We investigate spin-dependent conductance across a magnetic tunnel junction (MTJ) including a ferromagnetic insulating barrier. The MTJ consists of two half-metallic ferromagnetic ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3$ ($\mathrm{LSMO}$) manganites as electrodes and ${\mathrm{La}}_2{\mathrm{NiMnO}}_6$ ($\mathrm{LNMO}$) double perovskite as a ferromagnetic insulating barrier. The resistance of the junction is strongly dependent not only on the orientation of the magnetic moments in $\mathrm{LSMO}$ electrodes, but also on the direction of the magnetization of the $\mathrm{LNMO}$ barrier with respect to that of $\mathrm{LSMO}$. The ratio of tunnel magnetoresistance reaches a maximum value of 24% at $10\mathrm{K}$, and it decreases with temperature until it completely disappears above the critical temperature of $\mathrm{LNMO}$ at $280\mathrm{K}$. The tunneling process is described using a mechanism which involves both empty and filled $e_g$ states of the $\mathrm{LNMO}$ barrier acting as a spin filter. A magnetic insulating barrier is an interesting path for achieving room temperature magnetoresistance in oxide-based heterostructures.

Ever since it was found that $\mathrm{Mo}{\mathrm{B}}_2$ holds the second-highest superconductive transition temperature ($T_c$, 32 K) amongst all known diborides, the argument on the donation of its two phases, the high-pressure $\alpha \text{−}\mathrm{Mo}{\mathrm{B}}_2$ phase (hexagonal, $P6$/mmm) and the low-pressure β-$\mathrm{Mo}{\mathrm{B}}_2$ (rhombohedral, $R\text{−}3m$) has never stopped, because the phase-transition pressure is approximately 50 GPa higher than the pressure that superconductivity takes place. We simulated the phase transition and the possible superconductive properties of the two phases, and found that the ordered stacking arrangement ensures the p-d hybridization near the Fermi level and induces significant electron-phonon coupling, which actually influences the superconductivity. Therefore, the $\alpha \text{−}\mathrm{Mo}{\mathrm{B}}_2$ is the only phase that could be a superconductor. With our simulation, instead of further increasing, the $T_c$ of $\mathrm{Mo}{\mathrm{B}}_2$ would decrease slowly after reaching the maximum with a $d$ $T_c$/dp of −0.014 K/GPa, primarily due to the expected decrease and saturation of the electron-phonon coupling constant (λ) mainly contributed by Mo, along with the higher-frequency moments provided by B atoms under higher pressure. Therefore, in a certain transition-metal boride, it is not only the relatively high-frequency moments induced by B atoms, but also the stacking structure of them drives the electrons around transition-metal core in good symmetry and strengthens electron-phonon coupling, which leads to good superconductivity. Therefore, to manipulate boron-layer stacking arrangements might be an important angle to design and develop cutting-edge superconducting borides.

The honeycomb magnet ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ has recently been argued to realize an approximate hidden SU(2) symmetry that can be understood by means of a duality transformation. Using large-scale classical Monte Carlo simulations, we study the finite-temperature phase diagram of the pertinent Heisenberg-Kitaev-$\mathrm{\Gamma }\text{−}{\mathrm{\Gamma }}^{\prime }$ model near the hidden-SU(2)-symmetric point, in the presence of a six-spin ring exchange perturbation. At low temperatures, the model features collinear single-$\mathbf{q}$ zigzag and noncollinear triple-$\mathbf{q}$ ground states, depending on the sign of the ring exchange coupling. We show that in the vicinity of the hidden-SU(2)-symmetric point, the magnetic long-range orders melt in two stages. The corresponding finite-temperature transitions are continuous and fall into two-dimensional (2D) Ising and 2D Potts universality classes, respectively. The two fluctuation-induced phases at intermediate temperatures spontaneously break spin rotational and lattice translational symmetries, respectively, but both leave time-reversal symmetry intact. They are characterized by finite expectation values of a real, symmetric, traceless, second-rank tensor and are naturally understood as vestigial orders of the underlying magnetic states. We identify these vestigial orders as ${\mathbb{Z}}_3$ spin nematic and ${\mathbb{Z}}_4$ spin current density wave phases, respectively. For increasing ring exchange perturbations, the width of the vestigial phases decreases, eventually giving rise to a direct first-order transition from the magnetically ordered phase to the disordered paramagnet. We propose the ${\mathbb{Z}}_4$ spin current density wave phase, which is the vestigial phase of the primary triple-$\mathbf{q}$ magnetic order, as a natural candidate for the paramagnetic 2D long-range-ordered state observed in ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ in a small window above the antiferromagnetic ordering temperature.

We study a recently discussed XX spin chain with nonlocal dephasing [Y.-P. Wang et al., arXiv:2310.03069] in a steady-state boundary-driven setting, confirming superdiffusive magnetization transport in the thermodynamic limit. The emergence of superdiffusion is rather interesting as the Lindblad operators causing it are a coherent sum of two terms, each of which would separately cause diffusion. One therefore has a quantum phenomenon where a coherent sum of two diffusive terms results in superdiffusion. We also study perturbations of the superdiffusive model, finding that breaking the exact form of dissipators, as well as adding interactions to the XX chain, results in superdiffusion changing into diffusion.

We consider a Su-Schrieffer-Heeger (SSH) chain to each end of which we attach a semi-infinite undimerized chain (lead). We study the effect of the openness of the SSH model on its properties. A representation of the infinite system using an effective Hamiltonian allows us to examine its low-energy states in more detail. We show that, as one would expect, the edge states of the topological phase hybridize as the coupling between the systems is increased. As this coupling grows, these states are gradually suppressed, disappearing as the coupling goes to infinity. In the topologically trivial phase, in which no edge states exist in the isolated system, the opposite behavior is observed: as the coupling grows, a new type of edge state gradually emerges. These new states, referred to as phase-inverted edge states, are localized low-energy modes very similar to the edge states of the topological phase. Interestingly, localization occurs on a new shifted interface, moving from the first (and last) site to the second (and second to last) site. This suggests that the topology of the system is strongly affected by the leads, with three regimes of behavior. For very small coupling, the system is in a well-defined topological phase; for very large coupling, it is in the opposite phase; in the intermediate region, the system is in a transitional regime.

The control of the phonon-wave behavior is of prominent interest for applications in thermal management and the processing of quantum information. In this work, we design and characterize a phonon-based grating nanostructure in a suspended single-layer graphene. The anomalous transmission of lattice waves and the controllable angle of the transmitted wave can be achieved via a skillful design of the underlying nanostructure and the mode conversion can be monitored in situ. An interesting nonlocal effect emerges due to the inherently strong covalent bondings. These features distinctly differ from former scenarios with acoustic waves. This work paves the way for the regulation of phonon transport by designing phonon-based metagrating structures.

The paramagnetic phase diagrams of the half-filled Hubbard model on a twisted bilayer square lattice are investigated using the coherent potential approximation. In addition to the conventional metallic, band insulating, and Mott insulating phases, we find two site-selective insulating phases where certain sites exhibit band insulating behaviors while the others display Mott insulating behaviors. These phases are identified by the band gap, the double occupancy, the density of states, as well as the imaginary part of the self-energy. Furthermore, we examine the effect of on-site potential on the stability of the site-selective insulating phases. Our results indicate that fruitful site-selective phases can be engineered by twisting.

Signatures of the excitonic insulator in the mass-imbalance extended Falicov-Kimball model with the electron-phonon correlation are investigated. Based on the unrestricted Hartree-Fock approximation, we derive a set of self-consistent equations specifying the excitonic insulator order parameter and then construct the complex phase diagram of the excitonic insulator states. For a given intermediate Coulomb interaction, the excitonic insulator states at nearly zero temperature are found if the electronic hybridization-phonon coupling is sufficiently large. The window of the excitonic insulator states expands as suppression of the mass anisotropy. The influence of the mass imbalance and electron-phonon correlations on the nature of the BCS-BEC crossover of the condensation states is also addressed in the signatures of the optical conductivity. The preformed excitonic bound states in the normal semiconducting side are inspected in the features of the imaginary part of the dynamical excitonic susceptibility function. By increasing the electronic hybridization-phonon coupling or suppressing the mass imbalance, we find the dominance of the excitonic coherent bound states. The compact influence of the mass imbalance and electron-phonon correlations on the whole picture of the excitonic insulator state thus has been discussed.

The study of frustration-free Hamiltonians and their relation to finite bond-dimension matrix product states (MPS) has a long tradition. However, fractional quantum Hall (FQH) states do not quite fit into this theme since the known MPS representations of their ground states have infinite bond dimensions, which considerably obscures the relations between such MPS representations and the existence of frustration-free parent Hamiltonians. This is related to the fact that the latter necessarily are of infinite range in the orbital basis. Here, we present a theorem tailored to establishing the existence of frustration-free parent Hamiltonians in such a context. We explicitly demonstrate the utility of this theorem in the context of non-Abelian Moore-Read FQH states but argue the applicability of this theorem to transcend considerably beyond the realm of conformal-field-theory-derived MPSs or quasi-one-dimensional Hilbert spaces.

Spontaneous symmetry breaking is a foundational concept in physics. In condensed matter, it characterizes conventional continuous phase transitions but is absent at topological phase transitions such as the Berezinskii–Kosterlitz–Thouless (BKT) transition—as in the BKT case the expected norm (i.e., the magnitude) of the $U\left(1\right)$ order parameter vanishes in the thermodynamic limit at all nonzero temperatures. Phenomena consistent with low-temperature broken symmetry have been observed, however, in many different BKT experiments. Examples include recent experiments on superconducting films and the seminal work on two-dimensional arrays of Josephson junctions. While the inaccessibility of the above thermodynamic limit partially explains this paradox in finite systems, the full dynamical framework of symmetry breaking at the BKT transition remains unresolved. Here we provide this by introducing the broader concept of general symmetry breaking. This encompasses both spontaneous symmetry breaking and the BKT case by allowing the expected norm of the order parameter to go to zero in the thermodynamic limit, provided its directional phase fluctuations are asymptotically smaller. We demonstrate this asymptotically slow directional mixing in the low-temperature BKT phase. This explicitly shows that the order parameter arbitrarily chooses some well-defined direction in the thermodynamic limit, predicting negligible phase fluctuations compared to the expected norm in arbitrarily large experimental BKT systems. Our results provide a model for directional mixing timescales across the diverse array of experimental BKT systems. We suggest various experiments.

We develop the theoretical formalism and study the formation of valley trions in two-dimensional monolayers within the framework of a nonrelativistic potential model using the method of hyperspherical harmonics (HH) in four-dimensional space. We present the solution of the three-body Schrödinger equation with the Rytova-Keldysh (RK) potential by expanding the wave function of a trion in terms of the HH. We consider a long-range approximation when the RK potential is approximated by the Coulomb potential and a short-range limit when this potential is approximated by the logarithmic potential. In a diagonal approximation, the coupled system of differential equations for the hyperradial functions is decoupled in both limits. Our approach yields the analytical solution for binding energy and wave function of trions in the diagonal approximation for these two limiting cases: the Coulomb and logarithmic potentials. We obtain exact analytical expressions for eigenvalues and eigenfunctions for negatively and positively charged trions. The corresponding eigenvalues can be considered as the lower and upper limits for the trions binding energies. We apply the proposed theoretical approach to describe trions in transition metal dichalcogenides (TMDC) and address the energy difference between the binding energies of $X^{-}$ and $X^{+}$ in TMDC. Results of numerical calculations for the ground state energies with the RK potential are in good agreement with similar calculations and in reasonable agreement with experimental measurements of trion binding energies.

We present a perturbative approach to disordered systems in one spatial dimension that accesses the full range of phase disorder and clarifies the connection between localization and phase information. We consider a long chain of identically disordered scatterers and expand in the reflection strength of any individual scatterer. As an example application, we show analytically that in a discrete-time quantum walk, the localization length can depend nonmonotonically on the strength of phase disorder (whereas expanding in weak disorder yields monotonic decrease). More generally, we obtain to all orders in the expansion a particular nonseparable form for the joint probability distribution of the transmission coefficient logarithm and reflection phase. Furthermore, we show that for weak local reflection strength, a version of the scaling theory of localization holds: the joint distribution is determined by just three parameters.

We propose to realize the nonvolatile four-state antiferromagnetic (AFM) skyrmions in multiferroic oxide system with underlying asymmetry. Subsequently, through strain-mediated magnetic phase engineering, we find that AFM skyrmions can be obtained in bulk $\mathrm{PbV}{\mathrm{O}}_3$. Moreover, we also explore possible existence of AFM skyrmions in the $\mathrm{SrTi}{\mathrm{O}}_3/\mathrm{PbV}{\mathrm{O}}_3$ ($\mathrm{PbTi}{\mathrm{O}}_3/\mathrm{PbV}{\mathrm{O}}_3$) heterostructures via ab initio calculations, where AFM skyrmions can still survive. Eventually, we realize four-state AFM skyrmions (P↑,L↓), (P↑,L↑), (P↓,L↓), and (P↓,L↑) with different chirality induced by polarization (P) switching and polarity arising from magnetization (L) reversal in multiferroic $\mathrm{PbV}{\mathrm{O}}_3$. In this letter, we provide an avenue toward electric-field control of AFM skyrmions in multiferroic oxides and offer perspectives for designing AFM skyrmion-based spintronic devices.

Surface Dirac cones in three-dimensional topological insulators have generated tremendous and enduring interest for almost two decades owing to hosting a multitude of exotic properties. In this work, we unveil the existence of two types of anomalous surface Dirac cones in three-dimensional Dirac semimetals. These surface Dirac cones are located at the surfaces perpendicular to the rotation symmetry axis and are found to display a number of features remarkably different from that in topological insulators. The most prominent one is the absence of a singular Dirac node. In addition, the spin textures of these nodeless surface Dirac cones are found to exhibit a unique two-phase-angle dependence, leading to the presence of two different winding numbers in the orbital-resolved spin textures, which is rather different from the well-known spin-momentum locking in topological insulators. Despite the absence of a Dirac node, we find that the two types of surface Dirac cones are also characterized by quantized $\pi$ Berry phases, even though one of them takes a quadratic dispersion. In the presence of time-reversal-symmetry-breaking fields, we find that the responses of the surface and bulk Dirac cones display an interesting bulk-surface correspondence. The uncovering of these nodeless surface Dirac cones broadens our understanding of the topological surface states and bulk-boundary correspondence in Dirac semimetals and also lays down the basis for studying unconventional Dirac physics.

By an exact analytical approach we study the magnetothermal transport in the spin-$\frac{1}{2}$ easy-axis Heisenberg model, in particular the thermal conductivity and spin Seebeck effect as a function of anisotropy, magnetic field, and temperature. We stress a distinction between the common spin Seebeck effect with fixed boundary conditions and the one (intrinsic) with open boundary conditions. In the open boundary spin Seebeck effect we find exceptional features at the critical fields between the low field antiferromagnetic phase, the gapless one, and the ferromagnetic at high fields. We further study the development of these features as a function of easy-axis anisotropy and temperature. We point out the potential of these results to experimental studies in spin chain compounds—candidates for spin current generation in the field of spintronics.

We suggest an explanation based on the Blume-Capel model of why some layered compounds of the iron-intercalated transition metal dichalcogenides ${\mathrm{TaS}}_2\left({\mathrm{Se}}_2\right)$ exhibit spin-glass behavior, while another group of this family demonstrates low-temperature paramagnetism. In these materials, the doped Fe atoms either substitute the Ta atoms with losing their magnetic moments or sit between the ${\mathrm{TaS}}_2\left({\mathrm{Se}}_2\right)$ layers keeping their spin states. The Blume-Capel model allows us to introduce a chemical potential to control a balance of the intercalated elements of both types. The Ghatak-Sherrington theory of spin-glass behavior of this model predicts an existence of a tricritical point that means that there is a concentration threshold of Fe ions retaining their magnetic moments, above which spin-glass ordering occurs. Below the threshold, Fe ions behave as independent paramagnetic centers. We build temperature dependencies of magnetic susceptibility and field dependencies of magnetization to highlight specific features of the model related with a variable content of Fe ions in the high-spin state. A specific crystal structure of the layered transition metal dichalcogenides gives an opportunity to increase the concentration of ions with nonzero magnetic moments by co-intercalating non-Kramers $3d$ ions into the van der Waals gaps. This process may trigger spin-glass ordering in the initially paramagnetic Fe-doped ${\mathrm{TaS}}_2\left({\mathrm{Se}}_2\right)$ polytype complexes.

Topological charge pumping occurs in the adiabatic limit, and the nonadiabatic effect due to finite driving velocity reduces the pumping efficiency and leads to deviation from quantized charge pumping. In this paper, we discuss the relation between this deviation from quantized charge pumping and the entanglement generation after a pumping circle. In the simplest setting, we show that the purity $\mathcal{P}$ of the half-system reduced density matrix is equal to $\mathcal{R}$, defined as ${(1-\kappa )}^2+{\kappa }^2$, where $\kappa$ denotes the pumping efficiency. In generic situations, we argue that $\mathcal{P}<\mathcal{R}$ and the pumping efficiency can provide an upper bound for purity and, therefore, a lower bound for generated entanglement. To support this conjecture, we propose a solvable pumping scheme in the Rice-Mele-Hubbard model, which can be represented as a brick-wall-type quantum circuit model. With this pumping scheme, numerical calculation of charge pumping only needs to include at most six sites, and therefore the interaction and the finite-temperature effects can be both included reliably in the exact diagonalization calculation. The numerical results using the solvable pumping circle identify two regimes where the pumping efficiency is sensitive to driving velocity and support the conjecture $\mathcal{P}<\mathcal{R}$ when both interaction and finite-temperature effects are present.