A single-crystal compensated ferrimagnet ${\mathrm{Ni}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$, consisting of two antiferromagnetically coupled ferromagnetic (FM) $\mathrm{N}{\mathrm{i}}^{2+}$ layers with opposite moments in the ab plane, exhibiting noninterface exchange bias (EB), was investigated. The magnetization hysteresis loops $M\left(\theta \right)$, measured at a weak field H rotating in FM layers clockwise and then counterclockwise, show two successive M flips in opposite directions with a large hysteresis of the flip angle ${\theta }_{\mathrm{MR}}$. It was found that the angle ${\theta }_{\mathrm{MR}}$ shifts due to EB to higher or lower values, depending on the sign of the unidirectional anisotropy EB specified by the field cooling protocol. Furthermore, the EB effect also influences the temperature of spontaneous switching $T_{\mathrm{sw}}$ from negative to positive magnetization, which occurs below $T_{\mathrm{comp}}=33\mathrm{K}$, at a constant field H. Namely, $T_{\mathrm{sw}}$ decreases in the presence of positive unidirectional anisotropy and increases in the presence of negative anisotropy, indicating that the energy required to flip M depends on the sign of EB. The features discovered allow us to more fully comprehend the nature of the EB effect in coupled FM layers of compensated ${\mathrm{Ni}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$ ferrimagnet.

Rare-earth-ion-doped crystals have emerged as a promising platform for quantum technologies, owing to their narrow telecom-band optical emission, long spin memory, and compatibility with silicon integrated photonic architectures. However, the realization of scalable quantum devices requires host materials with intrinsically dilute spin environments to suppress decoherence. In this context, erbium $\left({\mathrm{Er}}^{3+}\right)$ doped cerium oxide $\left({\mathrm{CeO}}_2\right)$ is an attractive candidate due to the ultralow concentration of nuclear spins in the host matrix and its compatibility with silicon-based technologies. Here we perform a comprehensive investigation of the coherence properties of an ${\mathrm{Er}}^{3+}$ electron spin qubit in ${\mathrm{CeO}}_2$ via semiclassical as well as detailed cluster correlation expansion simulations. By systematically exploring magnetic field strength, pulse sequences, erbium concentration, and spin temperature, we identify regimes where decoherence from the spin bath is strongly suppressed. Our investigations illustrate that at dilute doping concentration (of the order of 10 ppb) and sub-Kelvin temperatures, operation near clock transitions enables Hahn-echo coherence times to approach the second timescale even at natural isotopic abundance. Importantly, from a practical standpoint, coherence times on the order of $\sim 10$ ms are expected even at liquid helium temperature (about 2 K) for similar concentrations. Moreover, we demonstrate that an additional enhancement can be obtained with conventional multi-$\pi$-pulse dynamical decoupling protocols. Thus, our findings establish ${\mathrm{Er}}^{3+}$ doped ${\mathrm{CeO}}_2$ as a front-runner for realizing spin qubits, quantum memories, and integrated quantum networks.

We study scattering in noninteracting multiterminal quantum wire junctions and show that junctions with dihedral symmetry can exhibit exact zero-reflection points for $N\ge 4$ terminals. By analyzing the scattering matrix, we identify these reflectionless points in the $(E,t^{\prime })$ parameter space, where $E$ is the incident particle energy and $t^{\prime }$ is the junction hopping amplitude. These points exhibit an even-odd dependence on $N$ and converge asymptotically to a common limiting value in the large-$N$ limit. We show that the reflectionless points are characterized by an integer winding number associated with the phase of the reflection amplitude, providing a topological description for their stability against weak on-site disorder. We also consider junctions with broken time-reversal symmetry and find that a magnetic flux can induce additional reflectionless points, including for the $N=3$ case. For a four-terminal junction threaded by a $\pi$-flux, we identify a unique parameter regime in which the reflection amplitude vanishes over the entire energy band. Finally, we discuss experimental signatures through the behavior of Friedel oscillations and examine the stability of these reflectionless points in the presence of weak interactions.

The ultrahigh thermal conductivity of diamond, relevant to the momentum-conserving phonon normal process, holds exceptional promise for advanced micro/nanoscale thermal management. However, efficient modeling of collective phonon transport in complex three-dimensional diamond structures remains challenging. Here, we develop a computational approach for phonon transport in complex diamond micro- and nanostructures by directly solving the phonon Boltzmann transport equation under Callaway's dual relaxation model with first-principles inputs. As a result, our model provides a good quantitative explanation of the thermal conductivities of diamond nanowires, nanobeams and cuboids. Furthermore, we analyze the effects of size and temperature on mode-resolved phonon hydrodynamic transport in natural and isotopically purified diamond microfilms. Finally, the computational framework is applied to porous diamond structures to reveal the effects of neck width and isotopic concentration on nondiffusive phonon transport. This work thus promotes both efficient modeling and physical insights into collective phonon transport in diamond micro- and nanostructures, offering guidance for future experimental measurements and engineering applications.

We study a minimal square-lattice model to examine the relation between momentum-space altermagnetic band geometry and real-space Ising criticality in a coupled fermion-Ising setting. In this model, itinerant fermions with explicitly spin-dependent nearest-neighbor hopping are locally coupled to transverse-field Ising moments. The hopping anisotropy parameter $\alpha$ tunes a $d_{x^2-y^2}$-type altermagnetic band geometry in momentum space, whereas the transverse field $h$ controls real-space antiferromagnetic (AFM) fluctuations and drives the finite-temperature AFM–paramagnetic (PM) transition of the Ising sector. Using determinant quantum Monte Carlo simulations, we show for the representative $\alpha <1$ case studied in detail, $\alpha =0.5$, that the momentum-space band-geometry fingerprint remains visible in the coupled system: the spin-resolved occupation imbalance $n_{\uparrow }\left(\mathbf{k}\right)-n_{\downarrow }\left(\mathbf{k}\right)$ displays a sign-changing structure that follows the underlying spin-split Fermi-surface geometry. By contrast, finite-size scaling of the staggered Ising order parameter indicates that, in the representative comparison between $\alpha =0.5$ and $1.0$, the extracted thermal critical fields remain numerically close on the scale resolved by our finite-size analysis at both $T/t=0.5$ and $0.2$. We argue that this separation reflects the coexistence of a tunable momentum-space altermagnetic band geometry with the nesting structure of the half-filled nearest-neighbor bare kinetic term, which remains an important constraint on the AFM channel in this benchmark. Our results show that, in this controlled setting, a visible altermagnetic band-geometry fingerprint in momentum space need not imply a comparably large shift of the collective real-space magnetic phase boundary.

Exceptional points (EPs) and bound states in the continuum (BICs) are two distinct types of optical singularities that have attracted significant attention for their potential. While EPs are known for their enhanced sensitivity to perturbations, their performance in passive resonator systems is often limited by radiation losses. Conversely, BICs are ideal nonradiating resonances with theoretically infinite Q factors. Here, we explore the interplay between these singularities under time-reversal $\mathcal{T}$ symmetry breaking. We demonstrate the realization of an EP by merging two symmetry-protected BICs corresponding to the $E_2$ representation in a $C_{6v}$-symmetric photonic crystal slab composed of magneto-optical material. By applying an external magnetic field, we introduce an additional mechanism to break $\mathcal{T}$ symmetry, which competes with the intrinsic radiation losses of the open system. Our simulations show that the two BICs coalesce at the $\mathrm{\Gamma }$ point, forming an EP. This work unveils the interaction between different forms of $\mathcal{T}$ symmetry breaking and provides a controllable pathway for dynamically inducing EPs from BICs with infinite Q factors, with potential implications for sensing and tunable photonic devices.

Reduced dimensionality is generally expected to enhance charge density wave (CDW) order. However, experimental results in monolayer ${\mathrm{NbSe}}_2$ appear conflicting: while optical measurements reveal CDW signatures up to 145 K on oxide substrates, scanning tunneling microscopy (STM) studies on graphene-supported monolayers reveal no evidence of CDW above the bulk transition temperature (33 K), suggesting the absence of a dimensionality-driven enhancement. This discrepancy raises fundamental questions about the nature of CDW in the monolayer limit and the role of substrate interactions. Here, we use STM to investigate monolayer ${\mathrm{NbSe}}_2$ on graphene and directly resolve local $3\times 3$ CDW modulation persisting to 77 K, well above the bulk transition. Distinct from coherent long-range CDW orders, the observed CDW modulation is short-ranged, nucleates preferentially at atomic-scale defects, and exhibits defect-selective pinning analogous to bulk behavior. Compared to bulk ${\mathrm{NbSe}}_2$, the CDW correlation length is significantly enhanced but remains spatially inhomogeneous, varying across flakes with different twist angles relative to the graphene substrate. These results indicate that reduced dimensionality indeed promotes CDW correlation in monolayer ${\mathrm{NbSe}}_2$. However, the stronger dielectric screening provided by graphene, compared to insulating oxides, likely contributes to the suppression of phase-coherent, quasi-long-range CDW order. Overall, our findings highlight the roles of dimensionality and substrate in determining CDW behavior in the monolayer limit.

We theoretically investigate a tunable and suppressible non-Hermitian skin effect (NHSE) in lattice systems governed by non-Abelian SU(2) gauge fields, Zeeman field, and spin-dependent gain and loss. We show that non-Abelian hopping alone can generate a parity-dependent discrepancy between open- and periodic-boundary spectra, even in the absence of the NHSE. A uniform Zeeman field or spin-dependent gain and loss activates a direction-selective NHSEs, whereas their coexistence gives rise to a bipolar NHSE. Strikingly, we find that a spatially nonuniform Zeeman field generically suppresses the NHSE and drives boundary-accumulated skin modes back into the bulk, irrespective of the modulation profile. This suppression originates from the competition between skin-induced boundary accumulation and gradient-induced Wannier–Stark-like localization. Our results reveal versatile and experimentally accessible mechanisms for engineering, tuning, and suppressing non-Hermitian boundary localization, and provide a promising route toward exploring non-Abelian non-Hermitian physics in platforms such as electrical circuits.

Whether an antiferromagnet can host direction-robust spin-polarized transport without a conventional spin-selective band gap remains a central challenge in antiferromagnetic spintronics. Here, we establish a gapless, direction-robust spin-filtering mechanism in a compensated two-dimensional altermagnetic Weyl semimetal that requires neither a spin-selective band gap nor a large velocity contrast between spin projections. Using Janus monolayer ${\mathrm{Ta}}_2\mathrm{TeSeO}$ as a realistic platform, we combine symmetry analysis with first-principles calculations, full-Brillouin-zone Wannier interpolation, and semiclassical transport. Rotating the Néel vector removes a unitary-mirror constraint and shifts one Weyl-cone pair away from its parent high-symmetry line. For an in-plane Néel vector, the residual $C_{2z}\mathcal{T}$ symmetry forbids the independent ${\sigma }_y$ mass that would open a local gap, allowing the reconstructed cones to shift in momentum while remaining gapless. Breaking unitary $C_{2z}$ simultaneously lifts the energy equivalence of the remaining mirror-pinned Weyl cones. The resulting coexistence of a metallic spin-projected manifold and a low-density-of-states (DOS) Weyl-derived manifold produces a predominantly DOS-driven conductance imbalance. At charge neutrality and 20 K, the longitudinal conductivity polarization for $\mathbf{n}\parallel x$ remains positive for every in-plane current direction and ranges from $76.4\%$ to $82.0\%$. The symmetry-related in-plane Néel orientations provide two distinct spin-filtering states. This Néel-vector-driven symmetry mechanism provides a general route to direction-robust gapless spin filtering in compensated altermagnets.

The exploitation of crystal-lattice symmetries to engineer angular momentum transfer between structured light and magnons marks a novel frontier for optomagnonic research. In Brillouin light scattering, when focused light propagates parallel to an external magnetic field and interacts with ferromagnetic uniform magnons, only optical vortex scattering is expected to be permitted. Due to the combined effects of magneto-optical coupling and optical spin-orbit interaction, the transfer of magnon spin angular momentum to photon orbital angular momentum allows for this distinctive scattering phenomenon. Here, we experimentally demonstrate that, for a specific ferromagnetic crystal orientation, Gaussian beam scattering coexists with the optical vortex scattering, contrary to conventional expectations based on angular momentum conservation between magnons and photons. We show that the crystal lattice, via the rotational umklapp process, provides the missing angular momentum required for the Gaussian beam scattering. Furthermore, our theoretical model predicts a focus-dependent efficiency crossover between Gaussian beam and optical vortex scattering as the numerical aperture increases.

We present a simplified formalism for predicting infrared optical constants from first-principles calculations by considering resonant interaction between infrared photons and the crystal lattice. Addressing limitations of the widely used four-parameter semiquantum Lorentz model, the proposed approach bridges the gap between the harmonic three-parameter model and the rigorous many-body Green's-function-based approaches. By incorporating essential anharmonic effects including four-phonon scattering and phonon renormalization, the model provides an efficient and accurate alternative while maintaining low computational cost. The frequency-dependent refractive indices of MgO and rutile ${\mathrm{TiO}}_2$ are computed and compared with experimental data, demonstrating good quantitative agreement. The framework offers a practical approach for predicting optical properties of materials across a wide range of materials.

Triangular-lattice systems host a variety of ground states, ranging from quantum spin liquids to magnetically ordered phases, the latter of which can exhibit a sequence of magnetic phase transitions under applied magnetic fields. Here, we report magnetic and thermodynamic measurements, combined with powder and single-crystal neutron diffraction, on a high-spin, nearly isotropic ${\mathrm{Mn}}^{2+}$ triangular-lattice system ${\mathrm{K}}_2\mathrm{Mn}{\left({\mathrm{SeO}}_3\right)}_2$. The compound undergoes long-range magnetic ordering below $T_{\mathrm{N}}\sim 4\mathrm{K}$ in zero field. Contrary to expectations for an ideal Heisenberg system, an up-down-zero (UD0) magnetic structure is favored down to the lowest temperature (0.05 K), rather than the commonly expected Y-type structure. This UD0 state is, however, highly sensitive to external magnetic fields. For fields applied along the $c$ axis, it is readily destabilized and replaced by the Y-type structure, followed by an up-up-down (UUD) phase corresponding to the 1/3 magnetization plateau. In contrast, when the field is applied within the triangular plane, the system evolves into a canted Y state at a higher critical field. These results reveal that weak anisotropy, though small in magnitude, exerts a strongly orientation-dependent influence, playing a key role in selecting the field-induced phases in this frustrated magnet.

Ab initio calculations and angle-resolved photoemission experiments demonstrate that the bulk and surface electronic structure of Weyl semimetal candidate ${\mathrm{MoTe}}_2$ changes significantly by tuning the chemical potential by less than 0.4 eV. Calculations indicate that several Lifshitz transitions can occur among multiple electron and hole Fermi pockets of differing orbital character. Experiments show that 18% Nb-Mo substitution reduces the occupation of bulk and (001) surface bands, effectively producing a quasirigid band shift of $\approx 0.2\mathrm{eV}$. Orbital character and dimensionality of the bulk bands is examined by soft-x-ray angle-resolved photoemission with control of the excitation light polarization. Deposition of alkali-metal atoms on the surface produces electron binding energy shifts consistent with increased band filling. The results indicate that multiple regimes of electronic properties can be easily accessed in this versatile, layered material.

We investigate the dynamics of the Hubbard–Holstein model after a sudden switch-on of the electron–phonon interaction within nonequilibrium dynamical mean-field theory (DMFT). Using the self-consistent Migdal approximation for the electron–phonon coupling together with self-consistent second-order perturbation theory for the electron–electron interaction, we show that the relaxation dynamics exhibits a crossover between electron-dominated and phonon-dominated regimes, extending to finite Hubbard interaction the scenario previously identified in the Holstein model. To investigate the emergence of a self-consistent thermalization of the electron–phonon system, we analyze the DMFT dynamics by enforcing the self-consistency after each global time-evolution, rather than at every time step. This allows us to follow the build-up of a self-consistent thermal bath. In the plane of real time and DMFT iteration number, thermalization is marked by a sharp propagating front. This front appears in electronic observables already for weak quenches within the simulated time window, whereas the phononic sector exhibits a visible front only at sufficiently strong coupling. Thus, at weak coupling the local dispersionless phonons show a delayed onset of front formation, while near and beyond the crossover the front develops on comparable timescales in both the electronic and phononic sectors. Whenever both fronts are resolved, they propagate with the same velocity, showing that thermalization spreads coherently through the coupled electron–phonon system.

A recent study showed that, in spin-1 chiral fermion systems composed of two linearly dispersing bands and one trivial band, impurity scattering produces an asymmetric cusp in the spectral conductivity $\sigma \left(\epsilon \right)$. We demonstrate that this asymmetric cusp markedly enhances the electronic thermoelectric response. Using linear-response theory within the self-consistent Born approximation, we find low-temperature enhancements in both the Seebeck coefficient and the electronic figure of merit. Increasing the curvature of the trivial band further strengthens this cusp-induced enhancement, even though the corresponding density of states becomes smoother. To clarify this mechanism, we introduce a minimal cusp model for $\sigma \left(\epsilon \right)$ and show that the enhancement is most pronounced when the cusp is sharp and strongly asymmetric and when the spectral conductivity at the cusp energy is small.

We consider possible encodings of a qubit within the space spanned by the $p$-orbital manifold of an anisotropic silicon quantum dot, including the effects of alloy disorder. We find two types of avoided crossing in this manifold: one due to alloy disorder and one due to an out-of-plane magnetic field. Defining a qubit at such an avoided crossing offers prolonged coherence time and complete single-qubit control using baseband electrical control of the dot anisotropy. Using a phenomenological two-level fluctuator dipole noise model, we estimate an average dephasing time of $T_2^{*}\approx 10\mu \text{s}$ and a quality factor of $Q\sim {10}^4$. At the avoided crossing due to magnetic field, the qubit is a minor adaptation of the previously proposed $pO$ qubit, spanned by the ground and excited $p$ orbitals in the ground valley. However, at the avoided crossing due to alloy disorder, the qubit is qualitatively different, spanning the ground $p$ orbital in the excited valley and the excited $p$ orbital in the ground valley, and we dub it the $pOv$ qubit. While the charge noise robustness is similar for $pO$ and $pOv$ qubits, we find that $pOv$ qubits have desirable interaction properties. Namely, in contrast to the always-on transverse Coulomb coupling of $pO$ qubits, we show that transverse coupling between $pOv$ qubits is negligible, leaving only a longitudinal two-qubit interaction that can be tuned by modulating the orientation of adjacent elliptical dots.

Density functional theory (DFT) is widely used to model twisted bilayers, but the accuracy of the local stacking energetics underlying such models remains uncertain. Here, we benchmark the local-registry landscape of bilayer InSe using diffusion quantum Monte Carlo (DMC). DFT predicts that AB, AAr, and ABr stackings, which share the same interfacial Se registry, are nearly degenerate within 1.5 meV/f.u. DMC instead separates these stackings by 8(5) and 41(4) meV/f.u., while the energy difference between the most stable and least stable registries reaches 60(7) meV/f.u. The corresponding DMC density redistribution is also strongly registry-dependent, whereas the vdW-DF and rev-vdW-DF2 density responses examined here remain much more similar among these stackings. These large energy separations show that the stacking energetics are not determined by the interfacial atomic motif alone but depend on the full registry and its associated many-body electronic response. For bilayer InSe, the tested DFT approximations substantially underestimate the local stacking-energy corrugation, highlighting the need to validate the local-registry energy scale used in moiré modeling because errors in this scale can propagate into predictions of structural relaxation, domain formation, and electronic reconstruction.

We investigate the influence of Co intercalation and altermagnetic order on the lattice dynamics of the layered compound ${\mathrm{Co}}_{1/4}{\mathrm{NbSe}}_2$. Polarization-resolved Raman spectroscopy, supported by density-functional theory, enables identification of six Raman-active phonons. Co intercalation drives a substantial reconstruction of the vibrational spectrum through zone folding of ${\mathrm{NbSe}}_2$ phonons, producing hybridized modes with mixed zone-center and zone-boundary character. Despite this, Co atoms do not participate in any Raman-active modes by symmetry, which is in marked contrast to related $1/3$ compounds where intercalant modes do contribute to the Raman spectrum. Temperature-dependent Raman measurements across the altermagnetic transition show no discontinuities, which is consistent with magnetic fluctuations persisting above the Néel temperature. However, we find evidence for spin-phonon coupling in $A_{1g}$ symmetry modes owing to their out-of-plane Se displacements. Our work demonstrates the substantial impact of intercalation on the vibrational properties of transition metal dichalcogenides and the presence of spin-phonon interactions in a newly discovered altermagnetic material.

The magnetic toroidal monopole, a time-reversal-odd scalar, has attracted attention through its characteristic responses, such as electric-field-induced nonreciprocal directional dichroism observed in ${\mathrm{Co}}_2{\mathrm{SiO}}_4$. However, its evaluation in crystalline solids remains unresolved, as it cannot be defined within conventional multipole expansions or thermodynamic formulations. In this paper, we propose a theoretical framework to evaluate the magnetic toroidal monopole in periodic crystals based on the response of relativistic electric polarization to a magnetic field gradient. By incorporating the magnetic-field-gradient correction to the relativistic polarization, we derive an explicit expression for the magnetic toroidal monopole beyond symmetry arguments. The resulting expression is formulated in terms of geometric quantities such as Berry curvatures and orbital magnetic moment defined in an extended parameter space spanning momentum, magnetic field, and electric field. We further perform model calculations for an antiferromagnetic system hosting a magnetic toroidal monopole and confirm that the proposed quantity is finite. These results provide a practical route to characterize magnetic toroidal monopoles in crystalline solids and clarify their quantum geometric nature.