The “ferron” excitations of the electric-dipolar order carry energy as well as electric dipoles. Here, we predict a nonlocal ferron-drag effect in a ferroelectric on top of a metallic film: An electric current in the conductor generates a heat current in the ferroelectric by long-range charge-dipole interactions. The nonlocal Peltier and its reciprocal Seebeck effect can be controlled by electric gates and detected thermographically. We predict large effects for van der Waals ferroelectric films on graphene.

We investigate several distinct spectral crossovers amongst various integrable (Poissonian) and quantum-chaotic (Wigner-Dyson) limits of a $1\mathrm{D}$ disordered quantum spin ($\mathrm{S}=1/2$) model, by tuning the relative amplitudes of various Hamiltonian parameters to retain or break relevant unitary and antiunitary symmetries. The spin model consists of an isotropic and deterministic Heisenberg term, a random Ising term, an anisotropic and antisymmetric, but deterministic Dzyaloshinskii-Moriya (DM) term, and a Zeeman coupling to a random, inhomogeneous magnetic field. Since we are specially interested in crossovers involving a Gaussian symplectic ensemble (GSE) limit, we carry out all our calculations with an odd number of lattice sites (spins) that naturally results in eigenspectra with Kramers degeneracies (KD's). The various crossovers (viz., the reentrant Poissonian-to-GSE-to-Poissonian, Poissonian-to-GUE, GSE-to-GUE and the reentrant Poissonian-to-GOE-to-Poissonian crossovers) are investigated via detailed studies of both short-range (nearest-neighbour-spacings distribution, NNSD) and long-range [spectral rigidity ${\mathrm{\Delta }}_3\left(L\right)$ and number variance ${\mathrm{\Sigma }}^2\left(L\right)$ ] spectral correlations, where $L$ is the spectral interval over which the long-range statistic is examined. The short-range studies show excellent agreement with RMT predictions. One of the highlights of this study is the systematic investigation of the consequences of retaining both eigenvalues corresponding to every Kramers doublet, in a crossover involving the GSE limit, and see how it evolves to a limit where the KD is naturally lifted. This is seen most clearly in the NNSD study of the GSE-to-GUE (Gaussian unitary ensemble) transition, achieved by gradually lifting the KD, using the random magnetic field. The NNSD plot in the GSE limit here exhibits a Dirac delta peak at zero splitting and a renormalized GSE hump at finite splitting, whose general analytical form and its asymptotic limit are derived. With an increasing symmetry breaking magnetic field the NNSD shows an interesting, dynamic two-peaked structure that finally converges to the standard GUE lineshape. We explain this trend in terms of a competition between the splittings amongst distinct Kramers doublets (related to unitary symmetries) and the Zeeman-like splittings induced by a breaking of the antiunitary time-reversal symmetry (TRS). This is investigated via the NNSD, the marginal spectral density (MSD) and the densities of states (DOS) for both spin models and RMT crossover matrix models. The first and the final short-range studies involve reentrant Poissonian-to-GOE(GSE)-to-Poissonian crossovers, where the final Poissonian is obtained by a many-body localization of states in the strongly disordered limit, whereas the initial Poissonian regime involve much more delocalized eigenstates. In the long-range spectral correlation studies, we shed light on the extent of agreement between our physical spin systems and RMT predictions. We find that the spin systems depart from the ideal RMT predictions for relatively finite $L\sim 10-15$ at least, for the spectral rigidity and a much smaller $L\sim 2-4$ for the number variance. It is further seen that the departure is usually sooner at the uncorrelated (Poissonian) upper end compared to the correlated (Wigner-Dyson) lower end. We carry out a detailed comparison between the local and the global crossover points, associated with the short-range and the long-range statistics respectively, and find that in most cases they seem to agree reasonably well, but for a few exceptions. Our studies also show that the long-range correlations may serve to distinguish between the two Poissonian limits (nonlocalized and localized) in the reentrant crossovers, which the short-range correlations fail to distinguish.

Measurement-induced phase transitions in the presence of competition between projective measurement and random unitary evolution have attracted increasing attention due to the rich phenomenology of entanglement structures. However, in open quantum systems with free fermions, a generalized measurement with conditional feedback can induce the skin effect and render the system short-range entangled without any entanglement transition, meaning the system always remains in the “area law” entanglement phase. In this work, we demonstrate that power-law long-range hopping does not alter the absence of entanglement transition brought on by the measurement-induced skin effect for systems with open boundary conditions. In addition, for finite-size systems, we discover an algebraic scaling $S(L,L/4)\sim L^{3/2-p}$ when the power-law exponent $p$ of long-range hopping is relatively small. For systems with periodic boundary conditions, we find that the measurement-induced skin effect disappears and observe entanglement phase transitions among “algebraic law,” “logarithmic law,” and “area law” phases.

Spin-orbit torques in noncentrosymmetric polycrystalline magnetic heterostructures are usually described in terms of field-like and damping-like torques. However, materials with a lower symmetry point group can exhibit torques whose behavior substantially deviates from the conventional ones. In particular, based on symmetry arguments it was recently proposed that systems belonging to the ${\text{C}}_{3\mathrm{v}}$ point group display spin-orbit torques that can promote field-free switching [Liu et al., Nat. Nanotechnol. 16, 277 (2021)]. In the present work, we analyze the general form of the torques expected in ${\text{C}}_{3\mathrm{v}}$ crystals using the invariant theory. We uncover several new components that arise from the coexistence of the threefold rotation and mirror symmetries. Using both tight binding model and first principles simulations, we show that these unconventional torque components arise from the onset of trigonal warping of the Fermi surface and can be as large as the damping-like torque. In other words, the Fermi surface warping is a key indicator to the onset of field-free switching in low symmetry crystals.

We combine in situ electrical transport and Seebeck coefficient measurements with the ionic liquid gating technique to investigate superconductivity and the normal state of ${\mathrm{Fe}}_y{\mathrm{Se}}_{1-x}{\mathrm{Te}}_x$ (FST) films. We find that the pristine FST films feature a non-Fermi-liquid temperature dependence of the Seebeck coefficient, i.e., $S/T\sim A_{\mathrm{S}}lnT$, and $A_{\mathrm{S}}$ is strongly correlated with the superconducting transition temperature ($T_{\mathrm{c}}$). Ionic liquid gating significantly raises $T_{\mathrm{c}}$ of FST films, for which the Seebeck coefficient displays a different scaling behavior and retains the logarithmic temperature dependence. Moreover, a quantitative relationship between the slope of $T\text{−}\mathrm{linear}$ resistivity ($A_{\rho }$) and $T_{\mathrm{c}}$ for gated films is observed, i.e., ${\left(A_{\rho }\right)}^{1/2}\sim T_{\mathrm{c}}$, consistent with previous reports on cuprates and FeSe. The scaling behaviors of $A_{\mathrm{S}}$ and $A_{\rho }$ point to a spin-fluctuation-associated transport mechanism in gated ${\mathrm{Fe}}_y{\mathrm{Se}}_{1-x}{\mathrm{Te}}_x$ superconductors.

By using the full 3D generalized time-dependent Ginzbug-Landau equation, we study a long superconducting film of finite width and thickness under an applied transport current. We show that, for sufficiently large thickness, the vortices and the antivortices become curved before they annihilate each other. As they approach the center of the sample, their ends combine, producing a single closed vortex. We also determine the critical values of the thickness for which the closed vortex sets in for different values of the Ginzburg-Ladau parameter. Finally, we propose a model of how to detect a closed vortex experimentally.

The existing proposals for superfluidlike spin transport have been based on easy-plane magnets where the U(1) spin-rotational symmetry is spontaneously broken in equilibrium, and this has been limiting material choices for realizing superfluidlike spin transport to restricted classes of magnets. In this work, we lift this limitation by showing that superfluidlike spin transport can also be realized based on easy-axis magnets, where the U(1) spin-rotational symmetry is intact in equilibrium but can be broken in nonequilibrium. Specifically, we find the condition to engender a nonequilibrium easy-cone state by applying a spin torque to easy-axis magnets, which dynamically induces the spontaneous breaking of the U(1) spin-rotational symmetry and thereby can support superfluidlike spin transport. By exploiting this dynamic easy-cone state, we show theoretically that superfluidlike spin transport can be achieved in easy-axis magnets under suitable conditions and confirm the prediction by micromagnetic simulations. We envision that our work broadens the material library for realizing superfluidlike spin transport, showing the potential utility of dynamic states of magnets as venues to look for spin-transport phenomena that do not occur in static magnetic backgrounds.

Magnon eigenmodes of an artificial structure consisting of cylindrical nanometric particles arranged in a honeycomb array are theoretically investigated, in zero applied field. The dispersion curves of the fundamental mode are calculated for two principal directions of the first Brillouin zone. A Dirac-like behavior, consisting of linear band crossing and zero-width gap, is found for moderate interdot separation. A tight-binding model fits the calculated behavior; the corresponding Dirac velocity has been obtained.

We investigate the frequency nonreciprocity in CoFeB/Ru/CoFeB synthetic antiferromagnets near the spin-flop transition region, where the magnetic moments in the two ferromagnetic layers are noncollinear. Using conventional Brillouin light scattering, we perform systematic measurements of the frequency nonreciprocity as a function of an external magnetic field. At near zero magnetic field, where the antiparallel alignment of the magnetic moments in the two layers is established, we observe a significant frequency nonreciprocity of up to a few GHz, which vanishes when the relative magnetization orientation switches into the parallel configuration at a large magnetic field. For the intermediate values of the magnetic field, where the system transitions from the antiparallel to the parallel orientation, a nonmonotonous dependence of the frequency nonreciprocity is found, with a maximum frequency shift around the spin-flop critical point. This nontrivial dependence of the nonreciprocity versus field is attributed to the nonmonotonous dependence of the dynamic dipolar interaction, which is the main factor that causes asymmetry in the dispersion relation. Furthermore, we found that the sign of the frequency shift changes even without switching the polarity of the bias field. These results show that one can precisely control the nonreciprocal propagation of spin waves via field-driven magnetization reorientation.

In this paper, we focus on the thermal transport properties of antiferromagnetic spin chains cuprates. The chain magnetic excitations, the spinons, partake in heat transport at low temperature, but spinon heat transport decays well below room temperature, possibly because of a coupling with phonons. By means of inelastic neutron scattering, we thoroughly study the lattice dynamics of spin chain compounds ${\mathrm{Sr}}_2{\mathrm{CuO}}_3, {\mathrm{Ca}}_2{\mathrm{CuO}}_3$, along with double spin-chain compounds ${\mathrm{SrCuO}}_2$. We come to the conclusion that there are no obvious anomalies in the phonon dispersions, which suggests a weak spinon-phonon coupling regime.

A two-component superconductor may hypothetically support a vestigial order phase above its superconducting transition temperature, with rotational or time-reversal symmetry spontaneously broken while remaining nonsuperconducting. This has been suggested as an explanation for the observed normal state nematicity of the nematic superconductor $M_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$. We examine the condition for this vestigial order to occur within Ginzburg-Landau theory with order parameter fluctuations, on both the nematic and chiral sides of the theory. Contrary to prior theoretical results, we rule out a large portion of parameter space for possible vestigial order. We argue that very extreme anisotropy is one prerequisite for the formation of a stable vestigial phase via this mechanism, which is likely not met in real materials.

Average symmetry protects the topological surface states of topological (crystalline) insulators with time-reversal symmetry from disorder-induced localization. However, the nature of such average symmetry for magnetic topological insulators and, in particular, its connection to surface transport await inspection. Here, we investigate the impact of imperfect magnetic order on an antiferromagnetic topological insulator, which is a solid with a bulk axion field $\theta =\pi$. We find that the disordered topological surfaces of an antiferromagnetic topological insulator can be generally gapped and localized. The behavior of topological surfaces is now controlled by a mesoscopic average time-reversal symmetry that requires a magnetically imperfect system to be divisible into finite and magnetically neutral slabs. In the presence of this mesoscopic average time-reversal symmetry, the topological surface states will be gapless in the thermodynamic limit and tend to delocalize at a single energy similar to the delocalization transition in the chiral universality class. The notion of average-symmetry-induced delocalization is thus extended to account for magnetic topological insulators, and the spectroscopic and transport signatures clarified herein are relevant to future experimental investigations.

We investigate the oxygen isotope effect on the ${\mathrm{VO}}_2$ metal-insulator transition. Using an alternative method, we synthesize ${\mathrm{V}}^{16}{\mathrm{O}}_2$ and ${\mathrm{V}}^{18}{\mathrm{O}}_2$ crystals, finding a 1%–3% phonon softening and a 1.3 K increase in the metal-insulator transition temperature for the latter. A simple calculation, further confirmed by density functional theory, shows that this shift can be attributed to changes in the lattice internal energy. Our results show that lattice dynamics plays a key role in setting the electronic transition temperature, indicating that electronic and structural degrees of freedom remain strongly coupled at the transition.

The energy spectra of lithium-related donors in silicon are revisited by temperature-dependent infrared absorption spectroscopy. We determine the valley-orbit splitting of an interstitial isolated lithium donor and its complex with residual oxygen in float-zone grown crystals doped from the melt. Lithium-oxygen donors exhibit a temperature evolution of their absorption spectra similar to that of substitutional single-electron group-V donors, while its valley-orbit splitting is corrected upwards to +9.865(5) meV. For the lithium donor, the previously reported inverted valley-orbit splitting is not confirmed since the components of different symmetry are not spectrally resolved in our study. Instead, the chemical shift of the ground state of the donor, having a symmetry different from that of group-V donors, was corrected to a slightly larger value of 1.82(2) meV. Intracenter transitions from the split-off ground state of the Li donor exhibit an anomalous broadening. In addition, several Rydberg-like high excited states, up to $7h_{\pm }$, of lithium-related centers were determined from absorption spectra of moderately doped samples.

We study a topological superconducting nanowire that hosts gliding Majorana zero modes in the presence of a microwave cavity field. We show that the cavity decay rate depends on both the parity encoded by the Majorana zero modes and their motion, in the absence of any direct overlap of their wave functions. That is because the extended bulk states that overlap with both Majorana states facilitate their momentum-resolved microwave spectroscopy, with the gliding acting to modify the interference pattern via a momentum boost. Moreover, we demonstrate that these nonlocal effects are robust against moderate disorder in the chemical potential, and we confirm the numerical calculations with an analytical low-energy model. Our approach offers an alternative to tunneling spectroscopy to probe nonlocal features associated with the Majorana zero modes in nanowires.

We demonstrate the preparation of both armchair (AC) and zigzag (ZZ) epitaxial graphene nanoribbons (GNRs) on $4H$-SiC using a polymer-assisted, sublimation growth method. Historically, the preparation of GNRs depended on the quality or smoothness of the surface changes during growth. The physical phenomenon of terrace step formation introduces the risk of GNR deformation along sidewalls, but the risk is heavily mitigated by this polymer-assisted sublimation method. Two widths (100 and 50 nm) are examined electrically and optically for both AC and ZZ GNRs. Our electrical results support the expected behaviors of the GNRs, while the optical signatures of variable strain reveal the subtle differences among all the GNR species measured.

We study two-dimensional (2D) Dirac fermions in the presence of a periodic mass term alternating between positive and negative values along one direction. This scenario could be realized for a graphene monolayer or for the surface states of topological insulators. The low-energy physics is governed by chiral Jackiw-Rebbi modes propagating along zero-mass lines, with the energy dispersion of the Bloch states given by an anisotropic Dirac cone. By means of the transfer matrix approach, we obtain exact results for a piecewise constant mass superlattice. On top of Bloch states, two different classes of boundary and/or interface modes can exist in a finite-size geometry or in a nonuniform electrostatic potential, respectively. We compute the dispersion relation for both types of boundary and interface modes, which originate either from states close to the superlattice Brillouin zone (BZ) center or, via a Lifshitz transition, from states near the BZ boundary. In the presence of a potential step, we predict that the interface modes, the Bloch wave functions, and the electrical conductance will sensitively depend on the step position relative to the mass superlattice.

Magnetic topological semimetals have interesting anomalous behavior and can be manipulated by tuning the symmetry-protected nodal crossings. ${\mathrm{Co}}_2$-based full Heusler compounds serve as a fertile playground where various novel topological properties can be investigated. In this paper, we present a systematic investigation of the anomalous Hall effect (AHE) in the ferromagnetic Heusler compound ${\mathrm{Co}}_2\mathrm{CrGa}$ using combined experimental and theoretical studies. The anomalous Hall resistivity ${\rho }_{yx}^A$ is observed to scale nearly quadratically with the longitudinal resistivity ${\rho }_{xx}$, and further experimental analysis suggests that the AHE in ${\mathrm{Co}}_2\mathrm{CrGa}$ should be dominated by the intrinsic Karplus-Luttinger Berry phase mechanism. Experimental results also reveal that the anomalous Hall conductivity (AHC) is as large as $\sim 569$ S/cm at 10 K with an intrinsic contribution of $\sim 526$ S/cm and the observed AHC is nearly temperature independent. In addition to the large AHC, we also found an exceptionally large anomalous Hall angle of $\sim 8.5$% and a large anomalous Hall factor of $\sim 0.23 {\mathrm{V}}^{-1}$ simultaneously at room temperature. First-principles calculations suggest that the Berry curvature originates from a gapped nodal line and that Weyl nodes which are generated from the triple point near the Fermi level $E_F$ in the presence of spin-orbit coupling are responsible for the observed large AHC in this compound.

In the half-filled one-orbital Hubbard model on a square lattice, we find pseudogap-like features in the form of two-peak structures associated with the momentum-resolved spectral function. These features exist within the temperature window $T_N\lesssim T\lesssim T^{*}$, where $T_N$ is the Néel temperature and $T^{*}$ is the temperature below which there exists a well-formed dip in the density of state. Inside the window $T_N\lesssim T\lesssim T^{*}$, the peak-to-peak separation in the two-peak structure of the momentum-resolved spectral function grows on moving away from the point ($\pi /2,\pi /2$) along the normal state Fermi surface toward $(\pi ,0)$, a behavior with remarkable similarities to what is observed in the $d$-wave state and pseudogap phase of high-$T_c$ cuprates. We unveil these features by using a parallelized cluster-based Monte Carlo method for simulating the magnetic order parameter fields on a superlattice. The method enables us to access the momentum-resolved single-particle spectral function corresponding to a lattice size of $\approx 240 \times$ 240 with an almost negligible finite-size effect.

In the extreme quantum-limit states under high magnetic fields, enhanced electronic correlation effects can stabilize anomalous quantum states. Using band-tuning with a magnetic field, we realized a spin-polarized quantum-limit state in the field-induced semimetallic phase of a topological insulator ${\mathrm{Bi}}_{1\text{−}x}{\mathrm{Sb}}_x$. Further increase in the field injects more electrons and holes to this state and results in an unexpected reentrant insulator state in this topological semimetallic state. A single-particle picture cannot explain this reentrant insulator state, reminiscent of phase transitions due to many-body effects. Estimates of the binding energy and spacing of electron-hole pairs and the thermal de Broglie wavelength indicate that ${\mathrm{Bi}}_{1\text{−}x}{\mathrm{Sb}}_x$ may host the excitonic insulator phase in this extreme environment.