We study numerically the process of vortex nucleation in the wake of a moving object in superfluids using a generalized and nonlocal Gross-Pitaevskii model. The nonlocal potential is set to reproduce the roton minimum present in the excitation spectrum of superfluid helium. By applying numerically a Newton-Raphson method we determine the bifurcation diagram for different types of nonlinearities and object sizes which allow for determining the corresponding critical velocities. In the case of a nonlocal potential, we observe that for small object sizes the critical velocity is simply determined by the Landau criterion for superfluidity, whereas for large objects there is little difference between all models studied. Finally, we study dynamically in two and three dimensions how rotons and vortices are excited in the nonlocal model of superfluid.

A traditional concept proposes that the suppression of the transition temperature $T_c$ in an amorphous nanobelt is driven by enhanced disorder, which accounts for localized Cooper pairs. However, in this paper, we observe $T_c$ suppression in an amorphous molybdenum-silicide (MoSi) nanobelt, which scales as the inverse square of the width but contradicts disorder theory. Instead, the transition regime can be well described by Cooper pair diffusion in the proximity effect. Both the nonlinear reduction of the switching current density and the abnormal increase of the effective retrapping current density with the reduction of the width further verify the proximity-induced relation. Therefore, we attribute the main size dependence of the suppressed superconducting properties in the MoSi nanobelt to the proximity effect rather than disorder. We speculate that the competition between superconductivity and disorder only appears at the two narrow edge bands rather than the entire nanobelt. Subsequently, the reduction in width does not produce a significant impact on superconductivity for disorder, and only the proximity effect plays an overwhelming role in the MoSi nanobelt.

The electronic structure, lattice dynamics, and electron-phonon coupling (EPC) of the recently discovered $\left(\mathrm{Ba},\mathrm{K}\right)\mathrm{Sb}{\mathrm{O}}_3$ superconductors are investigated by first-principles calculations. The EPC of $\left(\mathrm{Ba},\mathrm{K}\right)\mathrm{Sb}{\mathrm{O}}_3$ is significantly enhanced by considering nonlocal electronic correlation using the Heyd-Scuseria-Ernzerhof hybrid exchange-correlation functional (HSE06). The EPC strength λ of ${\mathrm{Ba}}_{0.35}{\mathrm{K}}_{0.65}{\mathrm{SbO}}_3$ is strongly increased from 0.33 in local-density approximation calculations to 0.59 in HSE06 calculations, resulting in a superconducting transition temperature $T_c$ of about 14.9 K, which is in excellent agreement with experimental value of ∼15 K. Our findings suggest $\left(\mathrm{Ba},\mathrm{K}\right)\mathrm{Sb}{\mathrm{O}}_3$ are extraordinary conventional superconductors, where nonlocal electronic correlation expands the bandwidth, enhances the EPC, and boosts the $T_c$. Moreover, we find both λ and $T_c$ depend crucially on the K-doping level for $\left(\mathrm{Ba},\mathrm{K}\right)\mathrm{Sb}{\mathrm{O}}_3$ and $\left(\mathrm{Ba},\mathrm{K}\right)\mathrm{Bi}{\mathrm{O}}_3$ compounds. $\left(\mathrm{Ba},\mathrm{K}\right)\mathrm{Sb}{\mathrm{O}}_3$ have stronger EPC strength and higher $T_c$ than those of $\left(\mathrm{Ba},\mathrm{K}\right)\mathrm{Bi}{\mathrm{O}}_3$ at the same K-doping level.

Circular dichroism and helical dichroism are intriguing chiroptical phenomena with broad applications in optical sensing and imaging. Here, we generalize one of the phenomena-helical dichroism-to acoustics. We show that a one-dimensional lattice of chiral resonators with loss can induce differential absorption of helical sounds (i.e., acoustic vortices) carrying opposite orbital angular momentum (OAM). This acoustic helical dichroism strongly depends on the rotation symmetry of the chiral resonators. Breaking the $C_4$ rotation symmetry can induce coupling between the opposite chiral dipole modes of the resonators. This leads to OAM band gaps and non-Hermitian exceptional points near the Brillouin-zone center and boundaries, which together give rise to significantly enhanced helical dichroism. The underlying physics can be well captured by an effective Hamiltonian that quantitatively reproduces the complex band structures. The acoustic helical dichroism can find important applications in acoustic OAM manipulations and chiral sound-matter interactions.

We construct infinite families of new universality classes of fracton hydrodynamics with momentum conservation, both with multipole conservation laws and/or subsystem symmetry. We explore the effects of broken inversion and/or time-reversal symmetry at the ideal fluid level, along with momentum relaxation. In the case of one-dimensional multipole-conserving models, we write down explicit microscopic Hamiltonian systems realizing these new universality classes. All of these hydrodynamic universality classes exhibit instabilities and will flow to new nonequilibrium fixed points. Such fixed points are predicted to exist in arbitrarily large spatial dimensions.

Chiral phonons, which offer a possibility for information processing and storing, have been theoretically proposed and experimentally observed in two-dimensional hexagonal lattices. Here, we theoretically predict and study the interband chiral phonon transfer in a honeycomb lattice with an external magnetic field. The interband chiral phonon transfer induced by the magnetic field is accompanied by band inversions and the opening or closing of bandgaps. This can be attributed to the abrupt change of phonon band topology from the calculations of Berry curvature and Chern number. Moreover, the signs of phonon magnetic moments contributed by two optical or acoustic bands tend to be the same when a magnetic field is applied, which leads to the increasing phonon magnetic moment with the increasing strength of the magnetic field. The transfer of phonon pseudoangular momentum is also obtained due to the interband chiral phonon transfer in magnetic fields. Our findings enrich the exploration of chiral phonons and may offer theoretical guidance on the potential manipulation of chiral phonons by the external magnetic field.

We study the low-lying spectrum of the bilinear-biquadratic Heisenberg model in the dimerized and Haldane phases using a tensor renormalization method. At the critical point $\theta =-\pi /4$, the finite-size spectrum predicted by the Wess-Zumino-Witten (WZW) model can only partly be confirmed. We find a singlet-singlet gap which does not fit into the WZW systematics. The results obtained are compared to Bethe ansatz, exact diagonalization, and density-matrix renormalization group calculations for specific parameters.

We study the time evolution of spin squeezing in the one-dimensional spin-1/2 XY model subject to a sudden quantum quench of a transverse magnetic field. The initial state is selected from the ground state phase diagram of the model, consisting of ferro- and paramagnetic phases separated by a critical value of the transverse field. Our analysis, based on exact results for the model, reveals that by a proper choice of protocol, a quantum quench from an unsqueezed state can create spin-squeezed nonequilibrium states. Moreover, we identify a nonanalyticity in the long-time average of the spin-squeezing parameter when quenching to the equilibrium quantum critical point. This suggests that the ferro- and paramagnetic phases also define distinct phases for how the transverse field redistributes quantum fluctuations among the spin components away from equilibrium.

The canonical multiferroic material ${\mathrm{Ni}}_3{\mathrm{V}}_2{\mathrm{O}}_8$ with a kagome staircase structure of $S=1{\mathrm{Ni}}^{2+}$ spins has attracted much attention for its complex magnetic phase diagram, as well as the interplay between spin orders and ferroelectricity. In the present work, we report a comprehensive angular dependent ferroelectric polarization study on ${\mathrm{Ni}}_3{\mathrm{V}}_2{\mathrm{O}}_8$ with the field rotating within the $ac$ plane at low temperatures. The low field induced ferroelectric (LF) state along the crystallographic $b$ axis, which has been observed when the field is applied parallel to the easy axis ($a$ axis), is revealed to vanish suddenly as the field is rotating close to the $c$ axis. Of particular interest is the observation of the reentrant ferroelectric (HF) phase at high field for various orientations when the field is between the $a$ and $c$ axes within a small angle range. The field-angle phase diagrams are built based on the low-temperature angular dependent polarization results and composed of a continuous LF phase and a discontinuous HF phase both exhibiting symmetry with respect to the $a$ axis. The microscopic origins of the two phases are different due to the two decoupled sublattices in the kagome staircase structure, where the low-field polarization is proposed to originate from the spiral structure of the spine spins, while the high-field phase is ascribed to the ordering of the cross-ties under a tilting high magnetic field.

Here, we report on a hyperscaling analysis of the thermodynamic measurements as a function of temperature and magnetic field for ${\mathrm{Ni}}_{1-x}{\mathrm{Rh}}_x$ with $x=0.375$ where a ferromagnetic quantum critical point has been recently identified [Phys. Rev. Lett. 124, 117203 (2020)]. The obtained critical exponents agree well with the theory proposed by Belitz, Kirkpatrick, and Vojta for a disorder tuned quantum critical point in the preasymptotic region.

The spin-1/2 transverse field two-leg Ising ladder with nearest-neighbor exchange and plaquette four-spin interaction $J_4$ is studied analytically and numerically with the density matrix renormalization group approach. The quantum phase diagram in the transverse field $B$ versus $J_4$ plane has been obtained. There are three different phases: a paramagnetic (PM) phase for high values of the transverse field and, for low values of $B$, a ferromagnetic (FM) ordered phase for small $J_4$, and a dimerized-rung (DR) phase for large negative values of $J_4$. All phases are separated by quantum phase transition lines meeting at a multicritical point. The critical lines have been obtained by exploring the entanglement entropy. The results show that along the critical lines the central charge is $c=1/2$, while at the multicritical point one has $c=1$. The scaling dimension of the energy operator is $X_{\epsilon }=1$, in agreement with the universality class of the critical behavior of the quantum Ising chain. An effective field theory for the multicritical point is also discussed. The FM and the DR order parameters have also been computed and we found a region where the FM and the DR phases coexist.

We examine energy relaxation of nonequilibrium quasiparticles (QPs) in different vortex configurations in “dirty” $s$-wave superconductors (SCs). The heat flow from the electronic subsystem to phonons in a mesoscopic SC disk with a radius of the order of several coherence lengths is calculated both in the Meissner and in the giant vortex states using the Usadel approach. The recombination process is shown to be strongly affected by interplay of the subgap states, located in the vortex core and in the region at the sample edge where the spectral gap $E_{\mathrm{g}}$ is reduced by the Meissner currents. In order to uncover the physical origin of the results, we develop a semiquantitative analytical approximation based on the combination of homogeneous solutions of Usadel equations in Meissner and vortex states of a mesoscopic SC disk and analytically calculate the corresponding spatially resolved electron–phonon heat rates. Our approach provides important information about nonequilibrium QPs cooling by the magnetic field-induced traps in various mesoscopic SC devices.

Recent progress in the growth and characterization of thin-film ${\mathrm{VO}}_2$ has shown its electronic properties can be significantly modulated by epitaxial matching. To throw new light on the concept of “Mott engineering,” we develop a symmetry-consistent approach to treat structural distortions and electronic correlations in epitaxial ${\mathrm{VO}}_2$ films under strain, and compare our design with direct experimental probes. We find strong evidence for the emergence of correlation-driven charge order deep in the metallic phase, and our results indicate that exotic phases of ${\mathrm{VO}}_2$ can be controlled with epitaxial stabilization.

The multichannel Kondo model supports effective anyons on the partially screened impurity, as suggested by its fractional impurity entropy. It was recently demonstrated for the multi-impurity chiral Kondo model, that scattering of an electron through the impurities depends on the anyon's total fusion channel. Here we study the correlation between impurity spins. We argue, based on a combination of conformal field theory, a perturbative limit with a large number of channels $k$, and the exactly solvable two-channel case, that the interimpurity spin correlation probes the anyon fusion of the pair of correlated impurities. This may allow, using measurement-only topological quantum computing protocols, to braid the multichannel Kondo anyons via consecutive measurements.

We study the magnetism in ${\mathrm{NdFe}}_2{\mathrm{Ga}}_8$ by the neutron-diffraction and temperature-modulated linear Grüneisen parameter measurements. Previous thermodynamical measurements have demonstrated that there are two magnetic transitions at 10 and 14.5 K, respectively. Neutron-diffraction measurements confirm that the lower one is an antiferromagnetic (AFM) transition with a commensurate magnetic structure. Both the commensurate and the incommensurate (IC) magnetic peaks are found below the higher transition but their intensities only gradually increase with decreasing temperature. Below 10 K, the commensurate peak intensity increases quickly with decreasing temperature, signaling the AFM transition, while the IC peak intensity disappears below 5 K. The linear Grüneisen parameter along the $c$ axis, ${\mathrm{\Gamma }}_c$, shows a hysteresis behavior that is different from the hysteresis behavior for the magnetization $M$. We give a discussion of the origin of the magnetism in ${\mathrm{NdFe}}_2{\mathrm{Ga}}_8$.

In this paper, we provide state sum path integral definitions of exotic invertible topological phases proposed in the recent paper by Hsin, Ji, and Jian [arXiv:2105.09454 [cond-mat.str-el]. The exotic phase has time-reversal $\left(T\right)$ symmetry, and depends on a choice of the space-time structure called the Wu structure. The exotic phase cannot be captured by the classification of any bosonic or fermionic topological phases and thus gives a novel class of invertible topological phases. When the $T$ symmetry defect admits a spin structure, our construction reduces to a sort of the decorated domain wall construction, in terms of a bosonic theory with $T$ symmetry defects decorated with a fermionic phase that depends on a spin structure of the $T$ symmetry defect. By utilizing our path integral, we propose a lattice construction for the exotic phase that generates the ${\mathbb{Z}}_8$ classification of the $(3+1)$d invertible phase based on the Wu structure. This generalizes the ${\mathbb{Z}}_8$ classification of the $T$-symmetric $(1+1)$d topological superconductor proposed by Fidkowski and Kitaev. On oriented space-time, this $(3+1)$d invertible phase with a specific choice of Wu structure reduces to a bosonic Crane-Yetter TQFT which has a topological ordered state with a semion on its boundary. Moreover, we propose a subclass of $G$-SPT phases based on the Wu structure labeled by a pair of cohomological data in generic space-time dimensions. This generalizes the Gu-Wen subclass of fermionic SPT phases.

We have measured the electrical resistivity and thermopower of electron-doped perovskite cobaltites ${\mathrm{LaCo}}_{1-y}{\mathrm{Te}}_y{\mathrm{O}}_3$. In contrast to hole-doped systems such as metallic ferromagnets ${\mathrm{La}}_{1-x}{M}_x{\mathrm{CoO}}_3$ $(M=\text{Ca}, \mathrm{Sr}, \mathrm{Ba})$, the electron-doped samples show an insulating behavior even in a heavily doped range due to a spin-state blockade mechanism that an electron hopping from a high-spin ${\mathrm{Co}}^{2+}$ to low-spin ${\mathrm{Co}}^{3+}$ site is energetically suppressed. We find that, despite electron doping, the thermopower shows relatively large positive values above $y=0.05$, strikingly distinct from the hole-doped case where it comes close to zero with doping. This prominent electron-hole asymmetry seen in the thermopower originates from a bipolar conduction which consists of a slight number of mobile holes and the main immobile electrons, demonstrating the impact of a spin-state blockade on thermoelectric transport.

The cluster slave-spin method is implemented to research the ground state properties of the honeycomb lattice Hubbard model with doping $\delta$ and coupling $U$ being its parameters. At half filling, a single direct and continuous phase transition between the semimetal and antiferromagnetic (AFM) insulator is found at $U_{\text{AFM}}=2.43t$ that is in the Gross-Neveu-Yukawa universality class, where a relation between the staggered magnetization $M$ and the AFM energy gap ${\mathrm{\Delta }}_{\text{AFM}}$ is established as $M\propto {\mathrm{\Delta }}_{\text{AFM}}$, compared to $M\propto {\mathrm{\Delta }}_{\text{AFM}}{(ln{\mathrm{\Delta }}_{\text{AFM}})}^2$ in the square lattice case. A first-order semimetal to the underlying paramagnetic (PM) insulator Mott transition is corroborated at $U_{\text{Mott}}=8.36t$, which is responsible for a broad crossover around $U_c=5.4t$ between the weak- and strong-coupling regimes in the AFM state that increases with $\delta$, in contrast to the square lattice case. In the doped system, the compressibility $\kappa$ near the van Hove singularity at $\delta =1/4$ is suppressed substantially by the interaction before the semimetal to AFM transition occurs, whereas $\kappa$ near the Dirac points is very close to the noninteracting one, indicating that the Dirac cone structure of the energy dispersion is rather robust. An overall phase diagram in the $U\text{−}\delta$ plane is presented, consisting of four regimes: the AFM insulator at $\delta =0$ for $U>U_{\text{AFM}}$, the AFM metal with compressibility $\kappa >0$ or $\kappa <0$, and the PM semimetal; the AFM metal with $\kappa <0$ only exists in an extremely small area near the phase boundary between the AFM and PM state.

We consider a magnetic skyrmion crystal formed at the surface of a topological insulator. Incorporating the exchange interaction between the helical Dirac surface states and the periodic Néel or Bloch skyrmion texture, we obtain the resulting electronic band structure and discuss the constraints that symmetries impose on the energies and Berry curvature. We find substantive qualitative differences between the Néel and Bloch cases, with the latter generically permitting a multiband low energy tight-binding representation whose parameters are tightly constrained by symmetries. We explicitly compute the associated Wannier orbitals, which resemble the ringlike chiral bound states of helical Dirac fermions coupled to a single skyrmion in a ferromagnetic background. We construct a two-band tight-binding model with real nearest-neighbor hoppings which captures the salient topological features of the low-energy bands. Our results are relevant to magnetic topological insulators (TIs), as well as to TI-magnetic thin film heterostructures, in which skyrmion crystals may be stabilized.