${\mathrm{BaZrO}}_3$ is known to be one of the very few perovskites that retains its ideal centrosymmetric $Pm\overline{3}m$ structure down to 2 K, despite showing intense Raman modes over the entire temperature range. Arguably, the Raman signals are either due to localized octahedral tilts/phase transition or due to second-order Raman-active phonon modes. By combining ab initio atomistic thermodynamics within the framework of density functional theory and experimental observations, we show that local disorder mediated by charged oxygen vacancy lifts this inherent cubic symmetry. The vacancies become more stable at real growth conditions viz. higher temperatures and lower oxygen partial pressures and a pseudo $a^0{a}^0{c}^{-}$ tetragonal-type structure develops in the short-range vicinity. However, the average long-range structure of ${\mathrm{BaZrO}}_3$, as determined by Bragg's diffraction, remains cubic. The defect states manifest as a shallow donor level below the conduction band minima in the electronic density of states and are responsible for a secondary transition at $\approx 3.86$ eV in the UV-vis spectra. Our O $1s$ x-ray photoelectron spectra and radial distribution function analysis from extended x-ray absorption fine-structure, further validate the presence of stable anion vacancies in this perovskite. Thus, the key to the mystery of the intense Raman signal in this otherwise long-range-ordered “centrosymmetric” perovskite is related to local structural disorder created by lattice oxygen vacancies.

Exciton-polariton condensates are explored using coupled driven-dissipative Gross-Pitaevskii equations under the Keeling-Berloff approximation in the presence of photonic spin-orbit coupling (SOC). An extended dissipative variational approximation method uses analytical expressions for coupled vortex ring solitons as minimizing functions of a generalized Lagrangian framework. The results reveal that the amplitudes of the vortices are linked to losses and gain, while the topological charges of the vortices are tied to the width of the vortex rings. Numerically, using the proposed solution as initial conditions, the emergence of even-numbered multilobed rings takes place as the topological charges of the seeded vortices are increased. The analytical results are then confronted by a detailed numerical analysis that further elucidates the rich vortex dynamics inferred by SOC and other combinations of various system parameters.

The spin diffusion barrier is a well-known phenomenon that limits the efficiency of dynamic nuclear polarization (DNP) in enhancing nuclear magnetic resonance (NMR) sensitivity. In this study, we introduce a solution to overcome the nuclear spin diffusion barrier in DNP under magic-angle spinning (MAS) at high magnetic fields. Our finding highlights the crucial role of electron-electron ($e\text{−}e$) coupling interactions, specifically electron spin flip-flop processes, in accelerating nuclear spin diffusion in DNP polarizing agents with strong hyperfine couplings, which are essential for effective DNP. Through a combination of theoretical and numerical analyses on a four-spin system model, we demonstrate that $e\text{−}e$ interactions can overcome the spin diffusion barrier via a concurrent four-spin flip-flop mechanism, termed the electron-assisted spin diffusion (EASD) mechanism. This mechanism operates within the $\alpha \beta$ and $\beta \alpha$ manifolds of coupled electron spins under specific degeneracy conditions. Experimental DNP buildup profiles, measured at 14.1 T under MAS, corroborate the EASD model, revealing that radical systems with stronger inter- or intramolecular $e\text{−}e$ coupling achieve significantly faster DNP buildup. EASD insights open avenues for the development of advanced DNP transfer strategies under MAS, elucidating how polarization can be diffused out of polarizing agents that would otherwise remain trapped. This paper not only advances our understanding of DNP transfer dynamics but also offers practical guidelines for designing next-generation DNP polarizing agents. In particular, it informs the optimization of bis-nitroxides with tailored $e\text{−}e$ coupling for cross-effect DNP, as well as the development of narrow-line radicals for solid-effect DNP, both of which can leverage the EASD mechanism for enhanced performance in hyperpolarized NMR applications. EASD also paves the way for improving DNP of materials with intrinsic or doped paramagnetic centers and advancing quantum information science, particularly quantum sensing, by leveraging coupled electron spins.

We investigate the magnetic ground state of single crystals of the ruthenium-dimer-based hexagonal perovskite ${\mathrm{Ba}}_3{\mathrm{ZnRu}}_2{\mathrm{O}}_9$ using magnetic susceptibility and resonant inelastic x-ray scattering (RIXS) measurements. While a previous study on powder samples exhibited intriguing magnetic behavior, questions about whether the spin state within a ${\mathrm{Ru}}_2{\mathrm{O}}_9$ dimer is a conventional $S=3/2$ dimer or an orbital-selective $S=1$ dimer were raised. The RIXS spectra reveal magnetic excitations from Hund's intraionic multiplet and intradimer spin-triplet transitions. The observed transition energies of the Hund's intraionic multiplets align with the $S=3/2$ ground state, contrasting with the theoretically proposed orbital-selective $S=1$ dimer state. High-temperature magnetic susceptibility analysis confirms the realization of the spin $S=3/2$ dimer state, and the extracted intradimer coupling is consistent with the spin-triplet transition energy observed in the RIXS spectra. These results highlights the ability of “spectroscopic fingerprinting” by RIXS to determine the magnetic ground states of complex materials.

Recently, Patel et al. Science 381, 790 (2023) introduced a higher-dimensional version of the SYK model with random coupling in a Yukawa interaction to explain the linear-$T$ resistivity. We test the universality of this mechanism by replacing the scalar field with a vector field in various dimensions. We find that it applies to vector and scalar interactions, although the details differ significantly. However, this mechanism for linear-$T$ resistivity works only in $(2+1)$ dimensions and not in higher dimensions, regardless of the interaction type. Based on these results, we explore the role of spatial random disorder and provide a simple explanation of how such random scattering transforms a Fermi liquid into a strange metal by modifying the self-energies of the involved bosons and fermions.

Topologically protected edge states are the highlight feature of an interface between nonequivalent insulators. The robustness and sensitivity of these states to local single-particle perturbations is well understood, while their stability in the presence of various types of two-particle interactions remains unclear. To add to previous discussions of the Hubbard and unscreened Coulomb interactions, we address this problem from the point of view of infinite-range Hatsugai-Kohmoto interaction. Based on our numerical results for two models of Chern insulators, the Kane-Mele and spinful Haldane model, on a ribbon geometry with zigzag edges, we argue that any finite interaction strength $U$ is sufficient to open a charge gap in the spectrum of either Chern insulator. We explain the differences between the two cases and present how their edge states phase out as the system enters the strongly correlated phase. We show that the closing of the many-body gap in periodic variants of these models can be connected to the onset of hybridization between the edge and bulk modes in finite geometries. Providing an example of the bulk-boundary correspondence in systems where there is a topological phase transition without closing of the spectral gap.

We present a systematic study of the energy levels of twisted bilayer graphene (tBLG) quantum dots (QDs) and rings (QRs) under an external perpendicular magnetic field. The confinement structures are modeled by a circular dot-like- and ring-like-shaped site-dependent staggered potential, which prevents edge effects and leads to an energy gap between the electron and hole states. Results are obtained within the tight-binding model with interlayer hopping parameters defined by the Slater-Koster form for different interlayer twist angles $\theta$. Our findings show that, for $\theta$ around $0^{\circ }$ or ${60}^{\circ }$, the energy spectra exhibit features resulting from the interplay between characteristics of the AA and AB/BA stacking orders that compose the moiré pattern of such tBLG, while the low-energy levels are shown to be nearly independent on the rotation angle for ${10}^{\circ }\lesssim \theta \lesssim {50}^{\circ }$. In the absence of a magnetic field, the energy levels of the QR scale with its width $W$ according to a power law $W^{-\alpha }$, whose exponent $1⪅\alpha ⪅2$ depends on the twist angle. Most interestingly, the lowest-energy states of tBLG QRs oscillate as a function of its average radius, with the oscillation period matching half of the moiré period. In the presence of an intense magnetic field, the lowest-energy levels for the tBLG QDs and QRs match almost perfectly, regardless of whether the external radius of the quantum confinement structure is smaller or on the order of the moiré period, which is due to the interplay of the trigonal warping effect and the preferential localization of the eigenstates. Our results reveal relevant information about the moiré pattern in tBLG and its role in charge particle confinement.

We propose a realistic semiconductor system containing bound states in the continuum (BICs) which allows for a practical realization. By varying the confinement strength of excitons in cuprous oxide quantum wells, we show that long-lived Rydberg states of the confined electron-hole pairs appear in the continuum background. The accuracy of calculations of the linewidths based on the coupled-channel Schrödinger equation with three channels and only few basis states is confirmed by a numerically exact solution employing a B-spline basis and the complex-coordinate-rotation method. We argue that finite-sized cuprous oxide crystals, due to their large exciton binding energies, are a convenient platform for experimental identification of BICs.

The recently discovered noncentrosymmetric (NCS) rhenium-based superconductor, NbReSi, has been demonstrated to possess an enhanced upper critical field ($H_{\mathrm{c}2}$) that approaches the Pauli-Clogston limit. In the regime of weak coupling, a high $H_{\mathrm{c}2}$ (0) value offers an optimal framework for investigating the properties of vortex matter and its phase diagram in the parity-violated background. In this work, we present a comprehensive study on the resistivity, magnetization, and magnetoresistance behavior of NbReSi and the V-substituted ${\mathrm{Nb}}_{1\text{−}x}{\mathrm{V}}_x\mathrm{ReSi}$ to derive their superconducting properties, including the lower and upper critical field, critical current density, flux pinning mechanisms, and the vortex phase diagram. The temperature dependence of $H_{\mathrm{c}2}$ establishes the multiband characteristics with repulsive intraband and dominant interband coupling parameters, which is consistent with the $s$-wave pairing symmetry. Analyzing the ρ ($T,H$) behavior around the transition region via thermally activated flux flow of vortices unveiled the field-induced crossover from single vortex pinning to collective pinning. Moreover, such a field-induced transformation from $\delta l$-type pinning to $\delta T_{\mathrm{c}}$-type pinning was also noticed in all these samples. Additionally, the pinning force density and pinning parameters suggest the coexistence of both point core and surface pinning mechanisms based on the framework of the Dew-Hughes model. Most importantly, we find the estimated flux creep rate to be very slow ($\sim {10}^{-3}$) and comparable to other NCS superconductors, while the magnetic relaxation behavior implies a strong bulk vortex pinning in these systems. Finally, combining the parameters and results obtained from the analysis of resistivity, magnetization, and critical current density, we present the global phase diagram of NbReSi, which reflects a narrow vortex fluctuation region, an important characteristic for practical application.

The superconducting diode effect (SDE) is a phenomenon that has attracted significant attention for its potential to establish new Josephson junction (JJ) physics and its application as a rectifier in Josephson circuits. While many experimental platforms for SDE have been reported, only a few demonstrate high efficiency. One promising system is a JJ coherently coupled with a second JJ embedded in a superconducting loop, where SDE is controllable in direction via the phase difference. Here we study SDE in such devices under microwave irradiation. With the appearance of Shapiro steps, SDE efficiency increases as the microwave power is raised. Additionally, a finite voltage difference appears without any direct bias current at high microwave powers. These results are explained by the dynamics of the JJ's washboard potential, which are common to all SDE systems. Our findings enhance the understanding of SDE dynamics and expand their potential applications.

The discovery of silicene has sparked extensive research into silicon-based two-dimensional (2D) materials due to their chemical and processing compatibility with contemporary silicon semiconductor technology. Although silicene exhibits numerous exotic properties, its lack of an intrinsic band gap and high reactivity with atmospheric conditions significantly impede its practical applications. Here, we successfully synthesized a 2D hexagonal $\mathrm{S}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$ ($H-\mathrm{S}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3, R\text{−}3m$) by applying pressure as an external stimulus. $H-\mathrm{S}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$ collapses to a higher coordinated three-dimensional phase ($C2/m$) at ∼6.5 GPa, accompanied by a semiconductor-metal transition, and then endures a displacive transition to another monoclinic phase ($C2/c$) at ∼13.9 GPa. The compressed $H-\mathrm{S}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$ exhibits highly tunable optical band gap characteristics, narrowing from 1.8 to 0.9 eV up to 31.8 GPa. Notably, the metallized $\mathrm{S}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$ retains significant optical band gap characteristics, in contrary to that in conventional metal systems. Our findings provide a viable strategy for designing silicon-based 2D polymorphs with unique optoelectronic properties through pressure modulation.

Based on the extended Su-Schrieffer-Heeger model and the nonadiabatic-dynamics method, we investigate the field-dependent polaron dynamics as well as its effect on the organic spin Hall effect (SHE) in three coupled organic chains with spin-orbit coupling. The results demonstrate a steplike transition of the polaron velocity with the increase of field strength, caused by the decoupling of the acoustic and optical deformations, and a dissociation of the polaron at stronger fields. The critical fields for decoupling and dissociation are much smaller than those reported for single-chain polymers due to the interchain coupling. Importantly, a distinct field dependence of the SHE and the charge-spin conversion efficiency is revealed. For subsonic polaron transport, the amplitude of the SHE increases with the field strength, whereas it shows a nonmonotonic field dependence in the supersonic regime. A large SHE is obtained even when the polaron is dissociated at strong fields. In contrast, the largest charge-spin conversion efficiency is found for the weak fields in the subsonic regime, caused by the small polaron velocity. The mechanism is analyzed in terms of the change of lattice distortions including both breathers and polaron distortions, which modulate the skew scattering from lattice distortions as well as the interchain spin transfer, and thus modify the SHE. In addition, the oscillations of the SHE in the various regimes are investigated by spectral analysis. This work sheds light on the microscopic mechanism of the electric-field effect on the SHE in organic polymers, which provides guidance for the pursuit of a strong SHE or a large charge-spin conversion efficiency via field manipulation.

Spin-phonon coupling is essential for explaining phenomena such as magnetoresistance, magnetoelectricity, magnetothermal transport, and structural and magnetic phase transitions. However, quantifying the spin-phonon coupling has remained challenging. Generally, in the absence of other phonon and magnon scattering mechanisms, deviation from expected trends in measured responses are often attributed to spin-phonon coupling. Here, we report mode-specific, orbital-resolved spin-phonon coupling-driven renormalization of both interatomic force constants and spin exchange interactions $J$ in archetypal antiferromagnet NiO that allow us to determine the changes in phonon and magnon energy, group velocity, and phonon-magnon scattering amplitude $\left(V_{\mathrm{ph}-\mathrm{mag}}\right)$. A comparison of the renormalized phonon energy of longitudinal and transverse optical modes with inelastic neutron scattering measurements shows reasonable agreement. However, in contrast to the widely used approximation of a linear dependence of $J$ on atomic perturbation, the renormalized $J$ is found to vary quadratically. Notably, the mode-specific contribution to $V_{\mathrm{ph}-\mathrm{mag}}$ is found to be large and comparable to the phonon-phonon scattering amplitude $V_{\mathrm{ph}-\mathrm{ph}}$. Our results provide an opportunity to quantify and decipher spin-phonon coupling and potentially control it using external stimuli, such as electric and magnetic fields to engineer spintronics devices.

We investigate the signature of quantum criticality in the long-time stationary state of the long-range Kitaev chain by performing various quench protocols. In this model, the pairing interaction decays with distance according to a power law with exponent $\alpha$. Using quantum information-theoretic measures, such as mutual information and logarithmic negativity, we show that irrespective of the values of $\alpha$, critical-to-critical quench displays quantum criticality even in the stationary state. Remarkably, in the presence of long-range pairing interactions, where fermionic correlators decay algebraically even at noncritical points, the signature of quantum criticality persists in the stationary state. Furthermore, the effective central charge, calculated from both mutual information and logarithmic negativity of the stationary state following a critical-to-critical quench, agrees with the central charge of the corresponding ground states for both $\alpha =0$ and $\alpha =2$. Therefore, information of the universality class can be inferred from the stationary state.

The origin of the $A$-type antiferromagnetic ordering, characterized by ferromagnetic layers coupling antiferromagnetically, in the prototype semiconductor altermagnet $\alpha$-MnTe, has been a topic of ongoing debate. Experimentally, $\alpha$-MnTe exhibits an in-plane ferromagnetic exchange interaction, whereas previous ab initio calculations predicted an antiferromagnetic interaction. In this paper, we resolve this discrepancy by considering an expanded set of magnetic configurations, which reveals a ferromagnetic in-plane exchange interaction in agreement with experimental findings. Additionally, we demonstrate that the tenth nearest-neighbor exchange interaction is directionally dependent, inducing a nonrelativistic chiral splitting in the magnon bands, as recently observed experimentally. We further show that applying a compressive strain may significantly enhance both nonrelativistic spin and chiral magnon splittings. The strain can also change the sign of the in-plane exchange interaction. Computing magnetic susceptibility, we show that strain enhances the Néel temperature, significantly. Our results highlight the critical importance of convergence in the number of magnetic configurations for spin interactions in antiferromagnetic materials.

We investigate the relativistic SO(2)- and SO(3)-invariant Gross-Neveu-Yukawa field theories for real, rank-two, symmetric, traceless tensor order parameters coupled to $N_{\text{f}}$ flavors of two-component Dirac fermions. These field theories arise as an effective description of fractionalized spin-orbital liquids. The two theories are the simplest and special cases of the more general class of field theories with $\mathrm{SO}(N$) symmetric tensor order parameter coupled to Dirac fermions, in which the symmetry is low enough to allow only one and not the usual two quartic self-interaction terms. Using a two-loop renormalization group near the upper critical dimension, we demonstrate that the theory exhibits an exotic critical fixed point for $N=3$ and the concomitant continuous phase transition for any value of $N_{\text{f}}$. For $N=2$ the theory is equivalent to the chiral XY model. We discuss the crucial role of the symmetry-allowed sextic self-interactions in the selection of the ground state configuration in the case of SO(3). The universal quantities such as the anomalous dimensions of order parameters and fermions, the correlation length exponent, and the mass gap ratio between order parameter and fermion masses are computed up to ${\epsilon }^2$ order.

We study the neutral excitations in the bulk of the fractional quantum Hall (FQH) fluids generated by acting with the Girvin-MacDonald-Platzman (GMP) density operator on the uniform ground state. Creating these density modulations atop the ground state costs energy, since any density fluctuation in the FQH system has a gap stemming from underlying interparticle interactions. We calculate the GMP density-mode dispersion for various bosonic and fermionic FQH states on the Haldane sphere using the ground-state static structure factor computed on the same geometry. Previously, this computation was carried out on the plane. Analogous to the GMP algebra of the lowest Landau level (LLL) projected density operators in the plane, we derive the algebra for the LLL-projected density operators on the sphere, which facilitates the computation of the density-mode dispersion. Contrary to previous results on the plane, we find that, in the long-wavelength limit, the GMP mode accurately describes the dynamics of the primary Jain states.

Using an efficient variational exact diagonalization method, we computed the electron removal spectral function within the framework of the Holstein-Hubbard model containing two electrons with opposite spins coupled to dispersive quantum optical phonons. We focus on how the interplay between the electron-phonon coupling and on-site Coulomb repulsion shapes the spectral function, a quantity directly relevant to angle-resolved photoemission spectroscopy (ARPES). Tuning the strengths of the electron-phonon coupling and the Hubbard interaction allows us to examine the evolution of the spectral properties of the system as it crosses over from a bound bipolaron to separate polarons. We find that phonon dispersion plays an important role: It substantially redistributes the spectral weight across the low- and the high-energy features and controls the emergence and the structure of distinct multiphonon continua. In regimes of strong electron-phonon coupling and low Coulomb repulsion, the spectral function exhibits a broad distribution reflecting bound bipolaronic states. Increasing the Coulomb repulsion weakens bipolaron binding, shifts the primary quasiparticle peak closer to polaronic behavior and confines the spectral weight near the Brillouin-zone center. Our results provide a framework for interpreting ARPES measurements in systems where phonon–mediated attraction competes with direct Coulomb repulsion.

The superconducting (SC) state ($T_{\mathrm{c}}=0.3\mathrm{K}$) of the heavy-fermion compound ${\mathrm{CeRh}}_2{\mathrm{As}}_2$, which undergoes an unusual field-induced transition to another high-field SC state, emerges from an unknown ordered state below $T_{\mathrm{o}}=0.55\mathrm{K}$. While an electronic multipolar order of itinerant $\mathrm{Ce}\text{−}4f$ states was proposed to account for the $T_{\mathrm{o}}$ phase, the exact order parameter has not been known to date. Here, we report on muon spin relaxation ($\mu \mathrm{SR}$) studies of the magnetic and SC properties in ${\mathrm{CeRh}}_2{\mathrm{As}}_2$ single crystals at low temperatures. We reveal a magnetic origin of the $T_{\mathrm{o}}$ order by identifying a spontaneous internal field below $T_{\mathrm{o}}=0.55\mathrm{K}$. Furthermore, we find evidence of a microscopic coexistence of local magnetism with bulk superconductivity. Our results suggest the possibility that the $T_{\mathrm{o}}$ phase involves both dipole and higher-order $\mathrm{Ce}\text{−}4f$ moments to account for the unusual shape of the $T_{\mathrm{o}}\left(B\right)$ phase boundary.