Three-dimensional doped Dirac insulators foster simply connected (in both topological and trivial regimes) and annular (deep inside the topological regime) Fermi surfaces (FSs) in the normal state, and allow on-site repulsions among fermions with opposite spin ($U_1$) and parity ($U_2$) eigenvalues. From an unbiased leading-order (one-loop) renormalization group analysis, controlled by a suitable $\epsilon$ expansion, we show that this system develops a strong propensity toward the nucleation of scalar $s$-wave and odd-parity pseudoscalar $p$-wave pairings, favored by repulsive $U_1$ and $U_2$ interactions, respectively, irrespective of the underlying FS topology. Our results can be pertinent for the observed superconductivity in various doped narrow gap semiconductors, and the theoretical foundation can readily be applied to investigate similar phenomenon in various doped topological materials.

We investigate the impact of optical excitation using two interfering ultrashort pulses on the process of all-optical magnetization reversal. Employing optical time-resolved techniques, we observe distinct differences in the process of light-induced magnetization switching in GdFeCo alloy when triggered with two noncollinear ultrashort pulses as compared to single-pulse excitation. Specifically, we find that the threshold fluence required for magnetization switching is significantly reduced when excited with interfering pulses compared to noninferometric excitation. We further discuss the potential modification of the magnetic dynamics of electrons as a result of optical excitation with interfering light as a possible explanation for the observed trends.

A nonlinearly enhanced electrical reactance $ImZ$ under a large AC current was measured to explore emergent inductors, which constitute a new class of inductors based on the spin-transfer torque effect. A nonlinear $ImZ$ was observed in conducting magnets that contain noncollinear magnetic textures and interpreted as the realization of an inductance due to current-induced spin dynamics. However, curious behavior was concomitantly observed. For instance, the nonlinear $ImZ$ always has a cutoff frequency of ${10}^0-{10}^4\mathrm{Hz}$, which is much lower than the resonance frequency of a ferromagnetic domain wall, $\sim {10}^7\mathrm{Hz}$. Furthermore, the magnitude of $ImZ$ is much greater than that theoretically expected, and the temperature and magnetic field dependences are complicated. This behavior appears to be difficult to understand in terms of the current-induced spin dynamics, and therefore, the earlier interpretation of the nonlinear $ImZ$ should be further verified. Here, we theoretically and experimentally show that time-varying Joule heating and its impact on the AC electrical response can naturally explain these observations. In the experimental approach, we investigate the nonlinear AC electrical response of two conducting materials that exhibit no magnetic order, ${\mathrm{CuIr}}_2{\mathrm{S}}_4$ and $1T^{\prime }\text{−}{\mathrm{MoTe}}_2$. Under time-varying Joule heating, a nonlinearly enhanced $ImZ$ is observed in both systems, verifying the concept of the Joule-heating-induced AC electrical response. We reconsider the nonlinear emergent inductance reported thus far and discover that the Joule-heating-induced AC electrical response approximately reproduces the temperature and magnetic field dependences, cutoff frequency, and magnitude of $ImZ$. Our study implies that the nonlinear $ImZ$ previously observed in conducting magnets that contain noncollinear magnetic textures includes a considerable contribution of the Joule-heating-induced apparent AC impedance.

The field of chiral phonons has developed rapidly in recent years, and two-dimensional materials provide an ideal platform for such research. However, as a pioneer of two-dimensional materials, chiral phonons in the graphene system have not been studied because the combined $\mathcal{PT}$ symmetry suppresses the existence of chiral phonons. In this work, we induce chiral phonons by adsorbing atoms on graphene. These chiral phonons can have quantized pseudoangular momentum at high-symmetry points $K$ and $K^{\prime }$. In addition to phonon chirality, adsorption can also induce nonzero phonon Berry curvature. Under a temperature gradient, the nonequilibrium phonon distribution can produce a total phonon angular momentum and nonlinear phonon Hall current in adsorbed graphene. Moreover, phonon chirality folding in graphene can be achieved through a specific adsorption way, which provides advantages for its Raman detection. Our work reveals the chiral phonon effect induced by adsorption in graphene, paving the way for further exploration of its chiral phonon physics.

We study theoretically a variety of collective quantum phases occurring in frustrated sawtooth chains of Josephson junctions embedded in a dissipationless transmission line. The basic element of a system, i.e., the triangular superconducting cell, contains two 0 and one $\pi$ Josephson junctions characterized by $E_J$ and $\alpha E_J$ Josephson energies, accordingly. In the frustrated regime the low-energy quantum dynamics of a single cell is determined by anticlockwise or clockwise flowing persistent currents (vortex or antivortex). The direct embedding of $\pi$ Josephson junctions in a transmission line allows us to establish a short or long-range interaction between (anti)vortices of well-separated cells. By making use of the variational approach, we map the superconducting circuit Hamiltonian to an effective $XX$ spin model with an exchange spin-spin interaction decaying with the distance $x$ as $x^{-\beta }$, and the local ${\widehat{\sigma }}_{x,n}$ terms corresponding to the coherent quantum beats between vortex and antivortex in a single cell. We obtain that, in long arrays such as $N\gg {\ell }_0\simeq \sqrt{C/C_0}$, where $C$ and $C_0$ are capacitances of a 0 Josephson junction and a transmission line, respectively, the amplitude of quantum beats is strongly suppressed. By means of exact numerical diagonalization, we study the interplay between the coherent quantum beats and the exchange spin-spin interaction leading to the appearance of various collective quantum phases such as the paramagnetic ($P$), compressible superfluid ($CS$), and weakly compressible superfluid ($w\text{−}CS$) states.

We investigate the nonequilibrium dynamics of the magnetization in an Ising chain subjected to a slowly rotating transverse field. The magnetization oscillations are found to be explained by the contributions from different particle excitations in the quantum $E_8$ model. We study the magnetization in the frequency domain in detail, uncovering a series of singular peaks for the $z$ (Ising) component. These singular peaks are split into two sets for the magnetization along the $x$ and $y$ directions with frequency shifts set by the rotational-field frequency. The peaks include both $\delta$-function type and edge-singularity type peaks. The $\delta$-function peaks can be attributed to particle excitations involving an $E_8$ particle with either the vacuum or a different particle. The edge-singularity peaks are contributed by particle excitations of two $E_8$ particles with either the vacuum or another particle, or by particle excitations that contain two sets of two particles with each set including at least a particle of the same type. We propose a Rydberg qubit array for possible experimental investigation.

${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, the first topological insulator with inherent magnetic ordering, has recently attracted significant attention for providing a platform to realize several exotic quantum phenomena at relatively higher temperatures. In this work, we have carried out an exhaustive investigation of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ and Sb-doped ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ thin films using THz time-domain spectroscopy. The extracted real THz conductivity displays a strong IR active ${\mathrm{E}}_u$ phonon absorption peak (at $\sim 1.5$ THz) merged on top of the Drude-like contributions from bulk and surface electrons. The extracted parameters from the THz conductivity data fitted to the Drude-Fano-Lorentz model show significant changes in their temperature dependence around the magnetic ordering Néel temperature, which is suggestive of the coupling between magnetic ordering and electronic band structure. The frequency of the $E_u$ phonon displays an anomalous blueshift with increasing temperatures by $\sim 0.1$ THz ($\sim 7\%$) for ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ and $\sim 0.2$ THz ($\sim 13\%$) for Sb-doped ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ between 7 and 250 K. The line shape of the $E_u$ phonon mode in Sb-doped ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ shows significant Fano asymmetry compared to that of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, indicating that Sb doping plays an important role in the Fano interference between the phonons and the electrons, in this system. These results indicate that the anomalous phonon behavior seen in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ arises mainly from a positive cubic anharmonicity-induced self-energy parameter, whereas both anharmonicity and the electron phonon coupling are at play in making the relatively higher anomalous blueshift of phonons in Sb-doped ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$. Our studies provide a comprehensive understanding of the phonon and electron dynamics of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ and Sb-doped ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ in the THz range using time-domain THz spectroscopy.

Analytical results on the correlation functions of strongly correlated many-body systems are rare in the literature and their importance cannot be overstated. We present determinant representations for the space-, time-, and temperature-dependent correlation functions of the strongly interacting one-dimensional Hubbard model in the presence of an external trapping potential. These representations are exact and valid in both equilibrium and nonequilibrium scenarios like the ones initiated by a sudden change of the confinement potential. In addition, they can be implemented numerically very easily, significantly outperforming other numerical approaches. As applications of our results we investigate the single-particle spectral functions of systems with harmonic trapping and show that dynamical quasicondensation occurs for both fermionic and bosonic spin-$1/2$ systems released from a Mott insulator state.

We explore the topological electronic band structure of the pyrochlore lattice in the strong spin-orbit coupling regime. Using angle-resolved photoemission spectroscopy, first-principles calculations, and symmetry analysis, we have investigated the bulk electronic structure of ${\mathrm{RbBi}}_2$, which has a Bi pyrochlore network. We observe the presence of 3D massless Dirac fermions enforced by nonsymmorphic symmetry, as well as a 3D quadratic band crossing protected by cubic crystalline symmetry. Furthermore, we identify an additional 3D linear Dirac dispersion associated with band inversion protected by threefold rotation symmetry. These observations reveal the rich band topology of itinerant pyrochlore lattice systems in the strong SOC limit and serve as a starting point to explore correlated topological phases in geometrically frustrated lattices.

We present extensive experimental and theoretical research aimed at thoroughly examining changes in the electronic structure of ${\mathrm{Cu}}_2\mathrm{O}$ under the influence of external factors. Hydrostatic pressure and temperature dependencies of optical properties were investigated experimentally using photoreflectance (PR), determining the pressure coefficients of as many as four direct optical transitions at the $\mathrm{\Gamma }$ point. Using state-of-the-art theoretical methods for band-structure prediction (including electron-phonon calculations), we obtained an excellent agreement of theory and experiment for both pressure and temperature properties. After theoretical analysis of excitonic properties, we claim that the change of the binding energy of the Wannier-Mott exciton under pressure turned out to be negligibly small up to applied pressure (17 kbar). We further describe the system in terms of the 12-band $k·p$ Hamiltonian, derived in the invariant expansion form, including the strain part. We find parameter values for the $k·p$ model based on our accurate band-structure calculations, including a set of absolute deformation potentials for band edges. As a result, we deliver a reliable and computationally efficient methodology for semiconductor device modeling.

Commensurability phenomena abound in nature and are typically associated with mismatched lengths, as can occur in quasiperiodic systems. However, not all commensuration effects are spatial in nature. In finite-sized Dirac systems, an intriguing example arises in tilted or warped Dirac cones wherein the degeneracy in the speed of right- and left-moving electrons within a given Dirac cone or valley is lifted. Bound states can be purely fast-moving or purely slow-moving, giving rise to incommensurate energy level spacings and a Vernier spectrum. In this work, we present evidence for this Vernier spectrum in Coulomb blockade measurements of ultraclean suspended carbon nanotube quantum dots. The addition-energy spectrum of the quantum dots reveals an energy-level structure that oscillates between aligned and misaligned energy levels. Our data suggest that the fast- and slow-moving bound states hybridize at certain gate voltages. Thus, gate-voltage tuning can select states with varying degrees of hybridization, suggesting numerous applications based on accessing this isospinlike degree of freedom.

The concepts of topology and geometry are of critical importance in exploring exotic phases of quantum matter. Although they have been investigated on various experimental platforms, to date a direct probe of the topological and geometric properties on a universal quantum computer even for a minimum model is still in vain. In this work, we first show that a density matrix form of the quantum geometric tensor (QGT) can be explicitly reconstructed from Pauli operator measurements on a quantum circuit. We then propose two algorithms suitable for IBM quantum computers to directly probe QGT. The first algorithm is a variational quantum algorithm particularly suitable for noisy intermediate-scale quantum era devices, whereas the second one is a pure quantum algorithm based on quantum imaginary time evolution. Explicit results obtained from IBM Q simulating a Chern insulator model are presented and analyzed. Our results indicate that transmon qubit-based universal quantum computers have the potential to directly simulate and investigate topological and geometric properties of a quantum system.

The emergence of negative longitudinal magnetoresistance in the topologically nontrivial transition-metal dichalcogenide ${\text{PtSe}}_2$is studied. Low-$T$, high-${\mu }_{0}H$ transport is performed for arbitrary field directions, and an analytical framework is established. The source of the negative longitudinal magnetoresistance is identified as the Kondo effect stemming from Pt vacancies contributing an uncompensated spin exclusively at the sample surface. The concentration of vacancies and the sample thickness are identified as tuning parameters. The findings are substantiated by density functional theory, which corroborates the proposed Pt-vacancy model.

We report strain induced modification of the topological surface band structure of layered transition metal dichalcogenide ${\mathrm{NiTe}}_2$, which hosts type-II Dirac points close to the Fermi level and topological surface states originating from band inversions along the $\mathrm{\Gamma }\text{−}A$ direction of the Brillouin zone. By means of first-principles density functional theory calculations, we predict the evolution of the surface states, analyzing their dispersion and spin texture, eventually showing a relevant modulation of their filling as a function of the uniaxial in-plane strain conditions. Synchrotron-based angle-resolved photoemission experiments, using an experimental setup to induce strain in two-dimensional layered materials, demonstrate a clear variation of the spin-polarized topological surface band structure of ${\mathrm{NiTe}}_2$, in agreement with theoretical predictions. Our study suggests the possibility of tuning ${\mathrm{NiTe}}_2$'s topological surface states with external uniaxial strain, leading to further studies on diverse strain conditions and spintronic applications.

Realizing fast energy dissipation in crystalline materials over macroscopic length scales is critical for energy-efficient devices and applications toward a carbon-neutral society but is usually dominated by electron-lattice interactions that cap the energy dissipation at the phonon velocity. Going beyond this velocity has been the focus of many studies, and the physical limit is the Fermi velocity where the energy is predominantly carried away by electrons throughout the materials. However, whether and how the Fermi velocity can be reached over macroscopic distances experimentally remain largely elusive. Here we show ultrafast energy dissipation at the Fermi velocity in the magnetocaloric metal ${\mathrm{LaFe}}_{10.6}{\mathrm{Co}}_{1.0}{\mathrm{Si}}_{1.4}$. Using time-resolved powder x-ray diffraction, we observe negative thermal expansion of the lattice throughout the micron-sized crystals in less than 600 fs with an incident optical fluence higher than 8 $\mathrm{J}{\mathrm{cm}}^{-2}$. The ultrafast timescale is in sharp contrast to the normal energy dissipation and shows the existence of a macroscopic momentum-relaxing electron mean free path immediately after the optical excitation. Our findings open a different regime in energy dissipation and demonstrate the possibility of manipulating macroscopic material properties by strong optical pulses.

This study explored the magnetic properties and dynamics of two, two-dimensional (2D) triangular-lattice Heisenberg antiferromagnets (2D-TLHAF), $h\text{−}(\mathrm{Lu},\mathrm{Y}){\mathrm{MnO}}_3$ and $h\text{−}(\mathrm{Lu},\mathrm{Sc}){\mathrm{FeO}}_3$, through neutron powder diffraction, single-crystal neutron scattering, and polarized neutron-scattering experiments. We identified that the magnetic structures of both materials are described by two irreducible representations ${\mathrm{\Gamma }}_1$ $\left(P{6}_{3}cm\right)$ and ${\mathrm{\Gamma }}_2$ $\left(P{6}_3{c}^{\prime }{m}^{\prime }\right)$ for $h\text{−}{\mathrm{Lu}}_{0.47}{\mathrm{Sc}}_{0.53}{\mathrm{FeO}}_3$, ${\mathrm{\Gamma }}_3$ $\left(P{6}_3^{\prime }c{m}^{\prime }\right)$, and ${\mathrm{\Gamma }}_4$ $\left(P{6}_3^{\prime }{c}^{\prime }m\right)$ for $h\text{−}{\mathrm{Lu}}_{0.3}{\mathrm{Y}}_{0.7}{\mathrm{MnO}}_3$. Polarized neutron scattering has shown that accurately describing the magnetic structures of a 2D triangular-lattice Heisenberg antiferromagnet (2D-TLHAF) requires the use of two irreducible representations, rather than relying on the assumption that the system undergoes spin reorientation when using only one irreducible representation. We then investigated the spin-wave dispersion of both materials on the basis of these magnetic structures. The branch in the lowest energy of the spin wave of $h\text{−}{\mathrm{Lu}}_{0.47}{\mathrm{Sc}}_{0.53}{\mathrm{FeO}}_3$ showed a flat dispersion along the $c$ axis, while ${\mathrm{Lu}}_{0.3}{\mathrm{Y}}_{0.7}{\mathrm{MnO}}_3$ displayed a distinct dispersion along the $c$ axis, suggesting the presence of interplanar interactions in the latter. We discuss the potential causes of spin reorientation and multiferroicity in the triangular antiferromagnetic on the basis of the parameters determined.

Topologically nontrivial patterns, such as quasiordered arrays of magnetic skyrmion bubbles and their electric analogs, have recently received the wide attention of researchers across disciplines. These structures and/or superstructures are considered as potential candidates for technological application, e.g., as the basis for new generations of memory devices. In this case, a manipulation of electric bubbles by external stimuli is of great essence. In the present work, we report ab initio-based simulations of ferroelectric $\mathrm{PbZ}{\mathrm{r}}_{0.4}{\mathrm{Ti}}_{0.6}{\mathrm{O}}_3$ ultrathin films, wherein quasiregimented arrays of electric bubbles in the form of quasihexagonal superstructures are formed under external electric-field bias at low temperatures. The emergence of these superstructures takes place in a range of external electric-field magnitudes and temperatures. The other structures, obtained by varying external electric-field magnitude, include vortex tubes and monodomains. An underlying mechanism of the pattern formation and their properties are discussed, with the primary focus put on the quasihexagonal superstructures. In particular, we find that the formation of these superstructures is associated with three “polarization waves” having reciprocal wave vectors of the same magnitude, and discuss mechanism of the superstructure formation. We also find that the external electric field modifies the average size of individual bubbles and the orientational order of quasihexagonal arrangement, and causes superstructure rotation with three distinct rotation angles identified.

${\mathrm{Al}}_2{\mathrm{O}}_3$, an excellent hydrogen permeation barrier (HPB) at ambient pressure, is also widely used in high-pressure experiments on hydrogen for protecting diamond anvil cells. In this paper, the diffusion behaviors of hydrogen in ${\mathrm{Al}}_2{\mathrm{O}}_3$ under high-pressure and high-temperature conditions are systematically calculated. Since the screw symmetric feature is suppressed by pressure, the hydrogen diffusion energy barriers in high-pressure ${\mathrm{Al}}_2{\mathrm{O}}_3$ with ${\mathrm{Rh}}_2{\mathrm{O}}_3\left(\mathrm{II}\right)$ and ${\mathrm{CaIrO}}_3$ phases are significantly smaller than that of the atmospheric corundum phase. For example, the diffusion energy barrier of ${\mathrm{H}}_2$ in the corundum phase is 2.56 eV at 80 GPa, and it drastically decreases to 0.24 eV in the ${\mathrm{Rh}}_2{\mathrm{O}}_3\left(\mathrm{II}\right)$ phase at 100 GPa. Because of intralayer diffusion behaviors, the hydrogen diffusion energy barriers in the ${\mathrm{CaIrO}}_3$ phase with layered postperovskite (PPv) structure are not sensitive to pressure ranging from 150 to 350 GPa. In this paper, we provide insights into select excellent HPBs under high-pressure and explore element diffusion behaviors in the low-mantle minerals, e.g., PPv-${\mathrm{MgSiO}}_3$.

${\mathrm{EuFe}}_2{\mathrm{As}}_2$, a parent compound of iron pnictide high-$T_{\mathrm{c}}$ superconductor, exhibits a twinned A-type antiferromagnetic order in the $\mathrm{E}{\mathrm{u}}^{2+}$ sublattice below 19 K, which can coexist with superconductivity under element substitution or high pressure. Although there has been considerable research on its static magnetic structures and phase transitions, its corresponding antiferromagnetic dynamics have rarely been explored. Here, we systematically studied magnetic dynamics in this twinned A-type antiferromagnetic single crystal. The left-handed spin-wave, optical spin-wave, and forced ferromagnetic modes can be observed and identified experimentally and further reproduced by micromagnetic simulation and the Landau-Lifshitz model. The experimentally determined exchange field $H_E=4.74$ kOe at 7 K is comparable to the magnetic anisotropy field $H_A=3.74$ kOe, which generates a narrow spin-flip region for one of the orthorhombic twinned antiferromagnetic domains and facilitates forming a magnetic stripe phase under an in-plane field. The identified antiferromagnetic dynamic modes and determined magnetic parameters will not only promote the development of antiferromagnet-based magnon spintronics but also provide an alternative way to reveal and understand the driven force of the magnetic stripe phase usually observed in antiferromagnet-based high-$T_{\mathrm{c}}$ pnictides and cuprates.