In this paper, we introduce the simplest, lowest-order Landau-like potential for ${\mathrm{BiFeO}}_3$ and La-doped ${\mathrm{BiFeO}}_3$ as an expansion around the paraelectric cubic phase in powers of polarization, ${\mathrm{FeO}}_6$ octahedral rotations, and strains. We present an analytical approach for computing the model parameters from density-functional theory. We illustrate our approach by computing the potentials for ${\mathrm{BiFeO}}_3$ and ${\mathrm{La}}_{0.25}{\mathrm{Bi}}_{0.75}{\mathrm{FeO}}_3$ and show that, overall, we are able to capture the first-principles results accurately, including properties that were not considered for the calculation of the model parameters. The computed models allow us to identify and explain the main interactions controlling the relative stability of the competing low-energy phases of these compounds.

The interplay between incommensurability and strong correlations is a challenging open issue. It is explored here via numerical renormalization-group (NRG) study of models of a magnetic impurity in a one-dimensional quasicrystal. The principal goal is to elucidate the physics at the localization transition of the Aubry-André Hamiltonian, where a fractal spectrum and multifractal wave functions lead to a critical Aubry-André Anderson (AAA) impurity model with an energy-dependent multifractal hybridization function. This goal is reached in three stages of increasing complexity: (1) Anderson impurity models with uniform fractal hybridization functions are solved to arbitrarily low temperatures $T$. Below a Kondo temperature, these models approach a fractal strong-coupling fixed point where impurity thermodynamic properties are oscillatory in ${log}_{b}T$ about negative average values determined by the spectrum's fractal dimension $D_F<1$, with $b$ set by the fractal self-similarity near the Fermi energy. (2) An impurity hybridizing uniformly with all conduction states of the critical AAA model is shown to approach the fractal strong-coupling fixed point corresponding to $D_F=0.5$ and $b\simeq 14$. (3) When the multifractal wave functions of the critical AAA model are taken into account, low-$T$ impurity thermodynamic properties are again negative and oscillatory, but with a more complicated structure than in (2). Under sample-averaging, the mean and median Kondo temperatures exhibit power-law dependencies on the Kondo coupling with exponents characteristic of different fractal dimensions. We attribute these signatures to the impurity probing a distribution of fractal strong-coupling fixed points with decreasing temperature. To treat the AAA model, the numerical renormalization group (NRG) is combined with the kernel polynomial method (KPM) to form a general, efficient treatment of hosts without translational symmetry in arbitrary dimensions down to a temperature scale set by the KPM expansion order. Implications of our results for heavy-fermion quasicrystals and other applications of the $\mathrm{NRG}+$KPM approach are discussed.

Topological transitions open a fundamentally new route of research with interesting optical phenomena in photonic metamaterials. In this work, we comprehensively investigate the topology of equifrequency surfaces in magnetized plasma. The full topological phase diagrams are shown and the critical points in the topological transitions are given. The equifrequency surfaces evolve from the closed-form ellipsoid to the open forms type-I or type-II hyperboloid in the topological transitions. Remarkably, a type of nonreciprocal optical isolator is demonstrated utilizing the transformation from topological surface states to leaky modes in magnetized plasma. Our work may provide additional insights into topological wave physics and the novel optical isolator has potential applications in light modulation.

Controlling the interplay between localized spins and itinerant electrons at the oxide interfaces can lead to exotic magnetic states. Here we devise ${\mathrm{SrTiO}}_3/{\mathrm{LaTiO}}_3/{\mathrm{SrTiO}}_3$ heterostructures with varied thickness of the ${\mathrm{LaTiO}}_3$ layer ($n$ monolayers) to investigate the magnetic interactions in the two-dimensional electron gas system. The heterostructures exhibit significant Kondo effect when the ${\mathrm{LaTiO}}_3$ layer is rather thin $(n =2,10)$, manifesting the strong interaction between the itinerant electrons and the localized magnetic moments at the interfaces, while the Kondo effect is greatly inhibited when $n =20$. Notably, distinct Shubnikov–de Haas oscillations are observed and a nonzero Berry phase of π is extracted when the ${\mathrm{LaTiO}}_3$ layer is rather thin $(n =2,10)$, which is absent in the heterostructure with thicker ${\mathrm{LaTiO}}_3$ layer $(n =20)$. The observed phenomena are consistently interpreted as a result of subband splitting and symmetry breaking due to the interplay between the interfacial Rashba spin-orbit coupling and the magnetic orderings in the heterostructures. Our findings provide a route for exploring and manipulating nontrivial electronic band structures at complex oxide interfaces.

We study the energy spectrum and energy levels of the extended Kane-Mele model with magnetic atoms on their zigzag edges. It is demonstrated that the edges of ferromagnetism or antiferromagnetism are enough to break the time-reversal symmetry and host one-dimensional gapped edge states. Thus, a second-order topological phase transition could happen, which leads to the emergence of topological in-gap zero-dimensional corner states. We also prove that the in-gap corner states are robust against corner defects and magnetic disorder. Our proposal based on the edge antiferromagnetism shows that the high-order topological states can be realized, although the net magnetization of the material is zero. In addition, we discuss the influence of spin magnetization orientation on the degeneracy and energy of in-gap corner states.

We evidence the possibility for coherent electrical manipulation of the spin orientation of topologically protected edge states in a low-symmetry quantum spin Hall insulator. By using a combination of ab initio simulations, symmetry-based modeling, and large-scale calculations of the spin Hall conductivity, it is shown that small electric fields can efficiently vary the spin textures of edge currents in monolayer 1T'-${\mathrm{WTe}}_2$ by up to a 90-degree spin rotation, without jeopardizing their topological character. These findings suggest a new kind of gate-controllable spin-based device, topologically protected against disorder and of relevance for the development of topological spintronics.

We study the spreading of quantum correlations and information in a one-dimensional quantum spin chain with critical disorder encoded in an infinite randomness fixed point. Specifically, we focus on the dynamics after a quantum quench of the Rényi entropies, mutual information, and entanglement negativity in the prototypical XXZ spin chain with random bonds and anisotropy parameters. We provide analytic predictions in the scaling regime based on real-space renormalization-group methods. We support these findings through numerical simulations in the noninteracting limit, where we can access the scaling regime.

Dirac point semimetals and related Fermi arc surface states are extremely important in topological electronic systems. In 2021, Chen et al. [Phy. Rev. Lett. 126, 185301 (2021)] proposed that Dirac points can appear on high-symmetry lines (HSLs) or at high-symmetry points (HSPs) in three-dimensional (3D) phonon systems as an extension from Dirac points in electronic systems. Inspired by this work, we present an exhaustive list of Dirac point phonons (DPPs) at HSPs in 230 space groups (SGs) by checking the encyclopedia of emergent particles in 3D crystals. The DPPs are divided into four categories: charge-zero (C-0) DPPs, charge-two (C-2) DPPs, quadratic DPPs (QDPPs), and cubic crossing DPPs (CCDPPs). 29 SGs, 6 SGs, 19 SGs, and 5 SGs are identified as candidate SGs for obtaining C-0 DPPs, C-2 DPPs, QDPPs, and CCDPPs at HSPs, respectively. Importantly, herein we contribute to the realization of materials with DPPs at HSPs and through the following: (i) we propose five realistic materials, namely, $Pna{2}_1$-type ${\mathrm{Li}}_3{\mathrm{AsS}}_3, P{2}_1$-type NiSbS, $P\overline{4}{2}_{1}c$-type ${\mathrm{NaBH}}_4$, and $Pm\overline{3}n$-type ${\mathrm{Ti}}_3\mathrm{Au}$ and ${\mathrm{Ta}}_3\mathrm{Sn}$ as materials with C-0 DPPs, C-2 DPPs, QDPPs, and CCDPPs at HSPs, respectively, based on the first-principles calculations. The surface states for these five materials are also examined in this work. (ii) We present 626 candidates with C-0 DPPs, 183 candidates with C-2 DPPs, 433 candidates with QDPPs, and 102 candidates with CCDPPs at HSPs, which were discovered by checking the phonon dispersions of 10 037 materials listed in the phonon database at Kyoto University. (iii) Due to the materials with SGs 103, 104, 106, 158, 184, 222, 223, 226, and 228 are not included in the phonon database at Kyoto University, we propose 18 candidates with C-0 DPPs, 5 candidates with QDPPs, and 8 candidates with CCDPPs at HSPs with above-mentioned SGs by screening the Inorganic Crystal Structure Database (ICSD). Therefore, our findings can be used to guide research into Dirac points at HSPs in 3D phonon systems.

We present a one-dimensional multicomponent model, known to be partially integrable when restricted to the subspaces made of only two components. By constructing fully antisymmetrized bases, we find integrable excited eigenstates corresponding to the totally antisymmetric irreducible representation of the permutation operator in the otherwise nonintegrable subspaces. We establish rigorously the breakdown of integrability in those subspaces by showing explicitly the violation of the Yang-Baxter equation. We further solve the constraints from the Yang-Baxter equation to find exceptional momenta that allows Bethe ansatz solutions of solitonic bound states. These integrable eigenstates have distinct dynamical consequence from the embedded integrable subspaces previously known, as they do not span their separate Krylov subspaces, and a generic initial state can partly overlap with them and therefore have slow thermalization. However, this novel form of weak ergodicity breaking contrasts with that of quantum many-body scars in that the integrable eigenstates involved do not have necessarily low entanglement. Our approach provides a complementary route to arrive at exact excited states in nonintegrable models: instead of solving towers of single-mode excited states based on a solvable ground state in a nonintegrable model, we identify the integrable eigenstates that survive in a deformation of the Hamiltonian away from its integrable point.

We predict that monolayer ${\mathrm{FeCl}}_2$ is a two-dimensional piezoelectric ferromagnet (PFM) with easy-axis magnetism and a Curie temperature of 260 K. Our ab initio calculations combined with data mining reveal 2H-${\mathrm{FeCl}}_2$ as the only easy-axis 2D monolayer PFM, and that its magnetic anisotropy increases many-fold with moderate hole doping. We develop an analysis based on magnetic anisotropies densities that explain the magnetic and doping-dependent behavior of ${\mathrm{FeCl}}_2$, as well as ${\mathrm{VSe}}_2$ and ${\mathrm{CrI}}_3$, and can enable the design of future 2D magnetically ordered materials.

Inertial effects in spin dynamics are theoretically predicted to emerge at ultrashort time scales, but their experimental signatures are often ambiguous. Here we calculate the spin-wave spectrum in ferromagnets and two-sublattice antiferromagnets in the presence of inertial effects. It is shown how precession and nutation spin waves hybridize with each other, leading to the renormalization of the frequencies, the group velocities, the effective gyromagnetic ratios, and the effective damping parameters. Possible ways of distinguishing between the signatures of inertial dynamics and similar effects explainable within conventional models are discussed.

Antisite disorder plays a significant role in determining the ground state of a magnetic material. The ternary chalcogenide material ${\mathrm{MnSb}}_2{\mathrm{Se}}_4$ with $\sim 26\%$ antisite disorder between Mn and Sb sites is known to be an antiferromagnet ($T_N=22.5\mathrm{K}$) and a type-II mulitferroic. In this paper, we report the effect of increased antisite disorder ($\sim 40\%$) on the magnetic and electrical transport properties of this compound. Magnetic susceptibility and heat-capacity measurements indicate the absence of long-range magnetic ordering. Furthermore, AC susceptibility measurements unfold its nonequilibrium magnetic dynamics at low temperatures. Data analysis with various dynamic scaling models, namely, power law, Arrhenius law, and Vogel-Fulcher confirm a cluster glass state. Moreover, the presence of magnetic memory effect and magnetic relaxation reveal the nonequilibrium dynamics of the system through many metastable states. In addition, a negative colossal magnetoresistance is observed below the freezing temperature due to competing ferromagnetic and antiferromagnetic interactions induced by increased antisite disorder. These results indicate that ${\mathrm{MnSb}}_2{\mathrm{Se}}_4$ is a new addition to a very small group of chalcogenides which show negative colossal magnetoresistance.

Anisotropy in the magnetic moments at the interface of a Co thin film sandwiched by Au layers is determined without thickness-dependent experiment, by using the depth-resolved x-ray magnetic circular dichroism (XMCD) technique, in which the fluorescence soft x rays are collected at different detection angles. By combining the depth-resolved XMCD technique with incidence angle-dependent XMCD sum rules, the spin magnetic moment, perpendicular and in-plane orbital magnetic moments, and perpendicular and in-plane magnetic dipole moments are separately estimated at the interface and inner layers of the Co thin film. It is revealed that the perpendicular orbital magnetic moment is significantly larger than the in-plane one at the interface, while the inner Co layers exhibit a small opposite tendency. It is verified that the interfacial Co has large interfacial anisotropy in the orbital magnetic moment even in the relatively thick-film region.

In the framework of the Landau-Ginzburg-Devonshire (LGD) approach, we studied the possibility of controlling the polarity and morphology of equilibrium domain structures by a homogeneous external electric field in a nanosized ferroelectric core covered with an ultrathin shell of screening charge. Under certain screening lengths and core sizes, the minimum of the LGD energy, which consists of Landau-Devonshire energy, Ginzburg polarization gradient energy, and electrostatic terms, leads to the spontaneous appearance of stable labyrinth domain structures in the core. The labyrinths evolve from an initial polarization distribution consisting of arbitrarily small randomly oriented nanodomains. The equilibrium labyrinth structure is weakly influenced by details of the initial polarization distribution, such that one can obtain a quasicontinuum of nearly degenerate labyrinth structures, whose number is limited only by the mesh discretization density. Applying a homogeneous electric field to a nanoparticle with labyrinth domains, and subsequently removing it, allows inducing changes to the labyrinth structure, as the maze polarity is controlled by a field projection on the particle polar axis. Under specific conditions of the screening charge relaxation, the quasistatic dielectric susceptibility of the labyrinth structure can be negative, potentially leading to a negative capacitance effect. Considering the general validity of the LGD approach, we expect that an electric field control of labyrinth domains is possible in many spatially confined nanosized ferroics, which can be potentially interesting for advanced cryptography and modern nanoelectronics.

Kinks interact with other kinks and antikinks elastically in ferroelastic domain walls and other interfaces in ferroic materials. In thin samples, suitable for transmission electron-microscopy, the interaction is purely dipolar with no indication of monopolar or higher order contributions. Kinks and antikinks attract each other while kink-kink interactions are repulsive. When the kinks are situated in two parallel twin walls, they display the same attraction/repulsion. We argue that this interaction constitutes, part or all, the elusive wall-wall interaction in ferroelastics. The dipolar interactions over distances $d$ between the kinks and between walls decay as $1/d^2$ when the samples have some nanoscale size. Nanoscale samples bend and tilt when kinks are introduced with typical bent regions of some 1 nm and tilt angles of some $1.2^{\circ }$. Multiple kinks will enhance the effect systematically and bent and modulated twin walls are predicted.

We develop the continuous matrix-product states approach for description of inhomogeneous one-dimensional quantum systems with long-range interactions. The method is applied to the exactly solvable Calogero-Moser model. We show the high accuracy of reproducing the ground-state properties of the many-body system and discuss potential errors that can originate from the approximation of the nonlocal interaction potentials with singularities.

Information in a chaotic quantum system will scramble across the system, preventing any local measurement from reconstructing it. The scrambling dynamics is key to understanding a wide range of quantum many-body systems. Here, we use Holevo information to quantify the scrambling dynamics, which shows a phase-transition-like behavior. When applying long random Clifford circuits to a large system, no information can be recovered from a subsystem of less than half the system size. When exceeding half the system size, the amount of stored information grows by two bits of classical information per qubit until saturation through another sharp unanalytical change. We also study critical behavior near the transition points. Finally, we use coherent information to quantify the scrambling of quantum information in the system, which shows similar phase-transition-like behavior.

The interface between a ferro- or ferrimagnetic insulator and a normal metal can support spin currents polarized collinear with and perpendicular to the magnetization direction. The flow of angular momentum perpendicular to the magnetization direction (“transverse” spin current) takes place via spin torque and spin pumping. The flow of angular momentum collinear with the magnetization (“longitudinal” spin current) requires the excitation of magnons. In this article we extend the existing theory of longitudinal spin transport [Bender and Tserkovnyak, Phys. Rev. B 91, 140402(R) (2015)] in the zero-frequency weak-coupling limit in two directions: We calculate the longitudinal spin conductance nonperturbatively (but in the low-frequency limit) and at finite frequency (but in the limit of low interface transparency). For the paradigmatic spintronic material system $\mathrm{YIG}|\mathrm{Pt}$, we find that nonperturbative effects lead to a longitudinal spin conductance that is ca. 40% smaller than the perturbative limit, whereas finite-frequency corrections are relevant at low temperatures $\lesssim 100\mathrm{K}$ only, when only few magnon modes are thermally occupied.