Topological states of matter, such as fractional quantum Hall states, are an active field of research due to their exotic excitations. In particular, ultracold atoms in optical lattices provide a highly controllable and adaptable platform to study such new types of quantum matter. However, finding a clear route to realize non-Abelian quantum Hall states in these systems remains challenging. Here we use the density-matrix renormalization-group (DMRG) method to study the Hofstadter-Bose-Hubbard model at filling factor $\nu =1$ and find strong indications that at $\alpha =1/6$ magnetic flux quanta per plaquette the ground state is a lattice analog of the continuum non-Abelian Pfaffian. We study the on-site correlations of the ground state, which indicate its paired nature at $\nu =1$, and find an incompressible state characterized by a charge gap in the bulk. We argue that the emergence of a charge density wave on thin cylinders and the behavior of the two- and three-particle correlation functions at short distances provide evidence for the state being closely related to the continuum Pfaffian. The signatures discussed in this letter are accessible in current cold atom experiments and we show that the Pfaffian-like state is readily realizable in few-body systems using adiabatic preparation schemes.

In systems in which interactions couple a central degree of freedom and a bath, one would expect signatures of the bath's phase to be reflected in the dynamics of the central degree of freedom. This has been recently explored in connection with many-body localized baths coupled with a central qubit or a single-cavity mode—systems with growing experimental relevance in various platforms. Such models also have an interesting connection with Floquet many-body localization via quantizing the external drive, although this has been relatively unexplored. Here we adapt the multilayer multiconfigurational time-dependent Hartree (ML-MCTDH) method, a well-known tree tensor network algorithm, to numerically simulate the dynamics of a central degree of freedom, represented by a $d$-level system (qudit), coupled to a disordered interacting one-dimensional spin bath. ML-MCTDH allows us to reach $\approx {10}^2$ lattice sites, a far larger system size than what is feasible with exact diagonalization or kernel polynomial methods. From the intermediate time dynamics, we find a well-defined thermodynamic limit for the qudit dynamics upon appropriate rescaling of the system-bath coupling. The spin system shows similar scaling collapse in the Edward-Anderson spin-glass order parameter or entanglement entropy at relatively short times. At longer timescales, we see slow growth of the entanglement, which may arise from dephasing mechanisms in the localized system or long-range interactions mediated by the central degree of freedom. Similar signs of localization are shown to appear as well with unscaled system-bath coupling.

Platinum diselenide (${\mathrm{PtSe}}_2$) is a recently-discovered extrinsic magnet, with its magnetism attributed to the presence of Pt vacancies. The host material to these defects itself displays interesting structural and electronic properties, some of which stem from an unusually strong interaction between its layers. To date, it is not clear how the unique intrinsic properties of ${\mathrm{PtSe}}_2$ will affect its induced magnetism. In this theoretical work, we show that the defect-induced magnetism in ${\mathrm{PtSe}}_2$ thin films is highly sensitive to: (i) defect density, (ii) strain, (iii) layer thickness, and (iv) substrate choice. These different factors dramatically modify all magnetic properties, including the magnitude of local moments, strength of the coupling, and even nature of the coupling between the moments. We further show that the strong interlayer interactions are key to understanding these effects. A better understanding of the various influences on magnetism can enable controllable tuning of the magnetic properties in Pt-based dichalcogenides, which can be used to design novel devices for magnetoelectric and magneto-optic applications.

Analytical solutions based on the Boltzmann transport equation within the relaxation time approximation are developed to relate the Nernst coefficient to materials band structure and relaxation times parameters in simple conductors. The dependence of the Nernst coefficient on the effective mass, anisotropy of bands, energy bandgap, and scattering parameters are investigated. The obtained relations are compared to the existing solutions presented in the past using different approximations.

We report neutron scattering and thermodynamic measurements of the quasi-one-dimensional Ising magnet $\mathrm{Sr}{\mathrm{Co}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ under transverse fields along the tetragonal $a/b$ direction. Our experiments reveal a Néel-type magnetic order in zero field, which successively changes into a coplanar antiferromagnetic order in applied fields. The detailed evolution of the noncollinear magnetic order in magnetic fields can be understood by considering the competition between the intrinsic magnetic interactions, uniform transverse field, and effective staggered field. Moreover, a series of gapped discrete confined-spinon modes are observed in zero field. With increasing field, the spin gap progressively softens, reaching a minimum value at $\sim 7\mathrm{T}$ where the Néel-type magnetic ordering moment is completely suppressed. This corresponds to the three-dimensional quantum critical point (QCP). The implications for multiple QCPs in this class of materials are discussed.

We report the successful synthesis of ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$ single crystals with $x$ ranging from 0 to 1 via a hydrothermal method. A complete phase diagram of ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$ has been obtained based on resistivity and magnetization measurements. The nematicity is suppressed with increasing $x$, and a small superconducting dome appears within the nematic phase. Outside the nematic phase, the superconductivity is continuously suppressed and reaches a minimum $T_{\text{c}}$ at $x$ = 0.45; beyond this point, $T_{\text{c}}$ slowly increases until $x$ = 1. Intriguingly, an anomalous resistivity upturn with a characteristic temperature $T^{*}$ in the intermediate region of $0.31\le x\le 0.71$ is observed. $T^{*}$ shows a domelike behavior with a maximum value at $x$ = 0.45, which is opposite the evolution of $T_{\text{c}}$, indicating competition between $T^{*}$ and superconductivity. The origin of $T^{*}$ is discussed in detail. Furthermore, the normal state resistivity evolves from non-Fermi-liquid to Fermi-liquid behavior with S doping at low temperatures, accompanied by a reduction in electronic correlations. Our study addresses the lack of single crystals in the high-S doping region and provides a complete phase diagram, which will promote the study of relations among nematicity, superconductivity, and magnetism.

We investigate the electronic structure of the $2H$ and $3R$ polytypes of $\mathrm{Nb}{\mathrm{S}}_2$. The Fermi surfaces measured by angle-resolved photoemission spectroscopy show a remarkable difference in size, reflecting a significantly increased band filling in $3R-{\mathrm{Nb}}_{1+x}{\mathrm{S}}_2$ compared to $2H-\mathrm{Nb}{\mathrm{S}}_2$, which we attribute to the presence of additional interstitial Nb, which act as electron donors. Thus, we find that the stoichiometry, rather than the stacking arrangement, is the most important factor in the difference in electronic and physical properties of the two phases. Our high resolution data on the $2H$ phase shows kinks in the spectral function that are fingerprints of the electron-phonon coupling. However, the strength of the coupling is found to be much larger for the the sections of bands with Nb $4d_{x^2-y^2,xy}$ character than for the Nb $4d_{3z^2-r^2}$. Our results provide an experimental framework for interpreting the two-gap superconductivity and latent charge density wave in $2H-\mathrm{Nb}{\mathrm{S}}_2$.

Direct means of measuring the susceptibility toward an octupole order parameter are proposed via a sixth-rank tensor property. Equivalent derivatives of more conventionally measured tensor properties, including elastic stiffness, magnetic susceptibility, and elastoresistivity, are written in full, as constrained by the symmetry of the experimentally motivated $O_h$ point group. For simplicity, we consider the specific case of ${\text{Pr}}^{3+}$ ions in a cubic point symmetry with a ${\mathrm{\Gamma }}_3$ crystal-field ground state, but the ideas are somewhat general. The experimental feasibility of measuring these various derivatives of tensor quantities is discussed.

Confined modes at the edge arbitrarily inclined with respect to optical axes of nonmagnetic anisotropic 2D materials are considered. By developing the exact Wiener-Hopf and approximated Fetter methods we studied edge modes dispersions, field and charge density distributions. The 2D layer is described by the Lorentz-type conductivities in one or both directions, which is realistic for natural anisotropic 2D materials and resonant hyperbolic metasurfaces. We demonstrate that, due to anisotropy, the edge mode exists only at wave vectors exceeding the nonzero threshold value if the edge is tilted with respect to the direction of the resonant conductivity. The dominating contribution to field and charge density spatial profiles is provided by evanescent 2D waves, which are confined both in space near the 2D layer and along the layer near its edge. The degree of field confinement along the layer is determined by wave vector or frequency mismatch between the edge mode and continuum of freely propagating 2D modes. Our analysis is suitable for various types of polaritons (plasmon, phonon, exciton polaritons, etc.) at large enough wave vectors. Thanks to superior field confinement in all directions perpendicular to the edge these modes look promising for modern plasmonics and sensorics.

Nodal-line semimetals are typically characterized by a nodal ring, a closed line of Dirac points in the Brillouin zone, whose topology is characterized by a quantized Berry phase. The nodal loop is protected by the combined inversion and time-reversal ($\mathcal{PT}$) symmetry in the absence of spin-orbit coupling, exhibiting the parity anomaly. Introduction of the $\mathcal{PT}$-break mass term can lead to the transverse Hall response at each point in the nodal ring, but the net Hall current vanishes due to the existence of inversion-symmetry points that contribute to the transverse current with opposite signs. We show that the combination of an inversion-broken tilt term and finite Fermi energy can cause the nonzero anomalous Hall effect. We explore the DC and AC anomalous Hall conductivity in tilted nodal-line semimetals using Kubo formula and focus on the contribution of the free carriers to the Hall conductivity. We find that the interband transition of free carriers dominates the nonvanishing dynamic Hall conductivity over the filled band contribution. The resulting anomalous Hall conductivity exhibits the parity anomaly for the DC case, and rich characteristic frequencies for the AC case which is closely related to the change of the geometry of the Fermi surface. These features may serve as a diagnostic tool to characterize the topological property, tilting parameter, or the radius of the nodal line.

In this Letter, we study the effect of phonons on the position of the $1s$ excitonic resonance of the fundamental absorption transition line in two-dimensional transition metal dichalcogenides. We apply our theory to ${\mathrm{WS}}_2$, a two-dimensional material where the shift in the absorption peak position has been measured as a function of temperature. The theory is composed of two ingredients only: (i) the effect of longitudinal optical phonons on the absorption peak position, which we describe with second-order perturbation theory, and (ii) the effect of phonons on the value of the single-particle energy gap, which we describe with the Huang Rhys model. Our results show excellent agreement with the experimentally measured shift of the absorption peak with the temperature.

We study the thermal transport properties of three ${\mathrm{CaF}}_2$ polymorphs up to a pressure of 30 GPa using first-principles calculations and an interatomic potential based on machine learning. The lattice thermal conductivity $\kappa$ is computed by iteratively solving the linearized Boltzmann transport equation and by taking into account three-phonon scattering. Overall, $\kappa$ increases nearly linearly with pressure, and we show that the recently discovered $\delta$-phase with $P\overline{6}2m$ symmetry and the previously known $\gamma -{\mathrm{CaF}}_2$ high-pressure phase have significantly lower lattice thermal conductivities than the ambient-thermodynamic cubic fluorite ($Fm\overline{3}m$) structure. We argue that the lower $\kappa$ of these two high-pressure phases stems mainly due to a lower contribution of acoustic modes to $\kappa$ as a result of their small group velocities. We further show that the phonon mean free paths are very short for the $P\overline{6}2m$ and $Pnma$ structures at high temperatures, and we use the Cahill-Pohl model to assess the lower limit of thermal conductivity in these domains.

Multimode magnon dynamics affected by demagnetizing fields has been discussed in terms of a Hamiltonian approach. In the multimode dynamics involving magnetization relaxation, inter-mode correlation of magnons is found to be formed spontaneously. The correlation suppresses the transverse magnetization dynamics, giving rise to depolarization correlation (DPC) of magnons. DPC may subsist in equilibrium distribution at finite temperatures.

We demonstrate how fundamental properties of magnons may be manipulated using femtosecond laser pulses by performing ab initio real-time time-dependent density functional theory simulations. To illustrate this, we show how the spin-wave dynamics in ${\mathrm{Fe}}_{50}{\mathrm{Ni}}_{50}$ can be manipulated in three different ways by tailoring the applied laser pulse to excite optically induced intersublattice transfer (OISTR): (1) element-selective destruction of magnon modes depending on the laser intensity, (2) delay-dependent freezing of the magnon mode into a transient noncollinear state (where the delay is in the pulse peak with respect to the start of simulations), and (3) OISTR-driven renormalization of the optical magnon frequency. Harnessing such processes would significantly speed up magnonic devices.

We develop a detailed theory describing a nontrivial interplay between nonequilibrium effects and long-range quantum coherence in superconducting hybrid nanostructures exposed to a temperature gradient. We establish a direct relation between thermoelectric and Josephson effects in such structures and demonstrate that at temperatures exceeding the Thouless energy of our device both phase-coherent thermoelectric signal and the supercurrent may be strongly enhanced due to nonequilibrium low-energy quasiparticles propagating across the system without any significant phase relaxation. By applying a temperature gradient, one can drive the system into a well-pronounced $\pi$-junction state, thereby creating novel opportunities for applications of Andreev interferometers.

${\mathrm{Mn}}_{3}X$ compounds in which the magnetic $\mathrm{Mn}$ atoms form $AB$-stacked kagome lattices have received a tremendous amount of attention since the observation of the anomalous Hall effect in ${\mathrm{Mn}}_3\mathrm{Ge}$ and ${\mathrm{Mn}}_3\mathrm{Sn}$. Although the magnetic ground state has been known for some time to be an inverse triangular structure with an induced in-plane magnetic moment, there have been several controversies about the minimal magnetic Hamiltonian. We present a general symmetry-based model for these compounds that includes a previously unreported interplane Dzyaloshinskii-Moriya interaction, as well as anisotropic exchange interactions. The latter are shown to compete with the single-ion anisotropy which strongly affects the ground state configurations and elementary spin-wave excitations. Finally, we present the calculated elastic and inelastic neutron scattering intensities and point to experimental assessment of the types of magnetic anisotropy in these compounds that may be important.

In a recent paper by Lucas and Das Sarma [Phys. Rev. B 97, 115449 (2018)], a solvable model of collective modes in two-dimensional metals was considered in the hydrodynamic regime. In the current work, we generalize the hydrodynamic theory to three-dimensional (3D) metals for which the calculation of sound modes in a strongly coupled quantum Coulomb plasma can be made explicit. The specific theoretical question of interest is how the usual linearly dispersing hydrodynamic sound mode relates to the well-known gapped 3D plasmon collective mode in the presence of long-range Coulomb interaction. We show analytically that both the zero sound in the collisionless regime and the first sound in the hydrodynamic region become massive in three dimensions, acquiring a finite gap because of the long-range Coulomb interaction, while their damping rates become quadratic in momentum. We also discuss other types of long-range potential, where the dispersion of sound modes is modified accordingly. The general result is that the leading-order hydrodynamic sound mode is always given by the leading-order plasmon frequency in the presence of long-range Coulomb interaction, but the next-to-leading-order dispersion corrections differ in hydrodynamic and collisionless regimes.