Evidence for multiple magnetic transitions and unconventional spin exchange interactions in the ilmenite insulator ${\mathrm{MnTiO}}_3$ is provided via neutron scattering. On cooling, while G-type antiferromagnetic (AFM) order sets in first at 63 K with a $k_1=\left(000\right)$ characteristic wave vector, a weaker second magnetic transition with $k_2=\left(00\frac{3}{2}\right)$ appears near 42 K, giving rise to a noncollinear structure. Intrinsic buckling of the honeycomb lattice along $\mathit{c}$ creates bond anisotropy and a distorted crystal field that can lead to exchange paths that modulate orbital overlap and spin-orbit coupling. The inelastic spectrum is best described by magnetic exchange anisotropy that breaks the local symmetry of the honeycomb, with competing AFM Heisenberg, Dzyaloshinskii-Moriya, and alternate intraplanar ferromagnetic interactions, that may yield a weakly coupled ladder system.

The interplay between quantum geometry and electron correlation has emerged as a compelling paradigm in quantum many-body physics. Recent studies have highlighted the diagnostic utility of quantum geometry in identifying magnetic instabilities within itinerant electron systems. In the present work, we critically reexamine these theoretical proposals, which are commonly formulated within the random phase approximation (RPA). To this end, we reformulate the matrix-based RPA instability criterion in the channel representation for generic two-orbital systems, explicitly accounting for multiple channels of magnetic ordering. Our results demonstrate that magnetic phase transitions are intricately governed by the interplay between the bare susceptibility tensor and the spin interaction matrix. Consequently, prior assertions that instabilities can be predicted solely from the quantum geometric structure of a single-channel susceptibility are valid only under complete channel decoupling in both the interaction and susceptibility matrices. By adopting the channel representation, our formulation achieves greater physical transparency and computational tractability compared to the conventional orbital-space approach, thereby furnishing a promising alternative for advancing theoretical studies of complex multiorbital systems.

Altermagnets, characterized by time-reversal symmetry breaking without net magnetization and momentum-dependent spin-split bands, offer a promising platform for spintronics due to their anisotropic spin textures and potential for tunable magnetic interactions. Here, we theoretically investigate the slow phonon-renormalized Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in two-dimensional Rashba $d$-wave altermagnets, incorporating arbitrary Rashba spin-orbit coupling (RSOC) strength and spin-dependent static-Holstein electron-phonon coupling (EPC). Using a continuum model Hamiltonian that captures altermagnetic anisotropy, RSOC, and adiabatic lattice distortions, we compute the noncollinear RKKY exchange tensor via second-order perturbation theory and numerical momentum-space integration. Our results reveal that static lattice distortions provide a versatile tuning knob for engineering the magnitude, anisotropy, and chirality of RKKY couplings. Moderate EPC suppresses long-range coherence while inducing component-specific phase shifts and sign reversals, particularly in Dzyaloshinskii-Moriya (DM) and compasslike terms. Systematic parameter scans demonstrate enhanced oscillatory complexity with altermagnetic order, RSOC, and doping, establishing slow phonons as a practical route to dynamically control ferromagnetic/antiferromagnetic alignments and DM chiralities in altermagnet-based heterostructures.

Despite recent advances in orbitronics, the influence of disorder on the orbital Hall effect and orbital relaxation mechanisms remains poorly understood. In this work, we numerically investigate the role of disorder in orbital transport in mesoscopic two-dimensional square lattices hosting atomic orbitals that carry orbital angular momentum. Using the Landauer–Büttiker formalism in both square and rectangular device geometries, we study disorder-induced orbital Hall current generation and orbital relaxation on equal footing. We find that disorder can strongly enhance the orbital Hall response, with a disorder scaling consistent with a skew-scattering-like regime in the diffusive limit. The main calculations use a uniformly distributed Anderson potential, while additional calculations with a symmetrized disorder distribution, for which all odd moments vanish, produce essentially the same behavior. This response therefore differs from the conventional skew-scattering mechanism known from the anomalous Hall effect. In addition, we show that the orbital propagation length extracted from the spatial decay of the orbital current can extend over hundreds of lattice spacings, in stark contrast to recent theoretical works predicting orbital relaxation over only a few atomic spacings. We attribute this robustness to the two-dimensional nature of the system, where efficient orbital relaxation channels are strongly restricted, leading to long-ranged orbital transport despite substantial disorder.

Recent Scanning Tunneling Microscopy (STM) experiments measuring the superconducting gap features in thin films of superconducting bilayer nickelates ${\mathrm{La}}_2{\mathrm{PrNi}}_2{\mathrm{O}}_7$ at ambient pressure and compressive strain paved the way to study the Cooper-pairing models and the band-selective identification of the gap features in these systems. Here, using the realistic two-orbital bilayer model and the continuum Green's function formalism, we theoretically analyze orbital and band-selective local density of states as well as the corresponding STM spectra. We find that the multiorbital character and the spatial dependence of the Wannier functions lead to characteristic features in the spectra that depend on the position of the STM tip. This allows for a band-resolved analysis of the superconducting coherence peaks and scattering momenta. We identify a clear route for experimental measurements not only to determine whether the $\gamma$ band is incipient, but also to identify the band origins of the coherence peaks through tip-height-dependent measurements of the local density of states and its impurity-induced corrections.

Generalized symmetry extends the usual notion of symmetry to ones that are of higher form, acting on subsystems, noninvertible, etc. The concept was defined in the field theory context using the idea of topological defects. On the lattice, an immediate consequence is that a symmetry twist is moved across the system by a sequential quantum circuit. In this paper, we ask how to deduce the full, potentially noninvertible symmetry action from the unitary sequential circuit and how the connection to a sequential circuit constrains the properties of the generalized symmetries. We find that, for symmetries that contain the trivial symmetry operator as a fusion outcome, which we call annihilable symmetries, the sequential circuit fully determines the symmetry action and puts various constraints on their fusion. In contrast, for unannihilable symmetries, like those whose corresponding twist is a Cheshire string, a further one-dimensional sequential circuit is needed for the full description. Matrix-product-operator and tensor-network-operator representations play an important role in our discussion.

The coexistence of surface superconductivity and Fermi arcs in trigonal $\gamma \text{−}\mathrm{PtB}{\mathrm{i}}_2$ has recently attracted attention for possible realization of topological superconductivity. The Fermi arcs on the two different (0001) surface terminations have been associated with the set of Weyl points just above the Fermi energy (${\mathrm{E}}_{\mathrm{F}}$). Here using first-principles calculations to explore the band crossings over the full Brillouin zone between the nominally highest valence and lowest conduction bands in $\gamma \text{−}\mathrm{PtB}{\mathrm{i}}_2$, we study the Weyl nodal loop (WNL) and multiple sets of Weyl points (WPs). The main difference between the two reported experimental structural parameters is the magnitude of Bi-layer buckling. While the WNL, bulk gap region, and the set of Weyl points just above the ${\mathrm{E}}_{\mathrm{F}}$ are robust, the number and location of the other sets of WPs depend sensitively on the structural parameters with different magnitude of Bi-layer buckling. Besides calculating the 2D Fermi surface with Fermi arcs and quasiparticle interference (QPI) around the ${\mathrm{E}}_{\mathrm{F}}$ in good agreement with angle-resolved photoemission spectroscopy and experimental QPI, we also predict new Fermi arc features at higher energy.

Precise control of the surface acoustic wave (SAW) frequency is essential for the reliable operation of high-frequency acoustic devices, which is typically achieved by varying the transducer periodicity $P$. However, the effect of transducer width $L$, or equivalently, the $L/P$ ratio, on frequency remains largely unexplored. Here, we investigate the generation and detection of GHz SAWs using metallic transducers of varying width $L$ but having constant periodicity $P$ fabricated on a nonpiezoelectric silicon wafer. Coherent SAWs are optically excited using near-infrared femtosecond pulses and probed via time-resolved extreme ultraviolet diffraction, enabling simultaneous measurement of first- and second-order SAWs. We find that as $L$ decreases from 410 to 45 nm for constant $P=1000\mathrm{nm}$, the first- and second-order SAW frequencies stiffen by nearly 3% and 6.5%, respectively. The measured stiffening exceeds predictions from finite-element-based linear-elastic eigenvalue analysis by 1% and 1.5%, respectively, thus making the SAW frequency transducer width dependent. The analysis of displacements, strains, and strain gradients indicates that the additional stiffening on decreasing $L$ may arise from large strain gradients at the transducer edges. Importantly, this relatively small change in SAW frequency for a large variation in $L$ establishes transducer width as a sensitive parameter for fine and continuous tuning of SAW frequency by approximately 1%–6% in the vicinity of the desired frequency, enabling precise control beyond what is typically achievable with $P$ alone. Such fine-tuning capability is particularly relevant for applications in radio-frequency signal processing and quantum acoustic systems, where even small deviations can significantly impact device performance.

The voltage-driven breakdown of a Mott insulating state is a phenomenon where both interactions and nonequilibrium effects play crucial roles. This interesting effect has generated significant theoretical and experimental interest, and may also prove technologically relevant. Recent experiments have observed a sharp nonequilibrium insulator-to-metal transition accompanied by hysteresis, negative differential conductance, and lattice deformations, but the underlying breakdown mechanism is highly debated. Here, we describe a minimal lead-driven scenario in which the breakdown can arise from voltage-induced chemical pressure and explore its ramifications for a paradigmatic model of interacting spinless fermions on a chain coupled to metallic reservoirs (leads) and calculate the charge and current responses at the RPA level. We find a complex phase diagram where the nature of the breakdown depends on the strength of the interaction, leading to various current-carrying nonequilibrium phases. For weak to intermediate interaction strengths, we identify a conducting charge-density-wave (CDW) phase with a bias-dependent ordering wave vector. At large interaction strength, the breakdown leads to a charge-separated insulating phase. Additionally, we observe instances of hysteretic behavior, sharp current onset, and negative differential conductance. Our results can help shed light on recent experimental findings and unveil a clean reference mechanism for current-induced breakdown of electronic order.

Polar metals, a category of materials exhibiting the unexpected coexistence of polar structural distortion and metallicity, have attracted considerable research interest. Despite several experimental studies, the effect of polar distortions on the electronic structure (such as orbital configuration) of polar metals remains unclear. Here, we investigate the electronic band structure, crystal-field splitting, and orbital configuration of an artificial two-dimensional polar metal based on the ${\mathrm{BaTiO}}_3/{\mathrm{SrTiO}}_3/{\mathrm{LaTiO}}_3$ heterostructure. The interfacial crystal and electronic structures were investigated by scanning transmission electron microscopy with atomic layer-resolved electron energy-loss spectroscopy. Resonant x-ray linear dichroism spectra revealed an anomalous orbital configuration with a unique sequence $(d_{xy}<d_{xz}/d_{yz}<d_{3z^2-r^2}<d_{x^2-y^2})$, where the in-plane and out-of-plane orbital sequence of the $t_{2g}$ and $e_g$ states is reversed. More importantly, the $t_{2g}$ orbitals are nearly degenerate, which is a characteristic feature of polar distortion. The nearly degenerate $t_{2g}$ orbitals and the altered orbital sequence arise from the competition between epitaxial strain and polar distortion. Our work demonstrates that the electronic structure of polar metals can be strongly modulated by polar distortions, providing a general route for designing quantum states with tailored orbital configurations.

Altermagnets have recently emerged as a new platform for spintronics applications, offering spin-split electronic bands despite vanishing net magnetization. Here, we investigate spin-current generation in Dirac altermagnets and identify Klein tunneling as an efficient mechanism for enhancing spin transport. Using a low-energy Dirac model combined with scattering theory, we demonstrate that Klein tunneling in altermagnets is strongly spin dependent and can be used to effectively control the electronic spin-current polarization by, for instance, adjusting the height, width, and orientation of the potential barrier. Finally, we explore how the $\ell$-wave symmetry of the Dirac altermagnet shapes the spin-current polarization and transmission, focusing especially on the $d$- and $g$-wave cases. Particularly promising results are obtained for the $g$-wave Dirac altermagnet, as it is found that the presence of a potential barrier can significantly boost the spin-current polarization, even when the intrinsic polarization due to the spin-split band structure is vanishingly small. For a barrier implemented via electrostatic gating, such a mechanism would, in turn, allow the spin-current polarization to be switched on and off via a gate voltage.

We present a microscopic theory of an atomic spin diode. Our proposed system consists of two magnetic adatoms deposited on the surface of a two-dimensional electron gas with Rashba spin-orbit coupling. A local s-d type coupling between the local spins and the spins of the electrons induces a nonlocal Ruderman-Kittel-Kasuya-Yosida type interaction and a Dzyaloshinskii-Moriya interaction, in addition to dissipative interactions, between the spins. We derive the effective action for the spins using the Keldysh formalism. From the effective action, we also derive equations of motion for the spins which are shown to be of Landau-Lifshitz-Gilbert type, and give expressions for the effective field and Gilbert damping which appear in this equation. From our microscopic theory, we find that for an in-plane magnetic field perpendicular to the vector connecting the two atoms, the magnitude of the field and the distance between the atoms can always be tuned to engender perfectly diodic coupling. Our findings may pave the way for experimental realization of atomic spin diodes.

${\mathrm{PdTe}}_2$ has been synthesized with controllable thickness down to the monolayer limit. Based on first-principles calculations within the fully anisotropic Migdal–Eliashberg framework, this work reveals that alkali-metal intercalation markedly enhances the weak superconductivity of bilayer ${\mathrm{PdTe}}_2$, boosting the transition temperature from 1.4 K to 5.0–13.5 K and yielding a two-dome-like evolution of $T_c$. Rubidium intercalation induces the highest $T_c$ of 13.5 K, which can be further increased to 14.5 K under biaxial tensile strain. The strain-dependent evolution of $T_c$ also exhibits a two-dome-like behavior, reflecting the interplay between strain-induced band structure modifications and electron–phonon coupling (EPC). Moreover, a systematic correlation is identified between interlayer interaction and the superconducting gap. Lithium intercalation induces a distinct two-gap state, whereas intercalants with larger atomic radii (Na, K, Rb, and Cs) drive the system into a single-gap character. The two-gap to single-gap transition originates from the modulation of interlayer coupling through intercalation-induced interlayer expansion. In addition, pristine and Li/Na-intercalated bilayers exhibit nontrivial band topology, suggesting that layered ${\mathrm{PdTe}}_2$ provides a promising platform for realizing the coexistence of superconductivity and nontrivial topology. These results provide detailed anisotropic insights into EPC and offer viable pathways for enhancing $T_c$ and achieving diverse properties in layered ${\mathrm{PdTe}}_2$ systems.

Density functional theory has demonstrated remarkable predictive power in calculating magnetic properties at zero temperature. At finite temperatures, thermally excited phonons affect magnetism. Efficient ab initio methods for calculating the temperature dependence of magnetic exchange interactions are still lacking despite the importance of room temperature magnetism for applications. Here we present a method to calculate the exchange interactions at finite temperatures from first principles using only two supercell calculations per temperature. Focusing on magnetic insulators, we find that magnetic exchange interactions decrease with temperature in NiO, while in ${\mathrm{Cr}}_2{\mathrm{O}}_3$ a substantial temperature-induced enhancement of 10% at $T=300\mathrm{K}$ is uncovered. The exchange constant is governed by an interplay between metal-ligand hybridization, Hubbard repulsion, and the band gap. They all depend on equilibrium interatomic distances and bond angles, themselves modified by thermal phonons.

Spin-wave excitations provide a central probe of magnetic order and electronic correlations in strongly correlated materials. In this work, we develop an adiabatic theory of spin dynamics by combining the Niu-Kleinman formalism with Kotliar-Ruckenstein slave-boson theory. For each frozen spin configuration, the constrained slave-boson saddle point is solved self-consistently, allowing the Berry-curvature matrix and energy Hessian entering the linearized adiabatic equations of motion to be extracted directly. Applied to the half-filled single-orbital Hubbard model, the resulting spin-wave dispersion shows substantially improved agreement with determinant quantum Monte Carlo benchmarks compared with the random phase approximation and closely approaches results from the time-dependent Gutzwiller approximation. We further extend the method to a two-orbital model of ${\mathrm{La}}_2{\mathrm{NiO}}_4$, demonstrating its applicability to realistic multiorbital correlated systems. Because the approach only requires saddle-point solutions near the magnetic ground state, it remains computationally efficient while incorporating strong correlation effects beyond conventional weak coupling descriptions, providing a practical framework for studying low-energy spin excitations in correlated quantum materials.

Bound states in the continuum (BICs) enable strong light confinement and enhanced light–matter interactions, yet how lattice deformation governs their momentum-space evolution remains unclear. Here, we develop a lattice-engineering framework for silicon photonic crystal slabs by investigating the continuous evolution from a square lattice to rectangular and monoclinic lattices. A symmetry-constrained far-field expansion shows that rectangular deformation lifts the degeneracy between two orthogonal BIC branches, while the preserved mirror symmetries pin their trajectories to the principal axes. Near the merging point, $k_{\mathrm{BIC}}^2$ varies linearly with the aspect-ratio detuning. Monoclinic shear removes these mirror constraints and enables coupling between orthogonal momentum components, steering the BICs along off-axis trajectories. The slab thickness provides an additional degree of freedom to shift the BICs along these symmetry-allowed trajectories and drive their charge-conserving merging at the $\mathrm{\Gamma }\mathrm{point}$. Further breaking the in-plane $C_2$ symmetry of the unit cell splits integer-charge BICs into circular-polarization points carrying half-integer topological charges, accompanied by extended regions of high ellipticity. These results show how lattice symmetry constrains BIC trajectories and geometric tuning controls their positions, providing a route to engineering BIC merging and polarization singularities.