We analyze the spin transport through a finite-size one-dimensional interacting wire connected to noninteracting leads. By combining renormalization-group arguments with other analytic considerations such as the memory function technique and instanton tunneling, we find the temperature dependence of the spin conductance in different parameter regimes in terms of interactions and the wire length. The temperature dependence is found to be nonmonotonic in part of the parameter space. In particular, the system approaches perfect spin conductance at zero temperature for both attractive and repulsive interactions, in contrast with the static spin conductivity. We discuss the connection of our results to recent experiments with ultracold atoms and compare the theoretical prediction to experimental data in the parameter regime where temperature is the largest energy scale.

Motivated by recent developments on the fabrication and control of semiconductor-based quantum dots, we theoretically study a finite system of tunnel-coupled quantum dots with the electrons interacting through the long-range Coulomb interaction. When the interelectron separation is large and the quantum dot confinement potential is weak, the system behaves as an effective Wigner crystal with a period determined by the electron average density with considerable electron hopping throughout the system. For stronger periodic confinement potentials, however, the system makes a crossover to a Mott-type ground state where the electrons are completely localized at the individual dots with little interdot tunneling. In between these two phases, the system is essentially a strongly correlated electron liquid with intersite electron hopping constrained by strong Coulomb interaction. We characterize this Wigner-Mott-liquid quantum crossover with detailed numerical finite-size diagonalization calculations of the coupled interacting quantum dot system, showing that these phases can be smoothly connected by tuning the system parameters. Experimental feasibility of observing such a hopping-tuned Wigner-Mott-liquid crossover in currently available semiconductor quantum dots is discussed. In particular, we connect our theoretical results to recent quantum-dot-based quantum emulation experiments where a collective Coulomb blockade was demonstrated. We discuss realistic disorder effects on our theoretical findings. One conclusion of our work is that experiments must explore lower density quantum dot arrays in order to clearly observe the Wigner phase although the Mott-liquid crossover phenomenon should already manifest itself in the currently available quantum dot arrays. We also suggest a direct experimental electron density probe, such as atomic force microscopy or scanning tunneling microscopy, for a clear observation of the effective Wigner crystal phase.

One-dimensional atomic mixtures of fermions can effectively realize spin chains and thus constitute a clean and controllable platform to study quantum magnetism. Such strongly correlated quantum systems are also of sustained interest to quantum simulation and quantum computation due to their computational complexity. In this article, we exploit spectral graph theory to completely characterize the symmetry properties of one-dimensional fermionic mixtures in the strong interaction limit. We also develop a powerful method to obtain the so-called Tan contacts associated with certain symmetry classes. In particular, compared to brute-force diagonalization that is already virtually impossible for a moderate number of fermions, our analysis enables us to make unprecedented efficient predictions about the energy gap of complex spin mixtures. Our theoretical results are not only of direct experimental interest but also provide important guidance for the design of adiabatic control protocols in strongly correlated fermion mixtures.

In strictly one-dimensional systems, repulsive interactions tend to reduce particle mobility on a lattice. Therefore, the Drude weight, controlling the divergence at zero-frequency of optical conductivities in perfect conductors, is lower than in noninteracting cases. We show that this is not the case when extending to quasi-one-dimensional ladder systems. Relying on bosonization, perturbative and matrix product states (MPS) calculations, we show that nearest-neighbor interactions and magnetic fluxes provide a bias between back- and forward-scattering processes, leading to linear corrections to the Drude weight in the interaction strength. As a consequence, Drude weights counterintuitively increase (decrease) with repulsive (attractive) interactions. Our findings are relevant for the efficient tuning of Drude weights in the framework of ultracold atoms trapped in optical lattices and equally affect topological edge states in condensed matter systems.

The existence of out-of-plane-oriented oxygen vacancy dipoles was experimentally established in (001) perovskite epitaxial films of ferroelectric barium titanate by analyses of the crystal and electronic structure as well as the ferroelectric and optical properties. The formation of the defects was shown to occur in the presence of in-plane compressive strain and oxygen-deficient deposition conditions. The in-plane compressive strain was suggested to favor the formation of oxygen vacancies ${\mathrm{V}}_{\mathrm{O}}$ in the Ba-O planes and thus stabilize the out-of-plane orientation of the dipolar ${\left({{\mathrm{V}}^{+2}}_{\mathrm{O}}\text{−}\mathrm{T}{\mathrm{i}}^{3+}\right)}^{+}$ defects.

Illumination of circularly polarized light is an alternative to electrical methods for spin injection. Spins are injected at the point of light illumination, and then they diffuse radially due to the in-plane gradient of the spin density. This diffusing spin current is converted into a circular charge current by the inverse spin Hall effect (ISHE) and is usually not detectable. However, by shining a light on an asymmetric part of the sample, such as near the edges, we detected this current as a helicity-dependent component in the measured photocurrent. This is not due to the circular photogalvanic effect because it occurs even at normal incidence of the light. A finite-element-method simulation shows that the ISHE-induced circular charge current generates an electric dipole at the edge of the sample, causing the measured charge current. The asymmetric light illumination shown here is a simple way to control the output of the ISHE, which may lead to novel spintronic devices.

The discovery of the anomalous Hall effect (AHE) in bulk metallic antiferromagnets (AFMs) motivates the search of the same phenomenon in two-dimensional (2D) systems, where a quantized anomalous Hall conductance can, in principle, be observed. Here we present experiments on microfabricated devices based on ${\mathrm{Co}}_{1/3}{\mathrm{NbS}}_2$, a layered AFM that was recently found to exhibit AHE in bulk crystals below the Néel temperature $T_N=29$ K. Transport measurements reveal a pronounced resistivity anisotropy, indicating that upon lowering temperature the electronic coupling between individual atomic layers is increasingly suppressed. The experiments also show an extremely large anomalous Hall conductivity of approximately 400 S/cm, more than one order of magnitude larger than in the bulk, which demonstrates the importance of studying the AHE in small exfoliated crystals, less affected by crystalline defects. Interestingly, the corresponding anomalous Hall conductance, when normalized to the number of contributing atomic planes, is $\sim 0.6e^2/h$ per layer, approaching the value expected for the quantized anomalous Hall effect. The observed strong anisotropy of transport and the very large anomalous Hall conductance per layer make the properties of ${\mathrm{Co}}_{1/3}{\mathrm{NbS}}_2$ compatible with the presence of partially filled topologically nontrivial 2D bands originating from the magnetic superstructure of the antiferromagnetic state. Isolating atomically thin layers of this material and controlling their charge density may therefore provide a viable route to reveal the occurrence of the quantized AHE in a 2D AFM.

We study the two-dimensional kagome-ice model derived from a pyrochlore lattice with second- and third-neighbor interactions. The canted moments align along the local $\langle 111\rangle$ axes of the pyrochlore and respond to both in-plane and out-of-plane external fields. We find that the combination of further-neighbor interactions together with the external fields introduces a rich phase diagram with different spin textures. Close to the phase boundaries, metastable “snake” domains emerge with extremely long relaxation time. Our kinetic Monte Carlo analysis of the magnetic-field quench process from saturated state shows unusually slow dynamics. Although the interior spins are almost frozen in snake domains, the spins on the edge are free to fluctuate locally, leading to frequent creation and annihilation of monopole-antimonopole bound states. Once the domains are formed, these excitations are localized and can hardly propagate because of the energy barrier of snakes. The emergence of such snake domains may shed light on the experimental observation of dipolar spin ice under tilted fields and provide a new strategy to manipulate both spin and charge textures in artificial spin ice.

Topological invariants play a key role in the characterization of topological states. Because of the existence of exceptional points, it is a great challenge to detect topological invariants in non-Hermitian systems. We put forward a dynamic winding number, the winding of realistic observables in long-time average, for exploring band topology in both Hermitian and non-Hermitian two-band models via a unified approach. We build a concrete relation between dynamic winding numbers and conventional topological invariants. In one dimension, the dynamic winding number directly gives the conventional winding number. In two dimensions, the Chern number is related to the weighted sum of all the dynamic winding numbers of phase singularity points. This work opens a new avenue to measure topological invariants via time-averaged spin textures without requesting any prior knowledge of the system topology.

In atoms and molecules, electrons screen the nucleus from the external electric field. However, if the frequency of the electric field reaches the energy of atomic or molecular transition, the electric field at the nucleus may be resonantly enhanced by many orders in magnitude. In this paper, we study the mechanisms of screening or enhancement of electric field acting on the nuclei in solid states. We show that in dielectric crystals the phonon oscillations of the lattice play a crucial role for determining the electric field on nuclei within the MHz–THz region. As an application, we propose an experimental scheme for measuring the CP-violating nuclear electric dipole moment in a solid state based on the coherent excitation of acoustic phonons.

We analyze the dynamical nearest-neighbor and next-nearest-neighbor spin correlations in the four-site and eight-site dynamical cluster approximation to the two-dimensional Hubbard model. Focusing on the robustness of these correlations at long imaginary times, we reveal enhanced ferromagnetic correlations on the lattice diagonal, consistent with the emergence of composite spin-1 moments at a temperature scale that essentially coincides with the pseudogap temperature $T^{*}$. We discuss these results in the context of the spin-freezing theory of unconventional superconductivity.

We introduce a theoretical framework for the concept of three-dimensional (3D) twistronics by developing a generalized Bloch band theory for 3D layered systems with a constant twist angle $\theta$ between successive layers. Our theory employs a nonsymmorphic symmetry that enables a precise definition of an effective out-of-plane crystal momentum, and also captures the in-plane moiré pattern formed between neighboring twisted layers. To demonstrate topological physics that can be achieved through 3D twistronics, we present two examples. In the first example of chiral twisted graphite, Weyl nodes arise because of inversion-symmetry breaking, with $\theta$-tuned transitions between type-I and type-II Weyl fermions, as well as magic angles at which the in-plane velocity vanishes. In the second example of a twisted Weyl semimetal, the twist in the lattice structure induces a chiral gauge field $\mathcal{A}$ that has a vortex-antivortex lattice configuration. Line modes bound to the vortex cores of the $\mathcal{A}$ field give rise to 3D Weyl physics in the moiré scale. We also discuss possible experimental realizations of 3D twistronics.

Continuous phase transitions associated with the onset of a spontaneously broken symmetry are thought to be successfully described by the Landau-Ginzburg-Wilson-Fisher theory of fluctuating order parameters. In this work, we show that such transitions can admit new universality classes which cannot be understood in terms of a theory of order parameter fluctuations. We explicitly demonstrate continuous time reversal symmetry breaking quantum phase transitions of $3+1$-D bosonic systems described by critical theories expressed in terms of a deconfined gauge theory with massless Dirac fermions instead of the fluctuating Ising order parameter. We dub such phase transitions “Landau transitions beyond Landau description” (LBL). A key feature of our examples is that the stability of the LBL fixed points requires a crucial global symmetry, which is nonanomalous, unbroken, and renders no symmetry protected topological phase throughout the phase diagram. Despite this, there are elementary critical fluctuations of the phase transition that transform projectively under this symmetry group. We also construct examples of other novel quantum critical phenomena, notably a continuous Landau-forbidden deconfined critical point between two Landau-allowed phases in $3+1$-D.

The spinor condensate with spin states degenerated in the ground spin-space provides a unique platform for investigating the edge of quantum mechanics and statistical physics. We study the evolution of the condensate under the scattering of a coherent atom beam. The time-dependent magnetization, entanglement entropy, statistical entropy, and the entropy production rate are calculated. A novel spontaneous spin symmetry breaking is found during the evolution. It is shown that the stationary spin distribution can be controlled by the coherent spin state of the incident atom beam, therefore the atom-condensate scattering provides a new way to generate designable spin distributions of the condensate.

We present numerical results for finite-temperature $T>0$ thermodynamic quantities, entropy $s\left(T\right)$, uniform susceptibility ${\chi }_0\left(T\right)$, and the Wilson ratio $R\left(T\right)$, for several isotropic $S=1/2$ extended Heisenberg models, which are prototype models for planar quantum spin liquids. We consider in this context the frustrated $J_1\text{−}{J}_2$ model on kagome, triangular, and square lattice, as well as the Heisenberg model on a triangular lattice with the ring exchange. Our analysis reveals that typically in the spin-liquid parameter regimes the low-temperature $s\left(T\right)$ remains considerable, while ${\chi }_0\left(T\right)$ is reduced consistent mostly with a triplet gap. This leads to vanishing $R(T\to 0)$, being the indication of a macroscopic number of singlets lying below triplet excitations. This is in contrast to the $J_1\text{−}{J}_2$ Heisenberg chain, where $R(T\to 0)$ either remains finite in the gapless regime, or the singlet and triplet gap are equal in the dimerized regime.

We analyze a $XXZ$ spin-$1/2$ chain which is driven dissipatively at its boundaries. The dissipative driving is modeled by Lindblad jump operators which only act on both boundary spins. In the limit of large dissipation, we find that the boundary spins are pinned to a certain value and at special values of the interaction anisotropy, the steady states are formed by a rank-2 mixture of helical states with opposite winding numbers. Contrarily to previous stabilizations of topological states, these helical states are not protected by a gap in the spectrum of the Lindbladian. By changing the anisotropy, the transition between these steady states takes place via mixed states of higher rank. In particular, crossing the value of zero anisotropy a totally mixed state is found as the steady state. The transition between the different winding numbers via mixed states can be seen in light of the transitions between different topological states in dissipatively driven systems. The results are obtained by developing a perturbation theory in the inverse dissipative coupling strength and using the numerical exact diagonalization and matrix product state methods.

We measured the reflectivity of the multifold semimetal RhSi in a frequency range from 80 to $20000{\mathrm{cm}}^{-1}$ (10 meV–2.5 eV) at temperatures down to 10 K. The optical conductivity, calculated from the reflectivity, is dominated by the free-carrier (Drude) contribution below 1000 ${\mathrm{cm}}^{-1}$ (120 meV) and by interband transitions at higher frequencies. The temperature-induced changes in the spectra are generally weak: only the Drude bands narrow upon cooling, with an unscreened plasma frequency that is constant with temperature at approximately 1.4 eV, in agreement with a weak temperature dependence of the free-carrier concentration determined by Hall measurements. The interband portion of conductivity exhibits two linear-in-frequency regions below 5000 ${\mathrm{cm}}^{-1}$ ($\sim 600$ meV), a broad flat maximum at around 6000 ${\mathrm{cm}}^{-1}$ (750 meV), and a further increase starting around 10 000 ${\mathrm{cm}}^{-1}$ ($\sim 1.2$ eV). We assign the linear behavior of the interband conductivity to transitions between the linear bands near the band crossing points. Our findings are in accord with the predictions for the low-energy conductivity behavior in multifold semimetals and with earlier computations based on band structure calculations for RhSi.

The phase diagram of the one-dimensional Su-Schrieffer-Heeger model of spinless fermions coupled to quantum phonons is determined by quantum Monte Carlo simulations and differs significantly from previous work. In addition to Luttinger liquid and bond-order-wave (BOW) phases, we find an extended charge-density-wave (CDW) phase. Because of different broken symmetries, BOW and CDW phases are connected by a retardation-driven phase transition. Our results are consistent with the theory of the frustrated $XXZ$ chain, including unconventional power-law exponents at criticality, and with an interpretation in terms of deconfined quantum criticality via proliferation of solitons.

We consider interacting $(2+1)$-dimensional Dirac fermions with competing symmetry-breaking electronic instabilities, as described by relativistic quantum field theories of the Gross-Neveu-Yukawa flavor with anticommuting mass terms. We demonstrate, using a combination of nonperturbative field-theoretical analysis and an adapted quantum Monte Carlo approach, that such systems exhibit a strong-coupling quantum multicritical fixed point with an emerging enhanced symmetry. Moreover, an extended phase coexistence regime expands out from this high-symmetry point. Our results disagree with recent results on the presence of a deconfined quantum criticality in $(2+1)$-dimensional Dirac fermions for the particular case of $O\left(3\right)$ Néel and $Z_2$ Kekulé symmetry-breaking instabilities on the graphene lattice. The robustness of these phenomena with respect to the microscopic symmetries furthermore demonstrates their relevance for a wide range of Dirac materials of current interest, from both theory and ongoing experiments.

We present numerically exact results from sign-problem free quantum Monte Carlo simulations for a spin-fermion model near an O(3) symmetric antiferromagnetic (AFM) quantum critical point. We find a hierarchy of energy scales that emerges near the quantum critical point. At high-energy scales, there is a broad regime characterized by Landau-damped order parameter dynamics with dynamical critical exponent $z=2$, while the fermionic excitations remain coherent. The quantum critical magnetic fluctuations are well described by Hertz-Millis theory, except for a $T^{-2}$ divergence of the static AFM susceptibility. This regime persists down to a lower-energy scale, where the fermions become overdamped and, concomitantly, a transition into a $d$-wave superconducting state occurs. These findings resemble earlier results for a spin-fermion model with easy-plane AFM fluctuations of an O(2) spin density wave (SDW) order parameter, despite noticeable differences in the perturbative structure of the two theories. In the O(3) case, perturbative corrections to the spin-fermion vertex are expected to dominate at an additional energy scale, below which the $z=2$ behavior breaks down, leading to a novel $z=1$ fixed point with emergent local nesting at the hot spots [Schlief et al., Phys. Rev. X 7, 021010 (2017)]. Motivated by this prediction, we also consider a variant of the model where the hot spots are nearly locally nested. Within the available temperature range in our study ($T\ge E_F/200$), we find substantial deviations from the $z=2$ Hertz-Millis behavior, but no evidence for the predicted $z=1$ criticality.