactions, we obtain another type of absorption enhancements, which are distributed broadly in spin excitation energies. These findings indicate that superconducting and metallic states are sensitive to an ultrashort half-cycle pulse, leading to the transient activations of optical absorptions with their respective mechanisms.

Based on density functional plus dynamical mean-field theory (DFT $+$ DMFT) calculations, we explore the electronic reconstruction of ${\mathrm{La}}_{1+x}{\mathrm{Sr}}_{1-x}{\mathrm{NiO}}_4(0\le x\le 1)$. We show how Mott localization manifests itself due to sizable electron correlation effects in the $e_g$-shell one-particle spectral functions. Our results upon electron-doping the ${\mathrm{LaSrNiO}}_4$ layered compound provide a many-particle interpretation for the tenacious insulating behavior seen in experiments. Our findings call for future theory and experimental investigations to validate or rule out the proximity to marginal Fermi liquidness as the prominent candidate for pseudogapped transport properties of pressurized ${\mathrm{LaSrNiO}}_4$.

Unidirectional propagation based on surface magnetoplasmons (SMPs) has recently been realized at the interface of magnetized semiconductors. However, usually SMPs lose their unidirectionality due to nonlocal effects, especially in the lower trivial band gap of such structures. More recently, a truly unidirectional SMP (USMP) has been demonstrated in the upper topological nontrivial band gap, but it supports only a single USMP, limiting its functionality. In this work, we present a fundamental physical model for multiple, robust, truly topological USMP modes at terahertz (THz) frequencies, realized in a semiconductor-dielectric-semiconductor (SDS) slab waveguide under opposing external magnetic fields. We analytically derive the dispersion properties of the SMPs and perform numerical analysis in both local and nonlocal models. Our results show that the SDS waveguide supports two truly (even and odd) USMP modes in the upper topological nontrivial band gap. Exploiting these two modes, we demonstrate unidirectional SMP multimode interference (USMMI), being highly robust and immune to backscattering, overcoming the back-reflection issue in conventional bidirectional waveguides. To demonstrate the usefulness of this approach, we numerically realize a frequency and magnetically tunable arbitrary-ratio splitter based on this robust USMMI, enabling multimode conversion. We, further, identify a unique index-near-zero (INZ) odd USMP mode in the SDS waveguide, distinct from conventional semiconductor-dielectric-metal waveguides. Leveraging this INZ mode, we achieve phase modulation with a phase shift from $-\pi$ to $\pi$. Our work expands the manipulation of topological waves and enriches the field of truly nonreciprocal topological physics for practical device applications.

Altermagnetism, as a new category of collinear magnetism distinct from traditional ferromagnetism and antiferromagnetism, exhibits the spin splitting without net magnetization. Currently, researchers are focused on searching three-dimensional altermagnetism and exploring its novel physical properties. However, there is a lack of understanding of the physical origin and valley splitting of two-dimensional altermagnetic emergent behavior. Here, we propose an approach to realize the transition from Néel antiferromagnetism to altermagnetism in a two-dimensional system using an electric field, Janus structure, and ferroelectric substrate. In monolayer ${\mathrm{VPSe}}_3$, we demonstrate that multiple physical fields cause the upper and lower Se atoms unequally to break $PT$ symmetry, resulting in altermagnetic spin splitting. We note that monolayer ${\mathrm{VPSe}}_3$ produces a spontaneous valley splitting of 2.91 meV at the conduction band minimum. The electric field can tune the valley splitting magnitude, while the Janus structure not only changes the valley splitting magnitude, but also alters the direction. More interestingly, when the ferroelectric polarization of ${\mathrm{Al}}_2{\mathrm{S}}_3$ is $\mathrm{P}\uparrow$, the direction of valley polarization is switched and the magnitude is almost unchanged. However, the valley splitting significantly increases under the $\mathrm{P}\downarrow$. It is worth noting that the ferroelectric polarization can switch altermagnetic effect and realize anomalous valley Hall effect. Besides, we reveal the microscopic mechanism of valley splitting by an effective Hamiltonian. Our findings not only provide a method to designing the altermagnet, but also enrich the valley physics.

The development of manganese (Mn)-rich cathodes for Li-ion batteries promises to alleviate potential supply chain bottlenecks in battery manufacturing. Fundamental challenges in Mn-rich cathodes arise from phenomena such as structural changes due to cooperative Jahn-Teller (JT) distortions of ${\mathrm{Mn}}^{3+}$ in octahedral environments, Mn migration, and phase transformations to spinel-like order, all of which affect the electrochemical performance. These physically complex phenomena motivate an ab initio re-examination of the Li-Mn-O rock-salt space, with a focus on the thermodynamics of the prototypical, ${\mathrm{LiMnO}}_2$ polymorphs. It is found that the generalized gradient approximation (GGA-PBEsol) and meta-GGA (${\mathrm{r}}^2\mathrm{SCAN}$) density functionals with empirically fitted on-site Hubbard $U$ corrections yield spurious stable phases for ${\mathrm{LiMnO}}_2$, such as predicting a phase with $\gamma \text{−}{\mathrm{LiFeO}}_2$-like order ($\gamma \text{−}{\mathrm{LiMnO}}_2$) to be the ground state instead of the orthorhombic (Pmmn) phase, which is the experimentally known ground state. Accounting for antiferromagnetic order in each structure is shown to have a substantial effect on the total energies and resulting phase stability. By using hybrid-GGA (HSE06) and GGA with self-consistent Hubbard parameters (on-site $U$ and inter-site $V$) calculated from linear response theory, the experimentally observed ${\mathrm{LiMnO}}_2$ phase stability trends are recovered. The calculated on-site $U$ between Mn-$3d$ states in the experimentally observed orthorhombic, layered, and spinel phases are significantly smaller than $U$ in $\gamma \text{−}{\mathrm{LiMnO}}_2$ and disordered layered structures, by $0.5\text{–}0.6\mathrm{eV}$ within GGA. The smaller values of $U$ are shown to be correlated with a collinear ordering of JT distortions, in which all $e_g$ orbitals are oriented in the same direction. This cooperative JT effect can lead to greater electron delocalization from Mn along the $e_g$ states due to increased Mn-O covalency, which contributes to the greater electronic stability compared to the phases with noncollinear JT arrangements. The structures with collinear ordering of JT distortions also generate greater vibrational entropy, which helps stabilize these phases at high temperature. These phases are shown to be strongly insulating with large calculated band gaps $>3\mathrm{eV}$, which are computed using HSE06 and $G_0{W}_0$.

We present a low-energy effective theory to describe the localization-delocalization transition, which occurs for wave functions of electrons and holes injected individually by a voltage pulse with fractional flux quantum. We find that the transition can be described by an effective scattering matrix in a truncated low-energy space, which is composed of two parts. The first part describes the behavior of the scattering matrix in the infrared limit, which dominates the infrared divergence of the electron distribution function. The second part represents the high-energy correlation, which can be approximated by a constant. For short-tailed pulses, which decay faster than Lorentzian, the infrared part itself gives rise to an inverse linear divergence in the distribution function. The divergence is responsible for the dynamical orthogonality catastrophe, which leads to electron-hole pairs with delocalized wave functions. In contrast, the high-energy correlation leads to electron-hole pairs with localized wave functions. Although it does not give rise to divergence by itself, the interplay between the infrared part and the high-energy correlation can give rise to an additional logarithmic divergence in the distribution function. Due to the interplay between them, the wave functions can undergo a localization-delocalization transition, which occurs for electrons and holes injected individually by the voltage pulse. As a consequence, the localization-delocalization transitions for all short-tailed pulses can be described by the same effective scattering matrix, suggesting that they belong to the same universality class. For pulses with longer tails, the distribution function can exhibit additional infrared divergences. We show that a Lorentzian pulse gives rise to a logarithmic squared divergence, while a fractionally powered Lorentzian pulse gives rise to a power-law divergence. The additional divergence can lead to localization-delocalization transitions belonging to different universality classes. These results demonstrate the fine-tuning capabilities of the localization-delocalization transition in time-dependent quantum transport.

Hydrogen hydrates, formed from hydrogen and water under high-pressure and low-temperature conditions, exhibit a rich phase behavior governed by intricate intermolecular interactions. Their structural evolution under extreme conditions provides a unique platform to study the interplay between hydrogen bonding, lattice dynamics, and guest-host interactions. This study investigates the complex phase diagram of hydrogen hydrate up to 50 GPa using angle-resolved neutron and x-ray diffraction techniques. We focus on the behavior of key phases: ${\mathrm{C}}_1, {\mathrm{C}}_2$, and ${\mathrm{C}}_3$, examining their crystallographic transformations, elastic properties, and pressure-induced structural deformations. Our findings provide critical insights into phase transitions, stability regimes, and the fundamental physics of molecular interactions in dense hydrogen-water systems, enhancing our understanding of their thermodynamic and mechanical properties under extreme conditions. These data are also essential to quantify the role of hydrogen on the thermal state and chemical evolution of water-rich worlds of various sizes.

Building on recent progress in the study of Anderson and many-body localization via the renormalization group (RG), we examine the scaling theory of localization in the quantum random energy model (QREM). The QREM is known to undergo a localization-delocalization transition at finite energy density, while remaining fully ergodic at the center of the spectrum. At zero energy density, we show that RG trajectories consistently flow toward the ergodic phase, and are characterized by an unconventional scaling of the fractal dimension near the ergodic fixed point. When the disorder amplitude is rescaled, as suggested by the forward-scattering approximation approach, a localization transition emerges also at the center of the spectrum, with properties analogous to the Anderson transition on expander graphs. At finite energy density, a localization transition takes place without disorder rescaling, and yet it exhibits a scaling behavior analogous to the one observed on expander graphs. The universality class of the model remains unchanged under the rescaling of the disorder, reflecting the independence of the RG from microscopic details. Our findings demonstrate the robustness of the scaling behavior of random graphs and offer insights into the many-body localization transition.