The antiferromagnetic system on a rare-earth pyrochlore has been focused as a strong candidate of U(1) quantum spin liquid. Here, we study the phase transitions driven by external magnetic field and discuss the thermal Hall effect due to emergent spinon excitations with staggered gauge fields. Despite the spinons, the charge excitations of the effective action that carry spin-1/2 quantum number, do not couple to the external field, the emergent U(1) gauge field is influenced in the presence of external magnetic field. In particular, along the [111] and [110] directions, we discuss the possible phase transitions between U(1) spin liquids with different gauge fluxes are stabilized in fields. Beyond the cases where the gauge flux per plaquette is fixed to be either 0 or $\pi$, there exists a regime where the staggered gauge fluxes are stabilized without time-reversal symmetry. In such a phase, the thermal Hall conductivity ${\kappa }_{xy}/T\sim 4.6\times {10}^{-3}\text{W}/\left({\text{K}}^2\text{m}\right)$ is expected to be measurable below 1 K.

X-ray synchrotron diffraction (XRD) measurements of single-crystal and powder molybdenum disulfide $\mathrm{Mo}{\mathrm{S}}_2$ are performed at pressures (P) up to 78 GPa and temperatures (T) of 20 to 298 K in diamond-anvil cells. The results on single crystals demonstrate a sharp pressure induced isosymmetric phase transition of $2H_c$ to $2H_a$ modification at 23 GPa at 40 and 300 K. The structure of the high-pressure $2H_a$ phase previously inferred theoretically and from powder XRD data is confirmed by our single-crystal XRD data solution, which also definitively determines the atomic potions as a function of pressure. No additional periodicity (commensurate or incommensurate) or distortion is found in the whole P-T range of this study. These results suggest that a previously proposed hypothetic charge-density-wave phase does not host pressure induced superconductivity experimentally found above 90 GPa.

We investigate the temperature dependences of the magnetic properties, electrical resistivity, magnetoresistance, and Hall effect resistivity, ${\rho }_H\left(H\right)$, in thin films of the Heusler-type ${\mathrm{Ni}}_{47.3}{\mathrm{Mn}}_{30.6}{\mathrm{Ga}}_{22.1}$ (at.%) magnetic shape memory alloys epitaxially grown onto a MgO(001) substrate. The results reveal martensitic transformation at about 230 K, premartensitic transition around 285 K, and the Curie temperature of austenite around 380 K. We obtained the coefficients of normal Hall effect (NHE), $R_0$, and anomalous Hall effect (AHE), $R_s$, by fitting the total Hall resistivity curves ${\rho }_H=R_0{B}_z+4\pi R_s{M}_z$ in several magnetic field ranges (0.1–1, 0–5, 8–16, and 0–16 kOe), using experimental magnetization data. Both coefficients $R_0$ and $R_s$ strongly depend on the magnetic field. We also fit the Hall effect resistivity with the expression ${\rho }_H=R_0{B}_z+4\pi R_s{M}_z+\mathrm{\Delta }{\rho }_H$ using the coefficients $R_0$ and $R_s$ obtained from the high-field interval (8–16 kOe), where the last term, $\mathrm{\Delta }{\rho }_H$, was considered to correspond either to the topological Hall effect or to the antiferromagnetic Hall effect. The obtained temperature dependence and magnitude of $\mathrm{\Delta }{\rho }_H$ discard the presence of the skyrmions or antiskyrmions. We conclude that unconventional field dependences of the NHE and AHE coefficients are produced by the antiferromagnetic correlations and the influence of the magnetic field on the electronic structure.

The mixed-spin Ising-Heisenberg and Heisenberg branched chains whose magnetic backbone consists of regularly alternating spins 1/2 and 5/2, the latter of which are additionally coupled to an extra spin 1/2 providing lateral branching, are investigated using exact analytical and density matrix renormalization group (DMRG) methods. The proposed spin-chain models capture some relevant aspects of the heterotrimetallic coordination polymer $\left[\mathrm{Cu}\mathrm{Mn}\left(\mathrm{L}\right)\right]\left[\mathrm{Fe}\left(\mathrm{bpb}\right){\left(\mathrm{CN}\right)}_2\right]·\mathrm{Cl}{\mathrm{O}}_4·{\mathrm{H}}_2\mathrm{O}$. The mixed spin-$(1/2,5/2,1/2)$ Ising-Heisenberg branched chain is exactly solvable under the assumption of an Ising-like exchange coupling along the chain, while the lateral branching is treated as an anisotropic $XXZ$ Heisenberg exchange interaction. We determine the ground-state phase diagram and quantify a bipartite quantum entanglement between dimers at lateral branching. It is shown that the studied mixed-spin Ising-Heisenberg branched chain accurately fits available experimental data for temperature dependence of the magnetic susceptibility. The ground-state phase diagram of the analogous mixed spin-$(1/2,5/2,1/2)$ Heisenberg branched chain is obtained within the DMRG method. The ground-state phase diagrams of the Ising-Heisenberg and its full Heisenberg counterpart are contrasted. In particular, the ground-state phase diagram of the mixed-spin Heisenberg branched chain involves a special Gaussian critical point, for which a proper finite-size scaling analysis is provided to accurately estimate its location and the correlation length critical exponent.

We present a switching mechanism that utilizes the proximity coupling between the surface spin texture of a Weyl semimetal and a superconductor in a Weyl semimetal-superconductor Weyl semimetal trilayer heterostructure. We show that the superconductivity in the middle layer can be fully suppressed by the surface spin texture of the Weyl semimetals in the presence of an external magnetic field, but it can be recovered again by only changing the field direction. The restoration of the middle-layer superconductivity indicates a sharp transition to a low-resistance state. This sharp switching effect, realizable using a Weyl semimetal because of its strong spin-momentum locking and surface spin polarization, is a promising avenue for novel superconducting spin-valve applications.

Andreev bound states (ABSs), as a close resemblance and a prototype of Majorana bound states, attracted particular interest in semiconductor-superconductor hybrids. So far, ABSs were explored as an equilibrium behavior employing tunneling spectroscopy. Under nonequilibrium, theorists did predict that the local density of states (DOS) of the ABSs split and broaden. We investigated an InAsSb quantum dot coupled to two superconductors to explore the nonequilibrium dynamics of ABSs and their interplay with Kondo effect. Through tunneling spectroscopy, we demonstrated a full picture of the nonequilibrium dynamic splitting and broadening of ABSs, which interplays with the nonequilibrium Kondo DOS. Since the observed features might mimic those in the normal superconductor-topological superconductor or Majorana junctions, our discovery could provide a relevant comparison to benchmark the Majorana signal in future experiments.

Tungsten ditelluride ${\mathrm{WTe}}_2$ is a type-II Weyl semimetal with electronic properties highly sensitive to external pressure, as demonstrated by the superconductivity emerging under pressure. Here we study the optical conductivity of the type-II Weyl semimetal ${\mathrm{WTe}}_2$ under external pressure at room temperature. With increasing pressure, a pronounced spectral weight transfer occurs from the high-energy to the low-energy interband transitions, with drastic changes in the profile of the optical conductivity spectrum indicating a high sensitivity of the electronic band structure to external pressure. The detailed analysis of the pressure-dependent optical conductivity furthermore reveals anomalies at the pressures $\sim 2$ and $\sim 4.5\mathrm{GPa}$, where an electronic and a structural phase transition, respectively, were reported in the literature.

We study the dynamics of a periodically driven Rydberg chain starting from the state with zero Rydberg excitations (vacuum state denoted by $|0\rangle$) using a square pulse protocol in the high drive amplitude limit. We show, using exact diagonalization for finite system sizes $(L\le 26)$, that the Floquet Hamiltonian of the system, within a range of drive frequencies that we chart out, hosts a set of quantum scars that have large overlap with the $|0\rangle$ state. These scars are distinct from their counterparts having high overlap with the maximal Rydberg excitation state $\left(|{\mathbb{Z}}_2\rangle \right)$; they coexist with the latter class of scars and lead to persistent coherent oscillations of the density-density correlator starting from the $|0\rangle$ state. We also identify special drive frequencies at which the system undergoes perfect dynamic freezing, and we provide an analytic explanation for this phenomenon. Finally, we demonstrate that for a wide range of drive frequencies, the system reaches a steady state with subthermal values of the density-density correlator. The presence of such subthermal steady states, which are absent for dynamics starting from the $|{\mathbb{Z}}_2\rangle$ state, imply a weak violation of the eigenstate thermalization hypothesis in finite-sized Rydberg chains distinct from that due to the scar-induced persistent oscillations reported earlier. We conjecture that in the thermodynamic limit, such states may exist as prethermal steady states that show anomalously slow relaxation. We supplement our numerical results by deriving an analytic expression for the Floquet Hamiltonian using a Floquet perturbation theory in the high amplitude limit, which provides an analytic, albeit qualitative, understanding of these phenomena at arbitrary drive frequencies. We discuss experiments that can test our theory.

We study the thickness dependence of the electronic, magnetic, and optical properties of a ${\mathrm{NiPS}}_3$ thin film, which is an antiferromagnetic charge-transfer insulator. Utilizing state-of-the-art advanced density functionals, we find the antiferromagnetic zigzag order, the band gap, and the main peaks in the dielectric tensor are all in good agreement with the corresponding experimental values. Upon thinning, the zigzag antiferromagnetic order becomes virtually degenerate with a competing Néel order, consistent with the suppression of long-range order observed by Raman spectroscopy due to strong magnetic fluctuations. Additionally, due to the robustness of the electronic band gap observed by spectroscopic ellipsometry measurements above $T_N$, we suggest that the persistence of the band gap is driven by strong electronic correlations. Other systematic changes in electronic dispersion, effective mass, and Kerr angle with thickness are also discussed. Finally, an applied external electric field is found to suppress the band gap by up to 13%, until precipitating an insulator-metal transition at a critical field value of $0.7$ eV/Å.

We investigate thermoelectric transport properties of ferromagnetic graphene with CT-invariant quantum spin Hall (CT-QSH) effect. Considering a strong magnetic field, we calculate the charge Seebeck coefficient $S_c$, spin Seebeck coefficient $S_s$, charge Nernst coefficient $N_c$, and spin Nernst coefficient $N_s$ based on the nonequilibrium Green's function and Landauer-Büttiker formula. Due to the coexistence of the CT-QSH and quantum Hall (QH) effects in ferromagnetic graphene, thermoelectric coefficients are divided into the QSH and QH types appearing at the zeroth and nonzero Landau levels, respectively. We find both the charge thermoelectric coefficients are determined by the filling factor ${\nu }_{\sigma }$. The $n\mathrm{th}$ peak heights of the QH-type $N_c$ and $S_c$ satisfy $|S_{c,n}|=N_{c,n}=ln2/\left(\right|n|+\frac{1}{2})$, exhibiting the half-integer QH effect. However, the $m\mathrm{th}$ peak height of the QSH-type $N_c$ satisfies $N_{c,m}=ln2/\left|m\right|$, similar to the integer QH effect. The peak height of $N_s$ remains $N_s=\mathrm{sgn}\left(s\right)2ln2$, and its sign depends on the spin of Landau level, either the QH or QSH type. In addition, the peak height of the QH-type $S_s$ remains $2ln2$. In the clean system, the QSH-type $S_c$ and $S_s$ are zero, while the QSH-type $N_c$ and $N_s$ appear at the zeroth Landau levels, which is different from the zero $N_c$ and $N_s$ in the conventional QSH system. In the presence of disorders, the QH-type thermoelectric coefficients are more robust than the QSH. For the QH-type thermoelectric coefficients, $S_c$ and $S_s$ are more robust than $N_c$ and $N_s$. Notably, the QSH-type $S_c$ and $S_s$ are no longer zero in dirty systems.

Based on the electronic band structure obtained from first-principles Hubbard corrected $U$ density functional theory $(\mathrm{DFT}+U)$ calculations, the optical spectra of yttrium and neodymium nickelates are computed. We show that the results are in fair agreement with available experimental data. We clarify the electronic transitions at the origin of the first two peaks, highlighting the important role of transitions from $t_{2g}$ states neglected in previous models. We discuss the evolution of the optical spectra from small to large rare-earth cations and relate the changes to the electronic band structure.

We study a model for a quantum critical point in two spatial dimensions between a semimetallic phase, characterized by a stable quadratic Fermi node, and an ordered phase, in which the spectrum develops a band gap. The quantum critical behavior can be computed exactly, and we explicitly derive the scaling laws of various observables. While the order-parameter correlation function at criticality satisfies the usual power law with anomalous exponent ${\eta }_{\varphi }=2$, the correlation length and the expectation value of the order parameter exhibit essential singularities upon approaching the quantum critical point from the insulating side, akin to the Berezinskii-Kosterlitz-Thouless transition. The susceptibility, on the other hand, has a power-law divergence with non-mean-field exponent $\gamma =2$. On the semimetallic side, the correlation length remains infinite, leading to an emergent scale invariance throughout this phase.

We study the fate of many-body localization (MBL) in the presence of long-range hopping ($\sim 1/r^{\sigma }$) in a system subjected to an electric field (static and time periodic) along with a slowly varying aperiodic potential. We show that the MBL in the static electric field model is robust against arbitrary long-range hopping, in sharp contrast to other disordered models, where MBL is killed by sufficiently long range hopping. Next, we show that the drive-induced phenomena associated with an ac square wave electric field are also robust against long-range hopping. Specifically, we obtain drive-induced MBL, where a high-frequency drive can convert the ergodic phase into the MBL phase. Remarkably, we find that a coherent destruction of MBL is also possible with the aid of a resonant drive. Thus in both the static and time-periodic square wave electric field models, the qualitative properties of the system are independent of whether the hopping is short range or long range.

We develop a theory of circular photogalvanic effect (CPGE) for classically high photon energies which exceed the electron scattering rate but are small compared to the average electron kinetic energy. In this frequency range one can calculate the CPGE by using two different approaches. In the fully quantum-mechanical approach we find the photocurrent density by applying Fermi's golden rule for indirect intraband optical transitions with virtual intermediate states both in the conduction and valence bands. In the framework of the semiclassical approach, we apply a generalized Boltzmann equation with accounts for the Berry-curvature-induced anomalous velocity, side jumps, and skew scattering. The calculation is carried out for a wurtzite symmetry crystal. Both methods yield the same results for the CPGE current demonstrating consistency between the two approaches and applicability of the semiclassical theory for the description of nonlinear high-frequency transport.

Shot-noise measurements are widely used for the characterization of nonequilibrium configurations in electronic conductors. The recently introduced quantum tomography approach was implemented for the studies of electronic wave functions of few-electron excitations created by periodic voltage pulses in phase-coherent ballistic conductors based on the high-quality GaAs two-dimensional electron gas. Still relying on the manifestation of Fermi correlations in noise, we focus on the simpler and more general approach beneficial for local measurements of energy distribution (ED) in electronic systems with arbitrary excitations with well-defined energies and random phases. Using biased diffusive metallic wire as a test bed, we demonstrate the power of this approach and extract the well-known double-step ED from the shot noise of a weakly coupled tunnel junction. Our experiment paves the way for local measurements of generic nonequilibrium configurations applicable to virtually any conductor.

We theoretically investigate the Kondo effect of a T-shaped triple-quantum-dot structure, by means of the numerical renormalization group method. It is found that the system's entropy and conductance have opportunities to exhibit abundant transition processes for different interdot-coupling manners, with the decrease of temperature. All these results are caused by different Kondo physics mechanisms. Moreover, in the presence of appropriate parameters, the three-stage Kondo effect comes into being. Next when the electron-hole symmetry is broken or the structural parameters are changed, the Kondo resonance can also be observed in the conductance spectrum. However, it shows alternative dependence on the relevant quantities, i.e., the Coulomb interaction and interdot couplings. All these phenomena exhibit interesting Kondo physics in this system. We believe that this work can be helpful for further understanding of the Kondo effect in the triple-quantum-dot structures.

Quantum anomalous Hall effect (QAHE) has been experimentally observed in magnetically doped topological insulators. However, ultralow temperatures (usually below 300 mK), which are mainly attributed to inhomogeneous magnetic doping, become a daunting challenge for potential applications. Here, a nonmagnetic-doping strategy is proposed to produce ferromagnetism and realize QAHE in topological insulators. We numerically demonstrate that magnetic moments can be induced by nonmagnetic nitrogen or carbon substitution in ${\mathrm{Bi}}_2{\mathrm{Se}}_3,$ ${\mathrm{Bi}}_2{\mathrm{Te}}_3,$ and ${\mathrm{Sb}}_2{\mathrm{Te}}_3$, while only nitrogen-doped ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ system can exhibit long-range ferromagnetism and preserve large bulk band gaps. We further show that its corresponding thin film can harbor QAHE at temperatures of 17–29 Kelvin, which is two orders of magnitude higher than typical realized temperatures in similar systems. Our proposed nonmagnetic doping scheme may shed light on experimental realization of high-temperature QAHE in topological insulators.

A comprehensive analytical and numerical study of spin-wave (SW) modes localized on isolated defects in a two-dimensional array of dipolarly coupled magnetic nanodots is presented. Two limiting cases of a defect dot are considered: (i) pointlike defect—a dot having a different eigenfrequency, but the same magnetic moment; (ii) “dipolar defect”—a dot having a different magnetic moment. It is shown, that the appearance of a localized SW mode depends on the peculiarities of the bulk SW spectrum near its bottom or/and top boundaries. In the case of a smooth parabolic spectrum, a localized mode can be created by a defect of a vanishingly small strength, whereas in the case of a spectrum described by a nonanalytic function a localized mode can be created only by a defect of a finite strength, exceeding a certain threshold value. In contrast to the pointlike defects, the dipolar defects may lead to the formation of higher-order localized SW modes. In a case of a complex array having several spin-wave branches (e.g., an array existing in an antiferromagnetic stationary state), the localized SW modes may appear near some (or all) of the SW spectral branches, depending on the structure of the SW spectrum, and on the defect properties.

Superconducting (SC) fluctuations in cuprate superconductors have been extensively studied to gain a deep insight into preformed Cooper pairs above the SC transition temperature $T_{\mathrm{c}}$. While the various measurements, such as the terahertz (THz) optical conductivity, Nernst effect, angle-resolved photoemission spectroscopy (ARPES), and scanning tunneling microscopy (STM) measurements, have provided the signature of the SC fluctuations, the onset temperature of the SC fluctuations depends on the measurement scheme. Here, we shed light on the Higgs mode to investigate the SC fluctuations, as it is the direct fingerprint of SC order parameter and can help elucidate the development of SC phase coherence. We perform THz pump-optical probe spectroscopy for underdoped and overdoped ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{8+x}$ (Bi2212) thin films. The oscillatory behavior in the pump-probe signal (THz Kerr signal) observed in the SC phase has been identified as the Higgs mode in single crystals in our previous work [K. Katsumi et al., Phys. Rev. Lett. 120, 117001 (2018)], but two onset temperatures are identified above $T_{\mathrm{c}}$. Combined with the results of the single crystals in a wide range of doping, we find that the first onset ${T_1}^{\mathrm{ons}}$ is 10–30 K above $T_{\mathrm{c}}$. ${T_1}^{\mathrm{ons}}$ coincides with that of the superfluid density $N_s$ extracted from the THz optical conductivity. Hence, ${T_1}^{\mathrm{ons}}$ is interpreted as the onset of macroscopic SC phase stiffness. On the other hand, the second onset ${T_2}^{\mathrm{ons}}$ is identified at substantially higher than $T_{\mathrm{c}}$, whose origin is discussed in terms of preformed Cooper pairs.