We present the result of an ab initio search for new Dirac materials among inverse perovskites. Our investigation is focused on the less studied class of lanthanide antiperovskites containing heavy $f$-electron elements in the cation position. Some of the studied compounds have not yet been synthesized experimentally. Our computational approach is based on density functional theory calculations which account for spin-orbit interaction and strong correlations of the $f$-electron atoms. We find several promising candidates among lanthanide antiperovskites which host bulk Dirac states close to the Fermi level. Specifically, our calculations reveal massive three-dimensional Dirac states in materials of the class ${\mathrm{A}}_3\mathrm{BO}$, where A=Sm, Eu, Gd, Yb, and B=Sn, Pb. In materials with finite magnetic moment, such as ${\mathrm{Eu}}_3\mathrm{BO}$ (B=Sn, Pb), the degeneracy of the Dirac nodes is lifted, leading to appearance of Weyl nodes.

This theoretical paper offers an explicit expression for the binding energy of excitons in a two-dimensional semiconductor with a flat valence band. The formula has been derived quasiclassically assuming that the exciton is tightly bound; i.e., its ground-state radius is determined by the intrinsic polarizability of the semiconductor rather than by the dielectric properties of the environment. The model is relevant to a few two-dimensional semiconductors discovered recently, including distorted 1T-${\mathrm{TiSe}}_2$, with a supposedly unstable electronic ground state. The valence band flatness reduces the exciton binding energy, which also may have an effect on the phase transition to an excitonic insulator.

Among the various oxide thermoelectric materials, double perovskites provide more flexibility to maneuver interdependent thermoelectric parameters to achieve enhanced thermoelectric figure of merit (ZT), as octahedral ordering, i.e., arrangement of $B^{\prime }{\mathrm{O}}_6$ and $B^{″}{\mathrm{O}}_6$ octahedra, present in the $A_2{B}^{\prime }{B}^{″}{\mathrm{O}}_6$ structure is impacted by cation doping. In this work, we investigated the role of octahedral distortion on thermoelectric properties of $\mathrm{L}{\mathrm{a}}_{2-x}\mathrm{S}{\mathrm{r}}_x\mathrm{CoFe}{\mathrm{O}}_6$ (LSCF) double perovskites with $0.0\le x\le 1.0$, synthesized by the autocombustion route. Rietveld refinement of x-ray diffraction data revealed the phase transition with increasing Sr concentration ($x$) in LSCF from rhombohedral crystal structure with R-3c space group $(x\le 0.6)$ to monoclinic $P{2}_1/n$ ($0.8\le x\le 1.0$) space group. X-ray photoemission spectroscopy analysis further confirmed the presence of multiple oxidation states of Co and Fe, and shifts in oxidation states population driven by Sr content. These multivalent cations participated in the charge transport mechanism, which was explained by the small polaron hopping conduction model in these double perovskites. The electrical conductivity at room temperature was found to be increased by more than ${10}^7$ times in LSCF due to Sr doping, causing a large enhancement in the thermoelectric power factor. Gradual decrease in the octahedral tilt angle with increasing Sr content in LSCF, leading toward the change of crystal structure from disordered (R-3c) to rock-salt-ordered ($P{2}_1/n$) double perovskites, was found to be responsible for the large decrease in activation energy barrier for small polaron hopping conduction in the LSCF system, resulting in the phenomenal increase in electrical conductivity. Maximum thermoelectric figure of merit, $\mathrm{ZT}=0.11$ was obtained at 723 K for $\mathrm{L}{\mathrm{a}}_{2-x}\mathrm{S}{\mathrm{r}}_x\mathrm{CoFe}{\mathrm{O}}_6$ with $x=0.2$ composition.

The paper is concerned with the surface acoustic waves propagating in half-infinite one-dimensional piezoelectric phononic crystals of general anisotropy. The phononic crystal is formed of periodically repeated perfectly bonded layers, and its exterior boundary is one of the layer boundaries. The surface waves occurring in the so-called full stop bands are considered. It appears that the number of surface waves existing within a full stop band for a given layered structure is interrelated with their number in the same stop band for the phononic crystal different from the given one only due to the reversed ordering of layers within a period. A series of statements is proved on the maximum possible number of surface waves per full stop band for both these structures in total, i.e., embracing surface-wave occurrences in either one or the other structure. The analysis is performed for the electrically closed, electrically open, and electrically free types of boundary conditions on the mechanically free crystal surface, in which cases it admits a correspondingly different number of surface waves. A subsidiary instance of mechanically clamped surface is addressed as well. It is observed that the piezoelectric coupling can create new surface waves which disappear when piezoelectric coefficients turn to zero. Besides the general case of arbitrary anisotropy, we also consider two specific situations where the crystallographic symmetry allows the existence of sagittally polarized piezoactive surface waves and of shear horizontally polarized piezoactive surface waves. Numerical examples of surface-wave branches under various boundary conditions are provided.

We study the infinite-$U$ Hubbard model on ladders of two, four, and six legs with nearest- $\left(t\right)$ and next-nearest- $\left(t^{\prime }\right)$ neighbor hoppings by means of the density-matrix renormalization group algorithm. In particular, we analyze the stability of the Nagaoka state for several values of $t^{\prime }$ when we vary the electron density $\rho$ from half filling to the low-density limit. We build the two-dimensional phase diagram, where the fully spin polarized and paramagnetic states prevail. We find that the inclusion of a nonfrustrating next-nearest-neighbor hopping stabilizes the fully spin polarized phase up until $|t^{\prime }/t|=0.5$. Surprisingly, for this value of $t^{\prime }$, the ground state is fully spin polarized for almost any electron density $1\gtrsim \rho \gtrsim 0$, connecting the Nagaoka state to itinerant ferromagnetism at low density. Also, we find that the previously found checkerboard insulator phase at $t^{\prime }=0$ and $\rho =0.75$ is unstable against $t^{\prime }$.

Topological excitations in magnetically ordered systems have attracted much attention lately. We report on topological magnon bands in ferromagnetic Shastry-Sutherland lattices whose edge modes can be put to use in magnonic devices. The synergy of Dzyaloshinskii-Moriya interactions and geometrical frustration are responsible for the topologically nontrivial character. Using exact spin-wave theory, we determine the finite Chern numbers of the magnon bands which give rise to chiral edge states. The quadratic band crossing point vanishes due the present anisotropies, and the system enters a topological phase. We calculate the thermal Hall conductivity as an experimental signature of the topological phase. Different promising compounds are discussed as possible physical realizations of ferromagnetic Shastry-Sutherland lattices hosting the required antisymmetric Dzyaloshinskii-Moriya interactions. Routes to applications in magnonics are pointed out.

Studies of copper-based superconductors demonstrate how their phase diagram becomes more complex as experimental probes improve, able to distinguish among subtly different electronic phases. One of those phases, nematicity, has become a matter of great interest also in iron-based superconductors, where it is detected deep in the tetragonal state. Here, we present the evolution of in-plane Nernst effect anisotropy in strain-detwinned $\mathrm{Ca}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ single crystals. We interpret the observed behavior using an approach developed to describe the nematic order parameter in liquid crystals. We also apply the same model to data from other superconductors, $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ and ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$. We conclude the observed broken rotational symmetry of the electronic system is a consequence of the emerging thermodynamic electronic nematic order at a temperature much higher than the onset of the magnetic and structural transitions.

We find a lattice instability in the superconductor ${\mathrm{KCr}}_3{\mathrm{As}}_3$, corresponding to a distortion of the Cr metallic wires in the crystal structure. This distortion couples strongly to both the electronic and magnetic properties, in particular by making the electronic structure much more nearly one dimensional, and by shifting the compound away from magnetism. We discuss the implications of these results in the context of the possibly unconventional superconductivity of this phase.

In this paper we study the driven critical dynamics in the three-state quantum chiral clock model. This is motivated by a recent experiment [Keesling et al., Nature 568, 207 (2019)], which verified the Kibble-Zurek mechanism and the finite-time scaling in a reconfigurable one-dimensional array of ${}^{87}\mathrm{Rb}$ atoms with programmable interactions. This experimental model has the same universality class as the quantum chiral clock model and has been shown to possess a nontrivial noninteger dynamic exponent $z$. Besides the case of changing the transverse field as realized in the experiment, we also consider the driven dynamics under changing longitudinal field. For both cases, we verify the finite-time scaling for a noninteger dynamic exponent $z$. Furthermore, we determine the critical exponents $\beta$ and $\delta$ numerically. We also investigate the dynamic scaling behavior including the thermal effects, which are inevitably involved in experiments. From a nonequilibrium dynamic point of view, our results strongly support a direct continuous phase transition between the ordered phase and the disordered phase. Also, we show that the method based on the finite-time scaling theory provides a promising approach to determine the critical point and critical properties.

The stationary response of ferromagnetic nanoparticles with mixed uniaxial and cubic anisotropy to superimposed external ac and dc bias fields of arbitrary strength is investigated using Brown's continuous diffusion of orientations model [W. F. Brown Jr., Phys. Rev. 130, 1677 (1963)]. The susceptibility and dynamic magnetic hysteresis (DMH) loops of an individual nanoparticle are calculated in extensive ranges of the superimposed fields, their orientation, damping, temperature, and specific mixed anisotropy parameter. The DMH loops and nonlinear dynamic susceptibility strongly depend on the dc and ac field strengths, the polar angle between the easy axis of the particle and the external field vectors, as well as the temperature and damping, simultaneously displaying a marked dependence on the mixed anisotropy parameter.

We report on the magnetization dynamics of a square array of mesoscopic disks, fabricated from an iron palladium alloy film. The dynamics properties were explored using ferromagnetic resonance measurements and micromagnetic simulations. The obtained spectra exhibit features resulting from the interactions between the disks, with a clear dependence on both temperature and the direction of the externally applied field. We demonstrate a qualitative agreement between the measured and calculated spectra. Furthermore, we calculated the mode profiles of the standing spin waves excited during time-dependent magnetic field excitations. The resulting maps confirm that the features appearing in the ferromagnetic resonance absorption spectra originate from the temperature- and directional-dependent interdisk interactions.

Layered transition-metal compounds have received great attention owing to their physical properties. Here, we present the structural, electronic, thermal, and magnetic properties of $\mathrm{BaM}{\mathrm{n}}_2\mathrm{S}{\mathrm{b}}_2$ and $\mathrm{B}{\mathrm{a}}_2\mathrm{M}{\mathrm{n}}_3\mathrm{S}{\mathrm{b}}_2{\mathrm{O}}_2$ single crystals, both with the layered structure analogous to high-temperature Cu- and Fe-based superconductors. While the Mn moment in the $\mathrm{MnS}{\mathrm{b}}_4$ tetrahedral environment forms a long-range $G$-type antiferromagnetic (AFM) ordering in both $\mathrm{BaM}{\mathrm{n}}_2\mathrm{S}{\mathrm{b}}_2$ ($T_{\mathrm{N}1}\approx 443\mathrm{K}$) and $\mathrm{B}{\mathrm{a}}_2\mathrm{M}{\mathrm{n}}_3\mathrm{S}{\mathrm{b}}_2{\mathrm{O}}_2$ ($T_{\mathrm{N}1}\approx 314\mathrm{K}$), a short-range AFM order is found in the intercalated $\mathrm{Mn}{\mathrm{O}}_2$ layer at a much lower temperature $(T_{\mathrm{N}2}\approx 60\mathrm{K})$ coexisting with long-range $G$-type order in $\mathrm{MnS}{\mathrm{b}}_4$ layer in $\mathrm{B}{\mathrm{a}}_2\mathrm{M}{\mathrm{n}}_3\mathrm{S}{\mathrm{b}}_2{\mathrm{O}}_2$. The directions of the ordered moments in these two magnetic sublattices of $\mathrm{B}{\mathrm{a}}_2\mathrm{M}{\mathrm{n}}_3\mathrm{S}{\mathrm{b}}_2{\mathrm{O}}_2$ are perpendicular to each other, even though the system is electrically conductive. The $G$-type AFM order in $\mathrm{MnS}{\mathrm{b}}_4$ layers in both compounds is distinct from the stripe-like order in Fe-based superconductors, but the in-plane magnetic structure in $\mathrm{Mn}{\mathrm{O}}_2$ layers of $\mathrm{B}{\mathrm{a}}_2\mathrm{M}{\mathrm{n}}_3\mathrm{S}{\mathrm{b}}_2{\mathrm{O}}_2$ is found to be similar to that in the cuprates. Our study reveals the significant and distinct roles of the $\mathrm{Cu}{\mathrm{O}}_2$- and $\mathrm{FeA}{\mathrm{s}}_4$-type transition metal layers in the structure and their magnetic and electronic properties.

In this paper, we conducted various magnetotransport measurements on ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{1-x}{\mathrm{Se}}_x$ single crystals from which excess iron was sufficiently removed. Our results revealed that crossover from the incoherent to the coherent electronic state and the opening of the pseudogap occur at high temperatures ($\approx 150\mathrm{K}$ for $x=0.2$). This is accompanied by a more substantial pseudogap and the emergence of a phase with a multiband nature at lower temperatures (below $\approx 50\mathrm{K}$ for $x=0.2$) before superconductivity sets in. A comparison of these results with those of the as-grown (nonsuperconducting) samples implies that the coherent state accompanied by the pseudogap is needed for the occurrence of superconductivity in this system.

The Sachdev-Ye-Kitaev (SYK) model, in its simplest form, describes $k$ Majorana fermions with random all-to-all four-body interactions. We consider the SYK model in the framework of a many-body Altland-Zirnbauer classification that sees the system as belonging to one of eight (real) symmetry classes depending on the value of $kmod8$. We show that, depending on the symmetry class, the system may support exact many-body zero modes with the symmetries also dictating whether these may have a nonzero contribution to Majorana fermions, i.e., single-particle weight. These zero modes appear in all but two of the symmetry classes. When present, they leave clear signatures in physical observables that go beyond the threefold (Wigner-Dyson) possibilities for level spacing statistics studied earlier. Signatures we discover include a zero-energy peak or hole in the single-particle spectral function, depending on whether symmetries allow or forbid zero modes to have single-particle weight. The zero modes are also shown to influence the many-body dynamics, where signatures include a nonzero long-time limit for the out-of-time-order correlation function. Furthermore, we show that the extension of the four-body SYK model by quadratic terms can be interpreted as realizing the remaining two complex symmetry classes; we thus demonstrate how the entire tenfold Altland-Zirnbauer classification may emerge in the SYK model.

We consider two-dimensional states of matter satisfying a uniform area law for entanglement. We show that the topological entanglement entropy is equal to the minimum relative entropy distance from the reduced state to the set of thermal states of local models. The argument is based on strong subadditivity of quantum entropy. For states with zero topological entanglement entropy, in particular, the formula gives locality of the states at the boundary of a region as thermal states of local Hamiltonians. It also implies that the entanglement spectrum of a two-dimensional region is equal to the spectrum of a one-dimensional local thermal state on the boundary of the region.

Despite the success in describing a range of quantum many-body states using tensor networks, there is a no-go theorem that rules out strictly local tensor networks as topologically nontrivial ground states of gapped parent Hamiltonians with short-range (including exponentially decaying) couplings. In this paper, we show that for free fermions, strictly local tensor networks may describe nonzero temperature averages with respect to gapped Hamiltonians with exponentially decaying couplings. Parent Hamiltonians in this sense may be constructed for any dimensionality and without any obstructions due to their topology. Conversely, we also show that thermal averages with respect to gapped, strictly short-range free-fermion Hamiltonians can be calculated by tensor networks whose links decay exponentially with distance. We also describe a truncation-reconstruction scheme for such tensor networks that leads to a controlled approximation of exact averages in terms of a sequence of related thermal averages. We illustrate our scheme on the two-dimensional Haldane honeycomb model considering both topological and nontopological phases.

We investigate the thermal physics of a Bose-Hubbard model with Rashba spin-orbit coupling starting from a strong coupling mean-field ground state. The essential role of the spin-orbit coupling $\left(\gamma \right)$ is to promote condensation of the bosons at a finite wave vector ${\mathit{k}}_0$. We find that the bosons display either homogeneous or phase-twisted or orbital ordered superfluid phases, depending on $\gamma$ and the interspecies interaction strength $\left(\lambda \right)$. We show that an increase of $\gamma$ leads to suppression of the critical interaction $U_c$ for the superfluid to Mott insulator transition in the ground state and a reduction of the $T_c$ for superfluid to Bose-liquid transition at a fixed interaction strength. We capture the thermal broadening in the momentum distribution function, and the real space profiles of the thermally disordered magnetic textures, including their homogenization for $T\gtrsim T_c$. We provide a Landau theory based description of the ground state phase boundaries and thermal transition scales and discuss experiments which can test our theory.

We present observations of an anisotropic resistance state at Landau-level filling factor $\nu =5/2$ in a two-dimensional hole system (2DHS), which occurs for certain values of hole density $p$ and average out-of-plane electric field $E_{\perp }$. The 2DHS is induced by electric-field effect in an undoped GaAs/AlGaAs quantum well, where front and back gates allow independent tuning of $p$ and $E_{\perp }$, and hence the symmetry of the confining potential. For $p\approx 2\times {10}^{11}{\mathrm{cm}}^{-2}$ and $E_{\perp }\approx -2\times {10}^5$ V/m, the magnetoresistance along $\langle 01\overline{1}\rangle$ greatly exceeds that along $\langle 011\rangle$, suggesting the formation of a quantum Hall nematic or “stripe” phase. Reversing the sign of $E_{\perp }$ rotates the stripes by ${90}^{\circ }$. We suggest this behavior may arise from the mixing of the hole Landau levels and a combination of the Rashba and Dresselhaus spin-orbit coupling effects.

Surface states of topological insulators (TIs) have been playing the central role in the majority of outstanding investigations in low-dimensional electron systems for more than 10 years. TIs based on high-quality strained HgTe films demonstrate a variety of subtle physical effects. The strain leads to a bulk band gap but limits a maximum HgTe strained film thickness, and therefore, the majority of experiments were performed on films with a thickness of less than 100 nm. Since a spatial separation of topological states is crucial for the study of a single-surface response, it is essential to increase the HgTe thickness further. In this work, by combining transport measurements together with capacitance spectroscopy, we perform an analysis of a 200-nm partially relaxed HgTe film. The Drude fit of the classical magnetotransport reveals the ambipolar electron-hole transport with a high electron mobility. A detailed analysis of Shubnikov–de Haas oscillations in both conductivity and capacitance allows us to distinguish three groups of electrons, identified as electrons on top and bottom surfaces and bulk electrons. The indirect bulk energy gap value is found to be close to zero. It is established that the significant gap decrease does not affect the surface states, which are found to be well resolved and spin nondegenerate. The presented techniques allow investigations of other three-dimensional TIs, regardless of the presence of bulk conductivity.