${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$ is an emerging high-performance layered semiconductor with excellent stability. While experimental studies have explored carrier transport across various doping levels for both $n$-type and $p$-type conduction, a comprehensive theoretical understanding remains incomplete. In this work, we present parameter-free first-principles calculations of the electron and hole mobilities in ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$, based on iterative solution of the Boltzmann transport equation that includes electron-phonon scattering and ionized impurity scattering on an equal footing. Intriguingly, we find that ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$ exhibits high electron mobilities in both the in-plane and out-of-plane directions, whereas the hole mobilities are only significant in the in-plane direction, displaying a unique three-dimensional electron transport and two-dimensional hole transport behavior. At 300 K, the calculated intrinsic electron and hole mobilities along the in-plane direction are 447 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ and 29 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, respectively, which are primarily affected by Fröhlich electron-phonon interactions. Due to its large static dielectric permittivity, ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$ exhibits exceptionally high low-temperature electron mobilities above $1.0\times {10}^5{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, and its electron mobilities above 50 K are robust against ionized impurity scattering over a wide range of impurity concentrations. By incorporating the Hall effect into our analysis, we predict an in-plane electron Hall mobility of 517 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at 300 K, in excellent agreement with experimental data. These results provide valuable insights into the carrier transport mechanisms in ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$, and offer predictive benchmarks for future theoretical and experimental investigations.

Understanding and engineering phonon transport in GaN/AlN superlattices (SLs) is critical for the thermal management of GaN high-electron-mobility transistors (HEMTs). While most studies suggest that interfacial roughness reduces the thermal conductivity of SLs, a few have reported its unexpected enhancement, yet the underlying mechanism, particularly concerning phonon coherence, remains elusive. In this study, we address this gap by developing ametric—the concentration degree of energy (CDE) in $k$-space—to directly quantify the degree of phonon coherence from atomic displacements obtained through lattice dynamics. Our simulations reveal that interfacial roughness can enhance the thermal conductivity of SLs, provided that the inherent periodicity of the SL is not disrupted. A mode-resolved coherence analysis based on the CDE metric reveals that while increased roughness leads to a rise in incoherent modes, it simultaneously enables more modes to propagate. Meanwhile, the preservation of periodicity is the decisive factor that significantly enhances phonon coherence. This insight successfully explains why SLs with moderate, periodicity-preserving roughness can achieve superior phonon coherence and higher thermal conductivity than ideal SLs. The coherence analysis framework is applicable to general SL systems.

Manipulating the valley degree of freedom in monolayer transition metal dichalcogenides (TMDs) is essential for advancing valleytronics-based information processing and transport. However, significant intervalley scattering causes valley signals to vanish at room temperature, posing a major challenge. To mitigate this, strategies involving magnetic fields, intense circularly polarized light, and in-plane electric fields have been explored to enhance valley polarization. Recently, increasing the excitons lifetimes using photonic nanostructures has emerged as a promising approach. In this work, we present a phenomenological model based on rate equations that describes and regulates the valley depolarization process in TMDs embedded within engineered photonic environments. Our findings demonstrate the potential to achieve near-unity valley polarization and enable negative valley polarization of dark excitons. Additionally, we identify extrinsically chiral nanocavities as effective platforms for generating chiral emission and spatially separating valley states. This research offers valuable insights for the nanophotonic design of devices with enhanced valley control, including manipulation of valley-polarized dark excitons, and paves the way for the development of advanced integrated valleytronic systems.

We study the structural relaxation and electronic properties of a one-dimensional (1D) moiré system composed of a zigzag graphene nanoribbon (GNR) placed on a hexagonal boron nitride $\left(h\text{−}\mathrm{BN}\right)$ substrate. Using an effective grid model derived from continuum elasticity theory, we calculate the relaxed atomic structure of the GNR/$h$-BN system for various twist angles and ribbon widths. The relaxation gives rise to a characteristic 1D domain structure consisting of alternating commensurate ${\mathrm{AB}}^{\prime }$ regions and two distinct types of domain boundaries. At finite twist angles, the ribbon adopts a wavy shape, locally tracing the $h$-BN zigzag direction but occasionally sliding to adjacent atomic rows. The resulting moiré potential strongly modulates the electronic structure: the zero-energy zigzag edge states are modulated by the local stacking, leading to densely packed subbands in the ${\mathrm{AB}}^{\prime }$ domains and sharply localized domain-wall states in the energy gaps between the domain subbands, which together realize gate-tunable one-dimensional arrays of quantum-confined electronic states. Our results demonstrate that moiré modulation in GNR/$h$-BN heterostructures provides a versatile platform for electronic structure engineering and the design of 1D moiré nanodevices.

An external orbital magnetic field applied perpendicular to the separation vector of a pair of Weyl points can couple them and induce a gap in the electronic spectrum. In this work, we investigate the gap-opening behavior in the presence of a lattice, revealing rich phenomenology absent in the continuum picture. Specifically, we address the emergence of layered Chern insulating states, examining how the anisotropy of the Weyl cone dispersion influences the sequence of phase transitions, and establishing connections to the continuum limit. We analyze the evolution of surface Fermi-arc states across these regimes, highlighting their distinct behaviors during the gap-opening transitions.

Fundamental quantum transition timescales are accessible through the spin polarization of photoelectrons coming from initially spin-degenerate states for solid-state materials. In this work we investigate the modification of this timescale in the vicinity of a resonance in photoemission from a solid. We employ spin- and angle-resolved photoemission spectroscopy (SARPES) to study the valence band of $1T\text{−}{\mathrm{TiSe}}_2$ and $1T\text{−}{\mathrm{TiTe}}_2$, with an excitation photon energy coinciding with the Ti $3p\text{−}3d$ autoionization state. The energy derivative of the measured spin polarization, which is in the off-resonance case proportional to the transition time, reveals a sign reversal and significant magnitude decrease compared to off-resonance measurements. We show that this effect goes beyond the conventional semianalytical models used to translate spin polarization to the transition time delay. At the photoemission resonance, the underlying interference assumption of the model breaks down, and additional information about resonance strength is needed to extract the transition time delays.

We present a unified bond dipole theory for metal-semiconductor interfaces to explain the microscopic origin of interface dipoles and Fermi level pinning (FLP) in terms of Harrison's bond-orbital model. By combining first-principles calculations with tight-binding analysis, we show that localized bonding between semiconductor surface dangling bonds and metal orbitals is sufficient to generate a large interface dipole and induce strong FLP, even when only a single metal monolayer is present. Within this framework, metal-induced gap states (MIGS), dangling-bond-induced surface states (DBSS), and bonding states embedded in the valence band are all understood as different outcomes of the same underlying interface bonding mechanism, rather than as independent causes of FLP. We further establish that the key parameter governing FLP strength is the density of surface dangling bonds that can form new chemical bonds with the metal, which directly controls the magnitude of the bond-induced interface dipole. This picture naturally explains the weaker pinning observed in more ionic semiconductors than in covalent ones and provides practical guidance for engineering metal-semiconductor interfaces and tuning Schottky barrier heights.

The two-dimensional ferroelectric (FE) material ${\mathrm{CuInP}}_2{\mathrm{S}}_6$ (CIPS) has garnered considerable interest due to its out-of-plane ferroelectricity at room temperature. However, a notable discrepancy exists between experiments and density functional theory (DFT) calculations regarding the ground state of CIPS thin films: experiments suggest a state with net polarization, while DFT predicts an antiferroelectric (AFE) state as the lowest-energy state. Here, we investigate the stability of polarization states in CIPS thin films by combining first-principles calculations with the deep potential (DP) method. Our results reveal that for films thicker than the bilayer, an AFE state that has intralayer AFE ordering in the inner layers and intralayer FE ordering in the two surface layers, has the lowest electronic energy. This state is significantly lower than the uniform FE state. In addition, we find that a ferrielectric (FiE) state with pure intralayer FE ordering is very close to the AFE state in energy. By using the DP model, we calculated the phonon free energy of CIPS thin films. For the monolayer, the intralayer FE ordering possesses a lower phonon free energy than the intralayer AFE ordering. This energetic preference for the intralayer FE ordering maintains as the thickness grows. Consequently, when the phonon-free energy is incorporated, the FiE state becomes energetically favorable over the AFE state for multilayers CIPS. Our findings demonstrate that the inclusion of vibrational entropy stabilizes the FiE state as the ground state in multilayers CIPS at finite temperatures, reconciling the previous discrepancy between experimental observations and DFT predictions. This insight is vital for understanding the FE properties of CIPS and its potential applications in devices.

Random permutation circuits were recently introduced as minimal models for local many-body dynamics that can be interpreted both as classical and quantum. Standard dynamical complexity indicators such as damage spreading and out-of-time-order correlators (OTOCs), show that these systems exhibit sensitivity to initial conditions in the classical setting and operator scrambling in the quantum setting. Here, we address their quantum chaoticity—a stricter property—by studying the time evolution of local operator entanglement (LOE). We show that the behavior of LOE in random permutation circuits depends on the dimension of the local configuration space $q$. When $q=2$, i.e., the circuits act on qubits, random permutations are Clifford and the LOE of any local operator is bounded by a constant, indicating that they are not truly chaotic. On the other hand, when the dimension of the local configuration space exceeds two, the LOE grows linearly in time. We prove this in the limit of large $q$ and present numerical evidence that a three-dimensional local configuration space is sufficient for a linear growth of LOE. Our findings highlight that quantum chaos can be produced by essentially classical dynamics. Moreover, we show that LOE can be defined also in the classical realm and put it forward as a universal indicator chaos, both quantum and classical.

We investigate uniaxial-stress effects on the magnetic phase diagram of the square-lattice itinerant magnet ${\mathrm{EuAl}}_4$, where strong coupling among spin, lattice, and charge produces a variety of helimagnetic phases, including rhombic and square skyrmion lattices. Combining resistivity and magnetization measurements with neutron scattering, we find that compressive stresses of only several tens of megapascal along [010] enhance antiferromagnetic character and shorten the magnetic modulation period in the lowest-temperature single-$\mathbf{Q}$ spiral state, thereby driving the critical temperatures and fields of multiple phases to higher values. First-principles calculations show that increasing orthorhombic lattice distortion deforms the Fermi surface relevant to the magnetism, providing compelling evidence that Fermi-surface nesting plays a crucial role in stabilizing the helical magnetic modulations in ${\mathrm{EuAl}}_4$.

Photoinduced amorphization in chalcogenide phase-change materials plays a key role in enabling ultrafast photonic functionality. Here, real-time time-dependent density-functional theory simulations are employed to elucidate the microscopic mechanism of ultrafast amorphization in ${\mathrm{Sb}}_2{\mathrm{Se}}_3$ under femtosecond laser excitation. The results reveal that the occupation of Sb–Se antibonding states weakens the covalent network and initiates local melting that develops into a percolated amorphous structure. At low temperature, coherent phonons drive ordered Peierls-like distortions, whereas thermal phonons at room temperature disrupt coherence and accelerate structural disorder. By incorporating a Boltzmann factor $\tau$ to restore detailed balance, the simulations capture hot-carrier cooling and electron-phonon energy transfer. This approach reveals the intrinsic correlation among photoexcitation, energy relaxation, and structural evolution, offering microscopic insight into ultrafast phase transitions.

Non-Hermitian acoustic systems featuring balanced gain and loss have gained significant popularity owing to their potential applications in multifunctional devices. In this paper, we introduce the concept of parity-time ($\mathcal{PT}$) symmetry to zero index metamaterials (ZIMs) to investigate the transport of sound waves through a $\mathcal{PT}$-ZIM junction in a waveguide system. The $\mathcal{PT}$-ZIM junction is initially established by doping two acoustic cylinders with identical geometry and conjugate refractive indices into the ZIM, with one being gain and the other loss. It is found that the balanced gain and loss induce a perfect resonance transmission (PRT) against a backdrop of perfect reflection, with the resonant linewidth being controllable and determined by the gain/loss factor. Furthermore, we demonstrate the pseudo-$\mathcal{PT}$-symmetry and present a more comprehensive PRT in scenarios where the cylinder geometries are nonidentical, and feature distinct gain and loss factors. The fundamental principle remains a delicate equilibrium between gain and loss, however, the sound localization effect in the cylinders is significantly enhanced due to the differentiation in radii. This work offers a pathway for exploring non-Hermitian acoustics in ZIM, holding promising applications in sensing and nonlinear acoustics.

To investigate the emergent properties that may arise from the interplay of different instabilities in orbitally degenerate systems, we consider the representative case of mixed compounds of ${\mathrm{CaFeO}}_3$ and ${\mathrm{LaMnO}}_3$, having isoelectronic B cations. First-principles study of the mixed systems reveals an intricate interplay of charge transfer, Jahn-Teller distortion, and charge disproportionation, influencing the electronic and magnetic properties of the mixed compounds. Although the ground states of both parent compounds are insulating, one being nearly ferromagnetic (FM; ${\mathrm{CaFeO}}_3$) and another antiferromagnetic (AFM; ${\mathrm{LaFeO}}_3$), several of the mixed compounds among the studied compositions, e.g., ${\mathrm{Ca}}_{0.75}{\mathrm{La}}_{0.25}{\mathrm{Fe}}_{0.75}{\mathrm{Mn}}_{0.25}{\mathrm{O}}_3, {\mathrm{Ca}}_{0.625}{\mathrm{La}}_{0.375}{\mathrm{Fe}}_{0.625}{\mathrm{Mn}}_{0.375}{\mathrm{O}}_3, {\mathrm{Ca}}_{0.375}{\mathrm{La}}_{0.625}{\mathrm{Fe}}_{0.375}{\mathrm{Mn}}_{0.625}{\mathrm{O}}_3$, and ${\mathrm{Ca}}_{0.25}{\mathrm{La}}_{0.75}{\mathrm{Fe}}_{0.25}{\mathrm{Mn}}_{0.75}{\mathrm{O}}_3$, turned out to be FM half-metals. The computed $T_c$ values for ${\mathrm{Ca}}_{0.75}{\mathrm{La}}_{0.25}{\mathrm{Fe}}_{0.75}{\mathrm{Mn}}_{0.25}{\mathrm{O}}_3$ and ${\mathrm{Ca}}_{0.25}{\mathrm{La}}_{0.75}{\mathrm{Fe}}_{0.25}{\mathrm{Mn}}_{0.75}{\mathrm{O}}_3$ are found to be reasonably high, having potential applications for spintronics. The ${\mathrm{Ca}}_{0.5}{\mathrm{La}}_{0.5}{\mathrm{Fe}}_{0.5}{\mathrm{Mn}}_{0.5}{\mathrm{O}}_3$ and ${\mathrm{Ca}}_{0.125}{\mathrm{La}}_{0.875}{\mathrm{Fe}}_{0.125}{\mathrm{Mn}}_{0.875}{\mathrm{O}}_3$ compounds, on the other hand, exhibit insulating, AFM behavior, ${\mathrm{Ca}}_{0.875}{\mathrm{La}}_{0.125}{\mathrm{Fe}}_{0.875}{\mathrm{Mn}}_{0.125}{\mathrm{O}}_3$ being FM and insulating. This results in several electronic and magnetic phase transitions as a function of $x$ in ${\mathrm{Ca}}_x{\mathrm{La}}_{1-x}{\mathrm{Fe}}_x{\mathrm{Mn}}_{1-x}{\mathrm{O}}_3$ mixed compounds. Furthermore, an interesting atomic-scale microstructure appears from the competing nature of metal-oxygen covalency.

We present electrical-resistivity measurements on the prototypical heavy-fermion ferromagnet ${\mathrm{UGe}}_2$ under pulsed magnetic field up to 60 T. An anisotropic field-induced suppression of the electronic correlations is revealed. The electrical resistivity strongly decreases when a magnetic field $\mathbf{H}$ is applied along the easy magnetic axis $\mathbf{a}$, while it remains almost unchanged when $\mathbf{H}$ is applied along the hard magnetic axes $\mathbf{b}$ and $\mathbf{c}$. The field-induced destabilization of the ferromagnetic state is also anisotropic: the anomaly at the Curie temperature $T_C$ disappears in fields higher than $\gtrsim 1\mathrm{T}$ for $\mathbf{H}\parallel \mathbf{a}$ and in fields higher than $\gtrsim 20\mathrm{T}$ for $\mathbf{H}\parallel \mathbf{b},\mathbf{c}$. At temperatures below 2 K, we observe quantum oscillations in fields larger than 50 T applied along $\mathbf{b}$, which support the presence of a two-dimensional Fermi surface similar to that previously observed at low fields.

The interplay between epitaxial strain, magnetic compensation, Curie temperature, and anisotropy in ferrimagnetic insulators remains a central question with both fundamental and technological significance. Here, we present a comparative theoretical study of strain effects on ${\mathrm{Gd}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ (GIG) thin films grown along [001] and [111] orientations. Using density functional theory and an ab initio-based effective Hamiltonian, we study the evolution of the magnetic compensation temperature ($T_M$), Curie temperature ($T_C$), and perpendicular magnetic anisotropy (PMA) under epitaxial strain. Our results show a linear enhancement of both $T_M$ and $T_C$ under compressive strain, with (001)-oriented films exhibiting a notably higher strain sensitivity than (111)-oriented counterparts. PMA was also found to have a nearly linear strain dependence with a strong orientation dependence, and to reverse when Gd $4f$ electrons were treated as core states—hinting at a possible strong temperature dependence of PMA. Our results offer clarity to experimental variability in $T_M$ and provide a microscopic basis for tailoring magnetic compensation, Curie temperature and anisotropy in rare-earth-based spintronics.

Metal borides such as ${\mathrm{MgB}}_2$ distinguish themselves from cuprate, iron-based, and hydride superconductors by integrating structural stability, high hardness, and simple stoichiometry, making them exceptionally suited for practical applications. Here, we predict two hard superconducting phases of ${\mathrm{ZrB}}_6$ ($P6$/mmm and $\mathit{Cm}$) that are dynamically stable at ambient pressure. Sharing a characteristic three-boron-layer sandwich structure, these phases nevertheless exhibit strikingly distinct superconducting behaviors. The $P6$/mmm phase achieves a high $T_c$ of 34 K, the highest among transition metal borides, which is enabled by strong electron-phonon coupling from synergistic flat bands and high-frequency boron phonons. In contrast, the $\mathit{Cm}$ phase exhibits a much lower $T_c$ of 5.8 K due to the absence of such electronic features. Notably, both phases also possess high Vickers hardness values of 33.6 and 39.8 GPa, respectively. Our results not only identify ${\mathrm{ZrB}}_6$ as a promising platform for hard superconductors but also provide a design strategy toward multifunctional materials for high-field magnet and fusion energy technologies.

A central question in superconductivity is how a semimetal such as antimony, normally nonsuperconducting, can host a robust superconducting state upon Au incorporation. We investigate this problem in metastable ${\mathrm{Sb}}_{0.83}{\mathrm{Au}}_{0.17}$, which crystallizes in the simple cubic $\alpha \text{−}\mathrm{Po}$-type structure stabilized through high-pressure and high-temperature synthesis. Bulk measurements of magnetic susceptibility, resistivity, and heat capacity establish type-II superconductivity with $T_c\approx 8\mathrm{K}$. The superconducting state exhibits strong electron-phonon coupling (${\lambda }_{\mathrm{ep}}=0.91$) and the single-gap $s$-wave model yields a superconducting gap of $\mathrm{\Delta }=$ 2.02 $k_B{T}_c$, exceeding the BCS weak-coupling limit. Heat-capacity data reveal low-energy Einstein modes and a pronounced Boson peak in the $\frac{C_{\mathrm{phonon}}}{T^3}$ vs $T$ plot, highlighting phonon softening as a key factor in the enhanced coupling. We argue that the emergence of superconductivity is driven by the cooperative effect of soft phonons and strong electron-phonon interactions. Density-functional theory calculations indicate a finite density of states at the Fermi level, consistent with a metallic behavior in contrast to the semimetallic nature of pristine Sb.

We investigate the pressure-dependent magnetism of the quasi-one-dimensional solid solution ${\mathrm{Ca}}_{1\text{−}x}{\mathrm{Na}}_x{\mathrm{Cr}}_2{\mathrm{O}}_4$ using neutron diffraction, Cr K-edge x-ray absorption spectroscopy, and muon spin rotation/relaxation. Na substitution is known to increase oxygen-ligand hole density (partial ${\mathrm{Cr}}^{3+}$ to ${\mathrm{Cr}}^{4+}$ character) and to drive the evolution from incommensurate order in ${\mathrm{CaCr}}_2{\mathrm{O}}_4$ to commensurate antiferromagnetism in ${\mathrm{NaCr}}_2{\mathrm{O}}_4$, suggesting a complex interplay between charge doping and structural changes. Here we apply hydrostatic pressure to tune the lattice without altering the nominal hole count, thereby separating compression effects from ligand-hole physics. For $x=0$ and $x=0.5$ the magnetic transition temperature is essentially pressure independent within our explored range, whereas in ${\mathrm{NaCr}}_2{\mathrm{O}}_4$ long-range order is progressively suppressed with pressure, as seen by a decrease of $T_N$ and of the magnetic Bragg intensity. Over the same pressure range, Cr K-edge x-ray absorption spectroscopy shows no resolvable change in the average Cr valence or coordination, and the refined corner-sharing Cr–O–Cr geometry remains nearly invariant within uncertainty. Extrapolating the pressure dependence of the magnetic order parameter suggests a critical pressure $P_c=107\left(8\right)$ kbar for complete suppression of long-range order. These results support ligand-hole density as the primary control parameter across ${\mathrm{Ca}}_{1\text{−}x}{\mathrm{Na}}_x{\mathrm{Cr}}_2{\mathrm{O}}_4$, with pressure acting as a secondary electronic tuning knob that weakens ordering in the Na-rich end member without inducing a new magnetic phase.

We conduct a comprehensive density functional theory study to explore the intricate magnetic properties of frustrated Mn monolayer on the Ag(111) surface. Spin-polarized scanning tunneling microscopy demonstrates that a Néel magnetic state characterizes such an interface, which contradicts systematic ab initio predictions made in the last two decades indicating that the ground state is collinear row-wise antiferromagnetic (RW-AFM) state. Here, we employ the all-electron full-potential Korringa-Kohn-Rostoker Green's function method and find that the ground state is a chiral magnetic Néel state, with magnetic moments rotating in the surface plane following a unique sense of rotation, as dictated by the underlying in-plane magnetic anisotropy and Dzyaloshinskii-Moriya interaction. Changing the stacking of the Mn monolayer from fcc to hcp switches the chiral nature of the Néel state. Once allowing disordered magnetic states, as described within the disordered local moment approach, we reveal the possibility of stabilization of a RW-AFM state. We conjecture that at low temperatures, the chiral Néel state prevails, while at higher temperatures, the magnetic exchange interactions are modified by magnetic disorder, which can then induce a transition towards a RW-AFM state. Our work addresses a long-term experimental-theoretical controversy and provides significant insights into the magnetic interactions and stability of Mn films on noble metal substrates, contributing to the broader understanding of the different magnetic facets of frustrated magnetism in thin films.