Migdal-Eliashberg theory of boson-mediated superconductivity contains a $\sqrt{\lambda }$ divergence in the critical temperature $T_c$ for strong electron-boson coupling $\lambda$. In the conventional Migdal-Eliashberg theory, the strong-coupling regime can be accessed only in the limit that ${\lambda }_E=\lambda {\omega }_D/{\varepsilon }_F\ll 1$, where ${\omega }_D$ is the Debye frequency and ${\varepsilon }_F$ is the Fermi energy. Here we go beyond this restriction in the context of the two-dimensional Yukawa-SYK (Y-SYK) model, which is solvable for arbitrary values of ${\lambda }_E$. We find that $T_c\approx 0.18{\omega }_D\sqrt{\lambda }$ for large $\lambda$, provided ${\lambda }_E$ remains small, and crosses over to a universal value of $T_c\approx 0.04{\varepsilon }_F$ for large ${\lambda }_E$. The saturation of $T_c$ is due to a self-consistent account of the boson dynamics for large ${\lambda }_E$ and remains valid provided the vertex corrections are negligible. Depending on the value of $\lambda$, this self-consistent approach leads to pairing that describes multiple classes of quantum critical electronic systems. These results demonstrate how the $\sqrt{\lambda }$ growth of $T_c$ in Migdal-Eliashberg theory saturates to a universal value independent of $\lambda$ and ${\omega }_D$, providing an upper bound on the critical temperature at strong electron-boson coupling.

Flux-closure domain is a prototypical chiral topological structure in ferroelectric square nanodots, distinguished by its characteristic cross-shaped 90° domain walls, in stark contrast to the continuous polarization rotation observed in vortex domain. In recent years, several studies have predominantly focused on the chiral switching of flux-closure domain, the feasibility of domain wall rotation has received comparatively little attention. However, the rotation mechanism of the 90° domain walls in flux-closure domain represents a critical yet unresolved issue in the broader context of domain wall dynamics. In this paper, the polarization configurations in ferroelectric nanodots under varying distinct depolarization fields and crystallographic orientations are systematically investigated via phase-field simulations. The results demonstrate that the formation and transformation of 90° domain walls of flux-closure domain in the nanodots are governed by the interplay between surface screening conditions and crystallographic orientation. Under weak screening (i.e., near open-circuit conditions), the domain walls remain pinned at a fixed 45° relative to the global geometric coordinates as a Saltire-cross shape, irrespective of the variation in crystallographic orientation. With increasing screening, a competitive balance arises between the depolarization field and crystallographic anisotropy, leading to the destabilization of the 90° domain walls and driving the transition from a flux-closure domain state to an ideal vortex domain state. Under strong screening (i.e., near short-circuit conditions), crystallographic orientation becomes dominant, driving the continuous reorientation of domain walls from a Saltire-cross shape to a Greek-cross shape with respect to the global coordinates. This work elucidates the coupled influence of electrostatic boundary screening and crystallographic anisotropy on domain morphology, emphasizing the competitive interplay between electrostatic, gradient, and elastic energy contributions. These findings elucidate the coupled electrostatic and crystallographic mechanisms underlying domain wall rotation, advance the fundamental understanding of topological polar structures, and offer new insights for domain-wall-based nanoelectronic applications.

We study the stationary bipartite entanglement in various integrable and nonintegrable models of monitored fermions evolving along quantum trajectories. We find that, for the integrable cases, the entanglement versus the system size is well fitted, over more than one order of magnitude, by a function interpolating between a linear and a power-law behavior. Up to the sizes we are able to reach, a logarithmic growth of the entanglement can also be captured by the same fit with a very small power-law exponent. For the nonintegrable cases, such as the staggered $t\text{−}V$ and the Sachdev-Ye-Kitaev (SYK) models, the numerical limitations prevent us from spanning different orders of magnitude in the system size. Here we fit the asymptotic entanglement versus the measurement strength with a generalized Lorentzian, finding a very good agreement, and then look at the scaling with the size of the fitting parameters. We find two different behaviors: for the SYK we observe a linear increase with the system size, while for the $t\text{−}V$ model we see the emergence of traces of an entanglement crossover. In the latter models, we study the localization properties in the Hilbert space through the inverse participation ratio, finding an anomalous-delocalization behavior with no relation with the entanglement properties. Finally, we show that our function also fits very well the system-size dependence of the fermionic logarithmic negativity of a quadratic model in a two-leg ladder geometry, with stroboscopic projective measurements.

Unveiling the interplay between spin density wave (SDW) and charge density wave (CDW) orders in correlated electron materials is important in obtaining a comprehensive understanding of their electronic, structural, and magnetic properties. Kagome lattice materials are interesting because their flat electronic bands, Dirac points, and Van Hove singularities can enable a variety of exotic electronic and magnetic phenomena. The kagome metal FeGe (the B35 phase), which exhibits a CDW order deep within an A-type antiferromagnetic (AFM) phase, was found to respond dramatically to postgrowth annealing—with the ability to tune the CDW repeatedly from long-range order to negligible order. Additionally, neutron scattering studies suggest that incommensurate magnetic peaks that onset at $T_{\mathrm{Canting}}=T_{\mathrm{SDW}}\approx 60\mathrm{K}$ in the system arise from a SDW order instead of the AFM double-cone structure. Here we use inelastic neutron scattering to show that two distinct spin excitations exist below $T_{\mathrm{Canting}}$ corresponding to two coexisting magnetic orders in the system in both sets of annealed samples with and without CDW. While CDW order or negligible order can dramatically affect the onset temperature of $T_{\mathrm{Canting}}$ and elastic incommensurate magnetic scattering, its impact on low-energy spin fluctuations is more limited. In both samples, a pair of gapless incommensurate spin excitations arising from the SDW order wave vector coexist with gapped commensurate spin waves from the A-type AFM order across $T_{\mathrm{Canting}}$. The low-energy spin excitations for both samples couple dynamically to the lattice through enhanced magnetic scattering intensity on cooling below $T_{\mathrm{CDW}}$, regardless of the status of the static long-range CDW order. The incommensurate SDW order in the long-range CDW ordered sample also induces a tiny in-plane lattice distortion of the kagome lattice that is absent in the negligible CDW ordered sample, in a way that is different from the previously known SDW and CDW ordering materials.

The current-carrying capability of superconductors, crucial for technological applications, is fundamentally limited by vortex motion, making effective vortex pinning paramount. While intrinsic defects can serve as potent pinning centers, their specific influence on vortex behavior remains poorly understood, particularly at the microscopic level. The stoichiometric superconductor ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$, free from extrinsic disorder, offers an ideal platform to address this knowledge gap. Here, we employ low-temperature magnetic force microscopy to directly correlate distinct intrinsic planar defects—${\mathrm{CaFe}}_2{\mathrm{As}}_2$ and ${\mathrm{KFe}}_2{\mathrm{As}}_2$ intergrowths—with vortex states in ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$. We reveal that ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ intergrowths induce significantly stronger pinning and more disordered vortex configurations than ${\mathrm{KFe}}_2{\mathrm{As}}_2$ intergrowths, with the vortex lattice exhibiting a squarelike symmetry at higher fields. Continuous field-cooling experiments uncover an inhomogeneous pinning landscape associated with ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ intergrowths, corroborated by microscale variations in deduced pinning energy and force per unit length, indicative of nanoscale superfluid density fluctuations. Crucially, correlating these microscopic observations with macroscopic critical current density yields a characteristic disorder length scale ($\sim 177\mathrm{nm}$) consistent with the lateral dimensions of ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ intergrowths. This study establishes a direct microscopic link between specific defect structures and vortex pinning mechanisms in ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$, providing fundamental insights essential for optimizing critical currents through targeted defect engineering in superconductors.

In this work, we use neural quantum states (NQS) to describe the high-dimensional wave functions of electron-phonon coupled systems. We demonstrate that NQS can accurately and systematically learn the underlying physics of such problems through a variational Monte Carlo optimization of the energy with minimal incorporation of physical information, even in highly challenging cases. We assess the ability of our approach across various lattice model examples featuring different types of couplings. The flexibility of our NQS formulation is demonstrated via application to ab initio models parametrized by density functional perturbation theory consisting of electron or hole bands coupled linearly to dispersive phonons. We compute accurate real-frequency spectral properties of electron-phonon via a response formalism based on NQS. Our work establishes a general framework for exploring diverse ground-state and dynamical phenomena arising in electron-phonon systems, including the nonperturbative interplay of correlated electronic and electron-phonon effects in systems ranging from simple lattice models to realistic models of materials parametrized by ab initio calculations.

Understanding thermal and electrical transport in topological materials is essential for advancing their applications in quantum technologies and energy conversion. Herein, we employ first-principles calculations to systematically investigate phonon and charge transport in the prototypical nodal-line semimetal ZrSiS. The results unveil that anharmonic phonon renormalization results in the pronounced softening of heat-carrying phonons and suppressed lattice thermal conductivity (${\kappa }_{\mathrm{L}}$). Crucially, anharmonic effects are found to noticeably weaken Zr-S interactions, triggering avoided-crossing behavior of low-frequency optical phonons. The combination of phonon softening and avoided crossing synergistically reduces phonon group velocities, yielding a 16% suppression in ${\kappa }_{\mathrm{L}}$ along the $c$ axis at room temperature. Contrary to conventional metals, we discover that the lattice contribution to thermal conductivity in ZrSiS is abnormally large, even dominating heat conduction along the $c$ axis. This unusual behavior results in a substantial deviation of the Lorenz number from the Sommerfeld value—exceeding it by up to threefold—thereby challenging the validation of standard Wiedemann-Franz law for thermal conductivity estimation. Moreover, our calculations demonstrate that ZrSiS exhibits exceptional electrical conductivity, attributed to its topological electronic Dirac states that account for both high Fermi velocities and weak electron-phonon coupling. This study provides critical insights into the electrical and thermal transport mechanisms in ZrSiS and highlights the importance of anharmonic effects in the lattice dynamics and thermal transport of metallic materials.

The one-dimensional Fröhlich model describing the motion of a single electron interacting with optical phonons is a paradigmatic model of quantum many-body physics. We predict the existence of an arbitrarily large number of bound excited states in the strong-coupling limit and calculate their excitation energies. Numerical simulations of a discretized model demonstrate the complete amelioration of the projector Monte Carlo sign problem by walker annihilation in an infinite Hilbert space. They reveal the threshold for the occurrence of the first bound excited states at a value of $\alpha \approx 1.73$ for the dimensionless coupling constant. This puts the threshold into the regime of intermediate interaction strength. We find a significant spectral weight and increased phonon number of the bound excited state at threshold.

The spin Seebeck effect is useful for probing the spin correlations and magnetic order in magnetic insulators. Here, we report a strong local spin Seebeck effect (LSSE) in antiferromagnetic ${\mathrm{V}}_2{\mathrm{O}}_3$ thin films. The LSSE response at cryogenic temperatures increases as a function of the external magnetic field until it approaches saturation. The response at a given power and field exhibits a nonmonotonic temperature dependence, with a pronounced peak that shifts toward higher temperatures as the field increases. Furthermore, the magnitude of the LSSE signal decreases consistently with increasing thickness, implying that the bulk SSE dominates any interfacial contribution. This negative correlation between the SSE and the thickness implies that the magnon energy relaxation length in ${\mathrm{V}}_2{\mathrm{O}}_3$ is shorter than the thickness of our thinnest film, 50 nm, consistent with the strong spin-lattice coupling in this material.

The search for effective techniques to manipulate valley logic states is crucial for the current development of valleytronics. In this study, we investigate the external electric and magnetic field controllable anomalous valley Hall effect (AVHE) in multiferroic $\mathrm{VInS}{\mathrm{e}}_3$ monolayer and bilayer using first-principles calculations. $\mathrm{VInS}{\mathrm{e}}_3$ monolayer is asymmetric ferroelectrics in which the polarization can be reversed between the ground phase with V coordinated by Se octahedron (V-O phase) and the metastable phase with tetrahedrally coordinated V (V-T phase). V-O phase possesses ferromagnetism with in-plane easy-axis, while V-T phase adopts out-of-plane easy-axis and intrinsic valley polarization. As a result, the AVHE is observed in V-T phase while absent in V-O phase, corresponding to logic states “ON” and “OFF” of AVHE. The V-T phase bilayer possesses interlayer polarization and interlayer ferromagnetism. The interlayer ferroelectric switching could control AVHE occurrence in top layer or bottom layer (layer-polarized AVHE). Magnetic field could switch the AVHE between spin-up channel and spin-down channel for both monolayer and bilayer. This approach to manipulate the AVHE by electric and magnetic field broadens the potential of multiferroics towards valleytronics and spintronics.

It is known that in contrast to homogeneous ferromagnetism, helical magnetism is compatible with superconductivity and has only a weak suppressive effect on superconducting critical temperature. Despite this fact, it induces $p$-wave triplet superconducting correlations in homogeneous superconducting systems with intrinsic helical magnetism. The combination of these two facts indicates a high potential for the application of such systems in dissipationless spintronics. For this reason, we investigate the proximity effect in atomically thin superconductor/helical (conical) magnet heterostructures (SC/HM). It is shown that in SC/HM heterostructures, the strength of the proximity effect and, in particular, the amplitude of $p$-wave triplet superconductivity and the degree of superconductivity suppression are complex functions of the magnet exchange field and filling factors of the magnet and the superconductor. Furthermore, we demonstrate that $p$-wave correlations ensure transport spin supercurrent flow in the SC/HM heterostructure with conical magnets, and we unveil the physical relationship between the transport spin supercurrent, the degree of magnet conicity, and the internal structure of $p$-wave correlations in momentum space.

N doping has emerged as a promising approach to achieving $p$-type conductivity in $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$, a leading ultrawide band gap semiconductor for high-power electronics. However, its impact on thermal transport remains insufficiently understood. In this study, we systematically investigate the effect of N substitution at three crystallographically distinct O sites, denoted as ${\mathrm{O}}_{\mathrm{I}}, {\mathrm{O}}_{\mathrm{II}}$, and ${\mathrm{O}}_{\mathrm{III}}$, in modifying phonon transport behaviors in $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$. The first-principles calculations combined with atomistic Green's function and Boltzmann transport theory reveal that N doping induces a local ionic-to-covalent bonding transition, characterized by enhanced orbital hybridization, reduced Bader charge transfer, and asymmetric electron delocalization. This transition perturbs local interatomic force constants and enhances lattice anharmonicity, with its intensity strongly dependent on the specific doping site. Moreover, substitution at the ${\mathrm{O}}_{\mathrm{II}}$ site, located within a distorted $\mathrm{G}{\mathrm{a}}_4\mathrm{O}$ tetrahedron, exhibits the most pronounced bonding transition, resulting in stronger phonon localization and enhanced phonon-defect scattering <10 THz. Consequently, the thermal conductivity is significantly reduced, particularly along the $b$ axis, with suppressions of up to 40% at a doping concentration of $1\times {10}^{20}\mathrm{c}{\mathrm{m}}^{-3}$, relevant for ohmic contacts in transistors. In contrast, doping at the ${\mathrm{O}}_{\mathrm{I}}$ site yields minimal disruption to bonding and heat conduction. These findings indicate the importance of doping-site selectivity in modulating phonon transport via local bonding effects and providing insights for thermal management in $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$-based devices.

The optical properties of Cr(III) impurities are well studied and exploited in $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$, which enables ruby lasers. Photoluminescence spectra resembling the famous 1.78 eV emission of Cr in $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$ have been observed in other wide-band-gap host materials, in particular AlN. However, it is unclear whether Cr would produce the same emission in the nitrides as in $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$, especially given the different local environment (tetrahedral vs octahedral coordination). We investigate the electronic and optical properties of transition metal substitutional impurities (Cr, Co, Ni, and Cu) in wurtzite GaN and AlN from first principles, utilizing hybrid density functional theory. We calculate the optical transition energies using a constrained occupation approach that accounts for the multideterminant nature of the excited states. Our results indicate that substitutional Cr(III) is not responsible for the emission near 1.78 eV observed in the nitrides. Among the examined impurities, only substitutional Co(II) produces emission near 1.78 eV. Based on the favorable properties of Co(II) (low formation energy, small Huang-Rhys factor, and an $S=3/2$ ground state) we propose it as a promising single-photon emitter or spin qubit. We additionally assign the PL peak observed near 1.05 eV in GaN and AlN to the ${}^{2}E\to {}^{4}A_2$ transition of the Ni(III) substitution.

Collinear compensated magnets with nonrelativistic spin splitting have attracted considerable interests, such as altermagnets and compensated ferrimagnets, offering new opportunities for spintronics beyond ferromagnets. In this work, we propose that the ${\mathrm{TaN}}_2{\mathrm{H}}_2$ bilayer is a room-temperature sliding multiferroic material with a compensated ferrimagnetic phase. The coexistence of ferroelectricity and compensated ferrimagnetism induces nonrelativistic spin splitting, magneto-optical Kerr effect, and valley polarization, all of which depend on ferroelectric polarization. Notably, we reveal that the valley polarization in the ${\mathrm{TaN}}_2{\mathrm{H}}_2$ bilayer originates from the combined effect of nonrelativistic spin splitting and spin-orbit coupling. The magnitude of the valley polarization in the ${\mathrm{TaN}}_2{\mathrm{H}}_2$ bilayer is directly related to the nonrelativistic spin splitting, rather than the spin-orbit coupling constant. External electrical fields can efficiently modulate the nonrelativistic spin splitting and valley polarization, introducing a phase transition from a valley semiconductor to a half-valley metal. Our findings provide a new perspective for spintronics and valleytronics based on multiferroic materials with compensated ferrimagnetism.

We use a combination of density functional theory calculations and optical modeling to establish that the electric field-induced Kerr rotation in metallic thin films has contributions from both nonequilibrium orbital moment accumulation (arising from the orbital Edelstein effect) and a heretofore overlooked surface Pockels effect. The Kerr rotation associated with orbital accumulation has been studied in previous works and is largely due to the dc electric field-induced change of the electron distribution function. In contrast, the surface Pockels effect is largely due to the dc field-induced change to the wave functions. Both of these contributions arise from the dual mirror symmetry breaking from the surface and from the dc applied field. Our calculations show that the resulting Kerr rotation is due to the dc electric field modification of the optical conductivity within a couple of nanometers from the surface, consistent with the dependence on the local mirror symmetry breaking at the surface. For thin films of Pt, our calculations show that the relative contributions of the orbital Edelstein and surface Pockels effects are comparable and that they have different effects on Kerr rotation of $s$ and $p$ polarized light, ${\theta }_K^s$ and ${\theta }_K^p$. The orbital Edelstein effect yields similar values of ${\theta }_K^s$ and ${\theta }_K^p$, while the surface Pockels effect leads to opposing values of ${\theta }_K^s$ and ${\theta }_K^p$.

Coupling between spins of conduction electrons and localized magnetic ions has potential to give rise to exotic magnetotransport through an impact of magnetism on the electronic structure. Particularly in a conducting frustrated magnet with magnetization plateaus, a gigantic transport response to a magnetic field is expected because of the accessibility to distinct electronic phases coupled to magnetism by magnetic field. Here, we report a study of magnetotransport of ${\mathrm{DyAgSb}}_2$ with magnetization plateaus. We observe longitudinal and transverse resistivities that show a characteristic enhancement in the field-induced transition regions as well as the intermediate field plateau states. We discuss this observation in the context of a two-band Drude model with magnetic-field-dependent mobilities. This result may highlight the significance of (semi)quantitative analysis of the transport coefficients for understanding physics of the charge-spin coupling in metals with magnetic-field-induced transitions.

To identify what types of non-Fermi liquid (NFL) behavior are most likely to occur in cubic metals due to doublet impurities, we derive every cubic symmetry-allowed, NFL, Kondo-type exchange coupling that does not need accidental degeneracy for its realization. We find three distinct types of NFL behavior: two-channel Kondo (2CK) behavior for a non-Kramers doublet impurity coupled to local ${\mathrm{\Gamma }}_8$ conduction electrons; topological Kondo physics for a Kramers doublet impurity and ${\mathrm{\Gamma }}_4$ or ${\mathrm{\Gamma }}_5$ conduction electrons; and lastly, spin-half impurity spin-$\frac{3}{2}$ conduction electron Kondo behavior for a Kramers doublet impurity and ${\mathrm{\Gamma }}_8$ conduction electrons. The first two critical behaviors are not straightforward to realize. In the first case, 2CK physics is not guaranteed, since cubic symmetry does not prevent an effective spatial anisotropy from exceeding the 2CK coupling, which restores a Fermi liquid behavior. In the second case, the topological Kondo interaction is guaranteed to dominate; however, the spin degeneracy of the conduction electrons needs to be lifted, e.g., by a magnetic field—so that they can be represented by ${\mathrm{\Gamma }}_4$ or ${\mathrm{\Gamma }}_5$ triplets—which then also lifts the degeneracy of the Kramers doublet. We find that the spin-half impurity spin-$\frac{3}{2}$ conduction electron, NFL, Kondo behavior has the greatest chance of existing in diluted, cubic compounds. We compute the thermodynamics of the topological Kondo model using the numerical renormalization group, and we discuss the thermodynamics of the spin-half impurity spin-$\frac{3}{2}$ conduction electron Kondo model. We also identify candidate materials where the corresponding NFL behaviors could be observed.

We study the boundary phase diagram and the low-temperature heat and magnetization transport at a $Y$ junction of XX spin chains. Depending on the magnetization axis anisotropy between the magnetic exchange interactions at the junction, the system exhibits two different strong-coupling regimes at low energies/temperatures, hosting a remarkable realization of, respectively, the overscreened (topological) four- and to the two-channel Kondo fixed points, both robust against anisotropies in the chain parameters and/or in the boundary couplings. Using renormalization group arguments combined with boundary conformal field theory methods, we show the instability of the former under any XY-type anisotropy at the junction. Our system allows for using the low-temperature spin and the heat conductances to evidence the fractionalization of the elementary excitations at the four-channel Kondo fixed point by means of the magnetic Wiedemann-Franz law. We caution that the instability under XY anisotropy may hinder the detection of the phenomenology related to the four-channel Kondo effect, therefore requiring careful control in experimental realizations.