A perturbed system relaxes towards an equilibrium given by a minimum in the potential-energy landscape. This often occurs by thermally activated jumps over metastable states. The corresponding dynamics is called creep and follows Arrhenius' law. Here we consider the situation where the equilibrium position itself depends on temperature. We show that this effect occurs in the vortex lattice of the anisotropic superconductor $2\mathrm{H}-\mathrm{Nb}{\mathrm{Se}}_2$ when the magnetic field is tilted away from the principal axes, and that it leads to the peculiar appearance of creep when cooling the sample. Temperature determines the system's equilibrium state and at the same time brings the system back to equilibrium, playing a dual and antagonistic role. We expect that cooling-induced creep occurs in correlated systems with many degrees of freedom, allowing one to tune the equilibrium state via heat treatment.

Floquet higher-order topological insulators (HOTIs) and superconductors (SCs) with an order-2 space-time symmetry or antisymmetry are classified. This is achieved by considering unitary loops, whose nontrivial topology leads to the anomalous Floquet topological phases, subject to a space-time symmetry/antisymmetry. By mapping these unitary loops to static Hamiltonians with an order-2 crystalline symmetry/antisymmetry, one is able to obtain the $K$ groups for the unitary loops and thus complete the classification of Floquet HOTIs and SCs. Interestingly, we find that for every order-2 nontrivial space-time symmetry/antisymmetry involving a half-period time translation, there exists a unique order-2 static crystalline symmetry/antisymmetry such that the two symmetries/antisymmetries give rise to the same topological classification. Moreover, by exploiting the frequency-domain formulation of the Floquet problem, a general recipe that constructs model Hamiltonians for Floquet HOTIs and SCs is provided, which can be used to understand the classification of Floquet HOTIs and SCs from an intuitive and complementary perspective.

We construct and study an intriguing model of one-dimensional interacting fermion wire, which is enforced to be in a topological phase by the spin-orbit locking structure in the interactions, regardless of the dimerization direction of the Peierls distortion, in sharp contrast to a conventional wisdom. Thorough analyses based upon the bosonization, the renormalization group technique, and the mean-field theory have been made. The novelty of being a pair-hopping fermionic wire lies in that symmetry breaking and the formation of (symmetry-protected) topological configurations are not independent, enriching our understanding of the interplay between topological phase and symmetry breaking. An experimentally feasible scheme is proposed for realizing the model with cold atoms and available techniques.

An emergent class of two-dimensional Dirac materials is $\alpha -T_3$ lattices that can be realized by adding an atom at the center of each unit cell of a lattice with $T_3$ symmetry. The interaction strength $\alpha$ between this atom and any of its nearest neighbors is a parameter that can be continuously tuned between zero and one to generate a spectrum of materials. We investigate the fundamentally important and practically relevant issue of quasiparticle confinement for the entire spectrum of $\alpha -T_3$ materials. Except for the two end points, i.e., $\alpha =0,1$, which correspond to the graphene and pseudospin-1 lattices, respectively, the time-reversal symmetry is broken, leading to the removal of level degeneracy and facilitating confinement. Taking the approach of quantum scattering off an electrically generated potential cavity in the quantum-dot regime, we characterize confinement by identifying and examining the scattering resonances. We study a number of cavities with characteristically distinct classical dynamics: circular, annular, elliptical, and stadium cavities. For the circular and annular cavities with classically integrable and mixed dynamics, respectively, the scattering matrix can be analytically obtained, so the scattering cross sections and the Wigner-Smith time delay associated with the resonances can be calculated to quantify confinement. For the elliptical and stadium cavities with mixed and chaotic dynamics in the classical limit, respectively, the scattering-matrix approach is infeasible, so we adopt an efficient numerical method to calculate the scattering wave functions and experimentally accessible measures of confinement such as the magnetic moment. The main finding is that, for all the cases, the regime of small $\alpha$ values offers the best confinement possible among the spectrum of $\alpha -T_3$ materials, which is general and holds regardless of the nature of the corresponding classical dynamics.

The Hubbard model is the simplest model that is believed to exhibit superconductivity arising from purely repulsive interactions and has been extensively applied to explore a variety of unconventional superconducting systems. Here we study the evolution of the leading superconducting instabilities of the single-orbital Hubbard model on a two-dimensional square lattice as a function of onsite Coulomb repulsion $U$ and band filling by calculating the irreducible particle-particle scattering vertex obtained from dynamical cluster approximation (DCA) calculations, and compare the results to both perturbative Kohn-Luttinger (KL) theory as well as the widely used random phase approximation (RPA) spin-fluctuation pairing scheme. Near half-filling, we find remarkable agreement of the hierarchy of the leading pairing states among these three methods, implying adiabatic continuity between weak- and strong-coupling pairing solutions of the Hubbard model. The $d_{x^2-y^2}$-wave instability is robust to increasing $U$ near half-filling as expected. Away from half-filling, the predictions of KL and RPA at small $U$ for transitions to other pair states agree with DCA at intermediate $U$ as well as recent diagrammatic Monte Carlo calculations. RPA results fail only in the very dilute limit, where it yields a $d_{xy}$ ground state instead of a $p$-wave state established by diagrammatic Monte Carlo and low-order perturbative methods, as well as our DCA calculations. We discuss the origins of this discrepancy, highlighting the crucial role of the vertex corrections neglected in the RPA approach. Overall, a comparison of the various methods over the entire phase diagram strongly suggests a smooth crossover of the superconducting interaction generated by local Hubbard interactions between weak and strong coupling.

By measuring the spin polarization of GeTe films as a function of light polarization we observed that the bulk states are fully spin polarized in the initial state, in strong contrast with observations for other systems with a strong spin-orbit interaction and the surface derived states in the same system. In agreement with state-of-the-art theory, our experimental results show that fully spin-polarized bulk states are an intrinsic property of the ferroelectric Rashba semiconductor $\alpha$-GeTe(111). The fact that the measured spin-polarization vector does not change with light polarization can be explained by the absence of a mixing of states with a different total angular momentum $J$.

We consider a strongly spin-orbit-coupled metal, one of whose Fermi surfaces is close to a Lifshitz (topological) transition. Via a renormalization group analysis of the square-lattice Hubbard model with strong Rashba spin-orbit coupling, we show that such a metal is generically unstable to the formation of mixed-parity superconductivity with a helical triplet component.

Electric dipole radiation can be controlled by engineering of the photonic environment. A coherent interaction between forward and backward emission depends interferometrically on the position of a nearby mirror. The transverse coherence of exciton emission in a single layer of ${\mathrm{MoSe}}_2$ and the highly radiatively broadened nature of our samples removes fundamental physical limitations of previous experiments employing pointlike dipoles. This enables full control over the exciton radiative coupling rate and total linewidth at cryogenic temperatures from near zero to $1.8\pm 0.4\mathrm{meV}$ and from 0.9 to $2.3\pm 0.1\mathrm{meV}$, respectively.

Superconductivity owes its properties to the phase of the electron pair condensate that breaks the $\text{U}\left(1\right)$ symmetry. In the most traditional ground state, the phase is uniform and rigid. The normal state can be unstable towards special inhomogeneous superconducting states: the Abrikosov vortex state and the Fulde-Ferrell-Larkin-Ovchinnikov state. Here we show that the phase-uniform superconducting state can go into a fundamentally different and more ordered nonuniform ground state, which we refer to as a phase crystal. This state breaks translational invariance through formation of a spatially periodic modulation of the phase, manifested by unusual superflow patterns and circulating currents, that also break time-reversal symmetry. We list the general conditions needed for realization of phase crystals. Using microscopic theory, we then derive an analytic expression for the superfluid density tensor for the case of a nonuniform environment in a semi-infinite superconductor. We demonstrate how the surface quasiparticle states enter the superfluid density and identify phase crystallization as the main player in several previous numerical observations in unconventional superconductors, and predict the existence of a similar phenomenon in superconductor-ferromagnetic structures. This analytic approach provides a unifying aspect for the exploration of boundary-induced quasiparticles and collective excitations in superconductors. More generally, we trace the origin of phase crystallization to nonlocal properties of the gradient energy, which implies the existence of similar pattern-forming instabilities in many other contexts.

We report small-angle x-ray scattering measurements of the skyrmion lattice in two 200-nm-thick ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ lamellae aligned with the applied magnetic field parallel to the out of plane [110] or [100] crystallographic directions. Our measurements show that the equilibrium skyrmion phase in both samples is expanded significantly compared to bulk crystals, existing between approximately 30 and 50 K over a wide region of magnetic field. This skyrmion state is elliptically distorted at low fields for the [110] sample, and symmetric for the [100] sample, possibly due to crystalline anisotropy becoming more important at this sample thickness than it is in bulk samples. Furthermore, we find that a metastable skyrmion state can be observed at low temperature by field cooling through the equilibrium skyrmion pocket in both samples. In contrast to the behavior in bulk samples, the volume fraction of metastable skyrmions does not significantly depend on cooling rate. We show that a possible explanation for this is the change in the lowest temperature of the skyrmion state in this lamellae compared to bulk, without requiring different energetics of the skyrmion state.

Recent work has shown that two seemingly different physical mechanisms, namely fracton behavior and confinement, can give rise to nonergodicity in one-dimensional quantum many-body systems. In this work, we demonstrate an intrinsic link between these two mechanisms by studying the dynamics of one-dimensional confining theories, such as a U(1) gauge theory and a quantum Ising model. We show that within certain parameter regimes, these models exhibit effective fracton dynamics, characterized by immobility of stable single-particle excitations and free motion of dipolar bound states. By perturbatively integrating out the linearly confining field, we obtain an effective fracton Hamiltonian for the confined charges which exhibits conservation of dipole moment. We discuss an intuitive understanding of these results in terms of the motion of the confining strings, leading to potential extensions to higher dimensions. We thereby interpret recent observations of nonthermal eigenstates and glassy dynamics in confining theories in terms of corresponding results in the fracton literature.

Steady-state thermoelectric machines convert heat into work by driving a thermally generated charge current against a voltage gradient. In this work, we propose a new class of steady-state heat engines operating in the quantum regime, where a quasiperiodic tight-binding model that features a mobility edge forms the working medium. In particular, we focus on a generalization of the paradigmatic Aubry-André-Harper (AAH) model, known to display a single-particle mobility edge that separates the energy spectrum into regions of completely delocalized and localized eigenstates. Remarkably, these two regions can be exploited in the context of steady-state heat engines as they correspond to ballistic and insulating transport regimes. This model also presents the advantage that the position of the mobility edge can be controlled via a single parameter in the Hamiltonian. We exploit this highly tunable energy filter, along with the peculiar spectral structure of quasiperiodic systems, to demonstrate large thermoelectric effects, exceeding existing predictions by several orders of magnitude. This opens the route to a new class of highly efficient and versatile quasiperiodic steady-state heat engines, with a possible implementation using ultracold neutral atoms in bichromatic optical lattices.

Crystallographic lattice defects strongly influence dynamical properties of magnetic materials at both microscopic and macroscopic length scales. A multiscale approach to magnetization dynamics, which is presented in this paper, accurately captures such effects. The method is illustrated using examples of systems with localized, nontrivial topological properties, e.g., in the form of skyrmions and domain walls that interact with lattice dislocations. Technical aspects of the methodology involve multiscale magnetization dynamics that connect atomistic and continuum descriptions. The technique is capable of solving the Landau-Lifshitz-Gilbert equations efficiently in two regions of a magnetic material—the mesoscopic and the atomistic regions, which are coupled in a seamless way. It is demonstrated that this methodology allows simulating realistically sized magnetic skyrmions interacting with material defects and complex physical effects.

The Bethe-Salpeter equation (BSE) is the standard computational method for optical excitations in solids, including excitonic effects. In this paper we explore ways to reduce the computational cost of the BSE by simplifying the dielectrically screened Coulomb interaction: Instead of calculating the dielectric function from first principles, we replace it by a momentum-dependent model dielectric function or just by a single parameter. Combined with a semilocal exchange-correlation kernel, this defines an alternative class of hybrid functionals for solids within generalized time-dependent density-functional theory. We perform a systematic assessment of these simplified approaches and find that they yield optical absorption spectra and exciton binding energies of semiconductors and wide-gap insulators in close agreement with the standard BSE and with experiment. We also present applications to the perovskite material ${\mathrm{CsPbBr}}_3$ as an example of a more complex system.

Quantum anomalies in Weyl semimetal (for either $\mathbf{E}·\mathbf{B}\ne 0$ or $\mathbf{\nabla }T·\mathbf{B}\ne 0$) leads to chiral charge and energy pumping between the opposite chirality nodes. This results in chiral charge and energy imbalance between the Weyl nodes which manifests in several intriguing magneto-transport phenomena. Here, we investigate the role of electrical-, thermal-, and gravitational chiral anomaly on magneto-transport in Weyl semimetals. We predict the planar Ettinghausen and Righi-Leduc effect to be a distinct signature of these quantum anomalies. We also demonstrate a significant enhancement in the thermoelectric conductivity, Seebeck effect, Nernst effect, and thermal conductivity with increasing temperature. Interestingly, this anomaly induced transport violates the Wiedemann-Franz law and Mott relation.

Hard-core bosons on honeycomb lattice ribbons with zigzag edges are studied using exact numerical simulations. We map out the phase diagrams of ribbons with different widths, which contain superfluid and insulator phases at various fillings. We show that charge domain walls are energetically favorable, in sharp contrast to the more typical occupation of a set of sites on a single sublattice of the bipartite geometry at $\rho =\frac{1}{2}$ filling. This “self-organized domain wall” separates two charge-density-wave regions with opposite Berry curvatures. Associated with the change of topological properties, superfluid transport occurs down the domain wall. Our results provide a concrete context to observe bosonic topological phenomena and can be simulated experimentally using bosonic cold atoms trapped in designed optical lattices.

The production of correlated Stokes (S) and anti-Stokes (aS) photons (SaS process) mediated by real or virtual phonon exchange has been reported in many transparent materials. In this work, we investigate the polarization and time correlations of SaS photon pairs produced in a diamond sample. We demonstrate that both S and aS photons have mainly the same polarization of the excitation laser. We also perform a pump-and-probe experiment to measure the decay rate of the SaS pair production, evidencing the fundamental difference between the real and virtual (phonon exchange) processes. In real processes, the rate of SaS pair production is governed by the phonon lifetime of $(2.8\pm 0.3)\mathrm{ps}$, while virtual processes only take place within the time width of the pump laser pulses of approximately $0.4\mathrm{ps}$. We explain the difference between real and virtual SaS processes by a phenomenological model, based on the probabilities of phonon creation and decay.

We consider a system of stacked tunnel-coupled two-dimensional electron- and hole-gas layers with Rashba spin-orbit interaction subjected to a staggered Zeeman field. The interplay of different intralayer tunnel couplings results in a topological insulator phase characterized by the presence of gapless helical surface states. The staggered Zeeman field further enriches the topological phase diagram by generating a second-order topological insulator phase characterized by the presence of gapless hinge states. The emergence of these topological phases is proven analytically in the regime of small Zeeman field and confirmed by numerical simulations in the nonperturbative region of the phase diagram. The topological phases are stable against external perturbations and disorder.

The molecular coordination complex ${\mathrm{NiI}}_2{(3,5-\mathrm{lut})}_4$ [where (3,5-lut) $=$ (3,5-lutidine) $=({\mathrm{C}}_7{\mathrm{H}}_9\mathrm{N}$)] has been synthesized and characterized by several techniques including synchrotron x-ray diffraction, electron-spin resonance, superconducting quantum interference device magnetometry, pulsed-field magnetization, inelastic neutron scattering, and muon spin relaxation. Templated by the configuration of 3,5-lut ligands the molecules pack in-registry with the Ni–$\mathrm{I}\cdots \mathrm{I}$–Ni chains aligned along the $c$ axis. This arrangement leads to an uncommon through-space $\mathrm{I}\cdots \mathrm{I}$ magnetic coupling which is directly measured in this work. The net result is a near-ideal realization of the $S=1$ Haldane chain with $J=17.5\mathrm{K}$ and energy gaps of ${\mathrm{\Delta }}^{\parallel }=5.3\mathrm{K} {\mathrm{\Delta }}^{\perp }=7.7\mathrm{K}$, split by the easy-axis single-ion anisotropy $D=-1.2\mathrm{K}$. The ratio $D/J=-0.07$ affords one of the most isotropic Haldane systems yet discovered, while the ratio ${\mathrm{\Delta }}_0/J=0.40\left(1\right)$ (where ${\mathrm{\Delta }}_0$ is the average gap size) is close to its ideal theoretical value, suggesting a very high degree of magnetic isolation of the spin chains in this material. The Haldane gap is closed by orientation-dependent critical fields ${\mu }_0{H}_{\mathrm{c}}^{\parallel }=5.3\mathrm{T}$ and ${\mu }_0{H}_{\mathrm{c}}^{\perp }=4.3\mathrm{T}$, which are readily accessible experimentally and permit investigations across the entirety of the Haldane phase, with the fully polarized state occurring at ${\mu }_0{H}_{\mathrm{s}}^{\parallel }=46.0\mathrm{T}$ and ${\mu }_0{H}_{\mathrm{s}}^{\perp }=50.7\mathrm{T}$. The results are explicable within the so-called fermion model, in contrast to other reported easy-axis Haldane systems. Zero-field magnetic order is absent down to $20\mathrm{mK}$ and emergent end-chain effects are observed in the gapped state, as evidenced by detailed low-temperature measurements.

The modified $XY$ model is a variation of the $XY$ model extended by a half-period term, exhibiting a rich phase structure. As the Goldstone model, also known as the linear O(2) model, can be obtained as a continuum and regular model for the $XY$ model, we define the modified Goldstone model as that of the modified $XY$ model. We construct a vortex, a soliton (domain wall), and a molecule of two half-quantized vortices connected by a soliton as regular solutions of this model. Then we investigate its phase structure in two Euclidean dimensions via the functional renormalization group formalism and full numerical simulations. We argue that the field dependence of the wave function renormalization factor plays a crucial role in the existence of the line of fixed points describing the Berezinskii-Kosterlitz-Thouless (BKT) transition, which can ultimately terminate not only at one but at two end points in the modified model. This structure confirms that a two-step phase transition of the BKT and Ising types can occur in the system. We compare our renormalization group results with full numerical simulations, which also reveal that the phase transitions show a richer scenario than expected.