Valleytronic, spintronic, and electronic devices primarily harness the valley, spin, and charge entities of an electron. Our study delves into the merger of these properties in Janus $MX{\mathrm{SiN}}_2$ ($M$ = Cr/Mo/W; $X$ = S/Se/Te) monolayers. Outperforming their pristine $M{\mathrm{Si}}_2{\mathrm{N}}_4$ monolayers, the Janus $MX{\mathrm{SiN}}_2$ monolayers exhibit concurrent vertical and horizontal electric polarization stemming from their unique geometry. The absence of out-of-plane mirror symmetry and inversion symmetry, in addition to strong spin-orbit coupling, leads to in-plane Rashba spin splitting (${\alpha }_R\sim 0.015$–0.624 eV Å), out-of-plane Zeeman-type valley spin splitting (${\mathrm{\Delta }}_{SO}^{c/v}\sim 4-374\mathrm{meV})$, and valley-contrasting Berry curvature (${\mathrm{\Omega }}_n\sim 6.22–13.30{\AA }^2)$ within the Janus $MX{\mathrm{SiN}}_2$ monolayer family. The tweaking of Rashba and valley spin splitting under biaxial strain is intricately influenced by changes in both the macroscopic charge distribution and the microscopic proportion of the atomic orbitals involved. Notably, at the Γ point of the highest valence band, the Rashba coupling is dominantly influenced by the out-of-plane atomic orbitals, while at the $K/K^{\prime }$ point of the highest valence band, the valley spin splitting is primarily controlled by the in-plane atomic orbitals. This work addresses the long-standing confusion of how orbital composition fundamentally influences spin-dependent phenomena. Further, this study identifies suitable Rashba descriptors like planar average potential difference (ΔΦ), the trace of the Born effective charge ($Z^{*}$), and the macroscopic static dielectric constant (${\varepsilon }_{11}$). Moreover, the Janus $MX{\mathrm{SiN}}_2$ monolayers exhibit moderate in-plane $d_{22}\sim 1.40–4.20\mathrm{pm}/\mathrm{V}$ and out-of-plane piezoelectricity $d_{31}\sim 0.16$–0.50 pm/V, governed by the electronegativity-difference ratio. This comprehensive exploration and integration of the valleytronic, spintronic, and piezoelectric properties serve as a foundation for self-sustained spin-based field-effect transistors, valley-coupled spin Hall effect, piezo-spintronics, etc., shaping the landscape of emerging technologies.

Recently, a direct connection between static structure factors and quantum ground states for two-spin interaction Hamiltonians was proven. This suggests the possibility of quantum state tomography from neutron scattering. Here, we investigate the associated fitness landscape numerically. We find a linear relationship between the mean-square distances of structure factors and associated state overlaps, implying a well-behaved fitness landscape. Furthermore, we find evidence suggesting that the approach can be generalized to thermal equilibrium states. We also extend the arguments to the cases of applied magnetic fields and finite clusters.

Recent strain-stabilized superconductivity at ambient pressure in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ films opens new avenues for nickelates research, in parallel with its pressure-induced counterpart. Using density functional theory calculations, we elucidate the critical factors bridging strain- and pressure-driven superconductivity in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ by comprehensively analyzing structural, electronic, magnetic, and density wave characteristics. Consistent with recent scanning transmission electron microscopy observations, we find an $I4/mmm$ structural transition at $-0.9\%$ strain, preceding superconductivity onset. Electronic analysis shows compressive strain lowers Ni-$d_{z^2}$ orbital energy levels, while interfacial Sr diffusion effectively reconstructs the $d_{z^2}$ pockets, quantitatively matching angle-resolved photoemission spectroscopy experiments. Crucially, the interlayer antiferromagnetic coupling $J_{\perp }$ under pressure and strain closely tracks experimental superconducting $T_c$ variation. The dome-shaped pressure dependence and monotonic strain dependence of $J_{\perp }$ mainly arise from modulations in the apical oxygen $p_z$ energy levels. Moreover, compressive strain suppresses both charge density wave (CDW) and spin density wave (SDW) instabilities analogous to pressure effects, with SDW vanishing concurrently with the structural transition and CDW disappearing at $\sim -3.3\%$ strain. Our results indicate that suppressed density waves and enhanced $J_{\perp }$ are crucial for both strain- and pressure-driven superconductivity. Accordingly, we propose several candidate substrates capable of achieving greater compressive strain, thereby potentially increasing $T_c$.

Unconventional orbital paramagnetism without enhanced density of states was recently discovered in the nodal-line semimetal ZrSiS. We propose an interband mechanism, linked to the negative curvature of energy dispersions, which successfully accounts for the observed anomalous response. This negative curvature originates from energy variation along the nodal line, inherent in realistic nodal-line materials. Our results suggest that such orbital paramagnetism provides strong evidence for the presence of nodal lines in ZrSiS and serves as a hallmark of other nodal-line materials.

We report the discovery of temperature-invariant ultralow thermal conductivity ($\kappa$) in monolayer ${\beta }^{\prime }\text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$, a two-dimensional ferroelectric crystal with in-plane polarization. Using a combination of generalized Wigner transport equation theory and machine-learning-assisted molecular dynamics simulations, we reveal that the balance between particlelike phonon propagating and wavelike tunneling transport mechanisms results in a propagating-tunneling-invariant ultralow thermal conductivity of approximately 0.6 W/mK (comparable to that of glass) over a broad temperature range ($150<T<800$ K). This behavior stems from intrinsic strong lattice anharmonicity driven by ferroelectric dipolar fluctuations, eliminating the need for extrinsic structural modifications. In contrast, the $\alpha \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$ monolayer, which shares the same stoichiometry, exhibits a temperature-dependent thermal conductivity typical of simple crystals. We show that the anharmonicity in ${\beta }^{\prime }\text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$ can be precisely modulated by an external electric field, enabling on-demand control of the temperature scaling behavior of heat conductivity. Furthermore, an electric-field-driven motion of the $\alpha /{\beta }^{\prime }$ phase interface is demonstrated, supporting a nonvolatile, large thermal switching ratio of $>3$. These findings provide fundamental insights into the interplay between field-tunable lattice anharmonicity, phonon dynamics, and thermal transport mechanisms.

Many microscopic models with the interaction between the next-nearest neighbors as a key parameter for cuprate physics have inspired us to study the diagonal superexchange interaction in a ${\mathrm{CuO}}_2$ layer. Our investigation shows that models with extended hopping provide a correct representation of magnetic interactions only in a hypothetical square ${\mathrm{CuO}}_2$ layer, where the diagonal superexchange interaction with the next-nearest neighbors always has an antiferromagnetic (AFM) nature. The conclusions are based on the symmetry prohibition on the ferromagnetic contribution to the diagonal superexchange between the next-nearest neighbors for a simple square ${\mathrm{CuO}}_2$ layer rather than for a real ${\mathrm{CuO}}_2$ layer, where diagonal AFM superexchange may be overestimated. We also discuss the reasons for magnetic frustration effects and high sensitivity of spin nanoinhomogeneity to square symmetry breaking.

While significant attention has been devoted to studying entanglement in photonic systems, solid-state spin lattices remain relatively underexplored. Motivated by this gap, we investigate the entanglement structure of one-dimensional ferrimagnetic chains composed of alternating spin-1/2 and spin-3/2 particles. We characterize the ground-state correlations using exact diagonalization and the density matrix renormalization group method. Although the bipartite entanglement is restricted to nearest neighbors, we reveal the presence of long-range genuine multipartite entanglement between spatially separated spin pairs. These findings advance our understanding of quantum correlations in ferrimagnetic materials. The micromagnon description allows us to provide fast approximation of ground states of ferrimagnets and emphasizes the presence of multipartite correlations not widely discussed thus far.

The atomic-scale determination of hydrogen positions in MoSH monolayers remains experimentally challenging, and existing studies are confined to Janus-type configurations. Here, we combine high-throughput structural screening with first-principles calculations to predict a novel non-Janus Hybrid 1T'-MoSH monolayer, which energetically surpasses all previously reported MoSH phases with a binding energy of −3.02 eV. This structure emerges as a hybrid of ${\mathrm{MoS}}_2$ and ${\mathrm{MoH}}_2$, featuring alternating S and H atoms on both sides of the Mo layer. Comprehensive stability analyses confirm its robustness in energy, mechanics, dynamics, and thermodynamics (stable up to 1600 K). Remarkably, anisotropic Migdal-Eliashberg theory predicts Hybrid 1T'-MoSH as a two-gap superconductor with a critical temperature $T_{\mathrm{c}}$ of 17.5 K, driven by strong electron-phonon coupling ($\lambda =1.33$). Substituting Mo with Hf, Ti, or Ta drastically suppresses $T_{\mathrm{c}}\sim (0.53-2.42\mathrm{K})$, highlighting Mo's unique role in enhancing superconductivity. Our work not only expands the family of 2D transition metal chalcogenides but also proposes a promising candidate for quantum technologies, bridging theoretical design to functional material discovery.

Nonlinear frequency conversion processes, such as optical rectification, difference-frequency generation, and sum-frequency generation, are fundamental for producing electromagnetic radiation at diverse frequencies. In this work, we demonstrate that coherently excited infrared-active phonons can act as transducers for generating nonlinear magnetizations through phonon-magnon interactions, analogous to nonlinear optical frequency conversion. We derive analytical solutions for the time-dependent magnetizations arising from the second-order response to the electric field component of an ultrashort laser pulse. These solutions enable us to define second-order nonlinear magneto-electric susceptibilities, which describe rectification, impulsive excitation, and sum-frequency excitation of coherent magnons. Our theoretical framework naturally incorporates the conventional magneto-optic and phonon inverse Faraday effects and predicts a hybrid magneto-opto-phononic inverse Faraday effect involving photon-phonon-magnon scattering. This work highlights nonlinear phononics as a pathway for controlling magnetization in solids.

Two-dimensional (2D) altermagnets have recently emerged as promising candidates for spintronic applications due to their intrinsic spin splitting in the absence of spin-orbit coupling (SOC). This often leads to the oversight of SOC effects in altermagnets containing heavy elements. In this work, we perform a comprehensive first-principles investigation of monolayer ${\mathrm{V}}_2{\mathrm{Te}}_2\mathrm{O}$, a 2D altermagnet containing heavy Te atoms, with a particular focus on its strain-tunable spin and valley properties in the presence of SOC. We reveal that its Néel-type antiferromagnetic ground state exhibits a unique spin-valley locking protected by the diagonal mirror symmetry, which can be tuned not only by applying uniaxial or biaxial strain, but also by the strain-induced change of easy magnetization axis with respect to the mirror plane. When SOC is included, the system exhibits rich phenomena such as a half-semiconducting state, large valley polarization, and 100% spin polarization. Accordingly, we propose a tunnel magnetoresistance (TMR)-like device constructed solely by monolayer ${\mathrm{V}}_2{\mathrm{Te}}_2\mathrm{O}$ and operated by strain, in which a high TMR ratio is expected due to the spin and valley selection mechanism. Furthermore, monolayer ${\mathrm{V}}_2{\mathrm{Te}}_2\mathrm{O}$ maintains a high Néel temperature exceeding 300 K both without strain and under several representative strains, confirming its potential applications for practical room-temperature spintronic and valleytronic devices.

The two-dimensional layered ferromagnet ${\mathrm{Fe}}_3{\mathrm{GaTe}}_2$, composed of a $\mathrm{Te}\text{−}{\mathrm{Fe}}_{\mathrm{I}}\text{−}{\mathrm{Fe}}_{\mathrm{II}}\text{/}\mathrm{Ga}\text{−}{\mathrm{Fe}}_{\mathrm{I}}\text{−}\mathrm{Te}$ stacking sequence, hosts two inequivalent Fe sites and exhibits a high Curie temperature and strong out-of-plane magnetic anisotropy, making it a promising platform for spintronic applications. Recent experiments have observed a pressure-induced switching of the magnetic easy axis from out-of-plane to in-plane near 10 GPa, though its microscopic origin remains unclear. Here, we employ first-principles calculations to investigate the pressure dependence of the magnetocrystalline anisotropy energy in ${\mathrm{Fe}}_3{\mathrm{GaTe}}_2$. Our results reveal a clear easy-axis switching at a critical pressure of approximately 10 GPa, accompanied by a sharp decrease in the magnetic moments arising from ${\mathrm{Fe}}_{\mathrm{I}}$ and ${\mathrm{Fe}}_{\mathrm{II}}$ atoms. As pressure increases, spin-up and spin-down bands broaden and shift oppositely due to band-dispersion effects, leading to a reduction in net magnetization. Simultaneously, the spin-orbit coupling (SOC) contribution from ${\mathrm{Fe}}_{\mathrm{I}}$, which initially favors an out-of-plane easy axis, diminishes and ultimately changes sign, thereby promoting in-plane anisotropy. The SOC contribution from the outer-layer Te atoms also decreases steadily with pressure, although it retains its original sign; this additional reduction further reinforces the in-plane magnetic easy axis. In contrast, ${\mathrm{Fe}}_{\mathrm{II}}$ atoms continue to favor an out-of-plane orientation, but their contribution is insufficient to counterbalance the dominant in-plane preference at high pressure. These findings elucidate the origin of magnetic easy-axis switching in ${\mathrm{Fe}}_3{\mathrm{GaTe}}_2$ and provide insights for tuning magnetic anisotropy in layered materials for spintronic applications.

Chiral anomalies are a quantum mechanical effect for massless Dirac fermions in both particle physics and condensed matter physics. Here we present a set of effective models for single massless Dirac fermions in one and three dimensions in the whole Brillouin zone from higher-dimensional Chern insulators, which uniquely capture both the chiral fermion behavior near the Dirac point and high-energy states at Brillouin zone boundaries. In the presence of electromagnetic fields, the chiral coefficient $C_5$ is found to be chemical potential dependent in general but quantized precisely at the Fermi surface where chiral symmetry is preserved. This result provides an alternative interpretation of a chiral anomaly on a lattice: The anomaly is caused by symmetry-broken states far below the Fermi surface and protected by the local chiral symmetry. Our analysis here might provide a potential theoretical foundation for applying the concept of chiral anomalies in condensed matter physics.

We establish a theoretical formulation of the orbital magnetic moment in multiorbital tight-binding models, focusing on the role of the electric dipole. We demonstrate that the total magnetic moment can be decomposed into several contributions in multiorbital tight-binding models generally. In particular, we reveal that the electric dipole moment of Wannier orbitals also contributes to the orbital magnetic moment, which is not included in the conventional expression for the orbital magnetic moment in lattice systems. The derived formulation for the magnetic moment is applied to the inverse Faraday effect (IFE), a phenomenon where circularly polarized light induces a magnetic moment. To account for all possible contributions, we adopt an $s-p$ tight-binding system as a minimal model for studying the IFE. Using an analytical approach based on the Schrieffer-Wolff transformation, we clarify the physical origins of these contributions. Additionally, we quantitatively evaluate each contribution on an equal footing through a numerical approach based on the Floquet formalism. Our results reveal that the orbital magnetic moment exhibits a significantly larger response compared with the spin magnetic moment, with all contributions to the orbital magnetic moment being comparable in magnitude. These findings highlight the essential role of orbital degrees of freedom in the IFE.

We present a comprehensive theoretical framework for quantum criticality in the non-Hermitian detuned PXP model, and establish the complete phase diagram, which had remained elusive in previous studies. Starting from a numerically identified phase transition point, we construct an exact second-order phase transition boundary through a similarity transformation in the real-energy regime. By introducing the biorthogonal entanglement entropy and biorthogonal Loschmidt echo, we demonstrate from both equilibrium and nonequilibrium perspectives that this transition belongs to the Ising universality class. Using the correlation function, we further distinguish between confined and deconfined phases within the $\mathcal{PT}$-symmetric region. In the complex-energy regime, we identify both a full $\mathcal{PT}$ transition and a first-excited-state $\mathcal{PT}$ transition, respectively. Moreover, we identify the location of the Yang-Lee edge singularity (YLES) using both the associated-biorthogonal and self-normal Loschmidt echoes, and extract the corresponding critical exponent, which agrees with the predictions of nonunitary conformal field theory. Finally, we propose an experimental scheme to observe the YLES in Rydberg atomic arrays, which offers a promising route to exploring non-Hermitian critical phenomena and singularities in future experimental settings.

Matrix-based quantum kinetic simulations have been widely used for the predictive modeling of electronic devices. Inelastic scattering from phonons and electrons are typically treated as higher-order processes in these treatments, captured using mean-field approximations. Carrier multiplication in avalanche photo-diodes (APDs), however, relies entirely on strongly inelastic impact ionization, making electron-electron scattering the dominant term requiring a rigorous, microscopic treatment. We go well beyond the conventional Born approximation for scattering to develop a matrix-based quantum kinetic theory for impact ionization, involving products of multiple Green's functions. Using a model semiconductor in a reverse-biased p-i-n configuration, we show how its calculated nonequilibrium charge distributions show multiplication at dead-space values consistent with energy-momentum conservation. Our matrix approach can be readily generalized to more sophisticated atomistic Hamiltonians, setting the stage for a fully predictive, first-principles theory of APDs.

We present a theoretical model for a coherent terahertz radiation source based on Bose-Einstein condensate of incoherently pumped microcavity photons. Energy relaxation is provided by inelastic photon scattering on a two-dimensional electron gas in a magnetic field. The proposed setup evades the standard lasing mechanisms: neither population inversion nor light wave amplification is utilized. We study the kinetics of photon condensation and describe a semiconductor-crystal-based device.

We theoretically investigate harmonic generation in graphene quantum dots under linearly polarized optical pulses, focusing on excitonic effects. Combining the tight-binding model and the single-particle density matrix approach, we derive semiconductor Bloch equations under a static-screened Hartree-Fock approximation. This framework characterizes the electron-electron interaction through local Hartree potentials for direct Coulomb interaction and nonlocal Fock potentials for exchange interaction. Distinct configurations of Hartree and Fock terms yield various approximation methods, including the independent-particle approximation, the mean-field approximation, the random phase approximation, and the excitonic effects. We thoroughly analyze how these approximation methods affect the electronic energy levels, linear optical absorption, and nonlinear harmonic generation. Within excitonic effects, we present the dependence of harmonic generation on the geometric variations of graphene quantum dots (sizes, triangular/hexagonal shapes, and armchair/zigzag edges) and the amplitude and polarization of electric fields. Our findings show that excitonic effects significantly enhance optical responses of graphene nanostructures. For a dot ensemble formed by randomly oriented graphene quantum dots, only odd-order harmonics exist along the polarization direction of the incident light. Crucially, harmonic generation in graphene quantum dots exhibits high tunability via geometric configuration, making them promising candidates for nonlinear optical nanodevices.

Spin-orbit coupling and quantum geometry are fundamental aspects in modern condensed matter physics, with their primary manifestations in momentum space being spin textures and Berry curvature. In this work, we investigate their interplay with high-harmonic generation (HHG) in two-dimensional noncentrosymmetric materials, with an emphasis on even-order harmonics. Our analysis reveals that the emergence of finite even-order harmonics necessarily requires a broken twofold rotational symmetry in the spin texture, as well as a nontrivial Berry curvature in systems with time-reversal invariance. This symmetry breaking can arise across various degrees of freedom and impact both spin textures and optical response via spin-orbit interactions. We also show that HHG is particularly sensitive to dynamical rotational-symmetry breaking, as even high-order components can be modulated by a time-dependent symmetry breaking. These findings underscore the potential of HHG as a tool for exploring electronic phases with broken rotational symmetry, as well as the associated phase transitions in two-dimensional materials, and provide novel perspectives for designing symmetry-dependent nonlinear optical phenomena.

Explicitly correlated methods, such as the transcorrelated method which shifts a Jastrow or Gutzwiller correlator from the wave function to the Hamiltonian, are designed for high-accuracy calculations of electronic structures, but their application to larger systems has been hampered by the computational cost. We develop improved techniques for the transcorrelated density-matrix renormalization group (DMRG), in which the ground state of the transcorrelated Hamiltonian is represented as a matrix product state (MPS), and demonstrate large-scale calculations of the ground-state energy of the two-dimensional Fermi-Hubbard model. Our developments stem from three technical inventions: (i) constructing matrix product operators (MPOs) of transcorrelated Hamiltonians with low bond dimension and high sparsity, (ii) exploiting the entanglement structure of the ground states to increase the accuracy of the MPS representation, and (iii) optimizing the nonlinear parameter of the Gutzwiller correlator to mitigate the nonvariational nature of the transcorrelated method. We examine systems of size up to $12\times 12$ lattice sites, four times larger than previous transcorrelated DMRG studies, and demonstrate that transcorrelated DMRG yields significant improvements over standard nontranscorrelated DMRG for equivalent computational effort. Transcorrelated DMRG reduces the error of the ground-state energy by $2.4\times$–$14\times$, with the smallest improvement seen for a small system at half filling and the largest improvement in a dilute closed-shell system.