The spin-dependence of the strongly correlated phenomena and many-body interactions play an important role in quantum information science. In particular, it is quite interesting but unclear how the spin degree of freedom ramifies the bandgap renormalization, one of the fundamental many-body phenomena. We report the first room-temperature observation of giant spin-selective bandgap renormalization (SS-BGR) in $\mathrm{Cs}\mathrm{Pb}{\mathrm{Br}}_3$ colloidal nanocrystals using time-resolved circularly polarized femtosecond pump-probe spectroscopy. The SS-BGR results from many-body interactions among carriers with the same spin that renormalize their joint density of states by $57\pm$ 1 meV, visualized here as photoinduced absorption (PIA) below band-edge transition energy when the pump and probe are co-polarized. The hallmark result of spectrally resolved SS-BGR is in stark contrast to the usually submerged signal in II-VI and III-V semiconductors and is 3 orders of magnitude larger than that observed in Ge/SiGe quantum wells, highlighting the unique and beneficial band structure of the metal-halide semiconductors. We propose that the PIA, due to SS-BGR, can be used to describe the spin-polarization and spin-relaxation dynamics. The experimental and theoretical findings open up new possibilities for optical manipulation of spin degrees of freedom and their many-body interactions in metal-halide perovskite nanocrystals for potential room-temperature applications.

We study sample-to-sample fluctuations of the gap ratio in the energy spectra in finite disordered spin chains. The chains are described by the random-field Ising model and the Heisenberg model. We show that away from the ergodic-nonergodic crossover, the fluctuations are correctly captured by the Rosenzweig-Porter (RP) model. However, fluctuations in the microscopic models significantly exceed those in the RP model in the vicinity of the crossover. We show that upon introducing an extension to the RP model, one correctly reproduces the fluctuations in all regimes, i.e., in the ergodic and nonergodic regimes as well as at the crossover between them. Finally, we demonstrate how to reduce the sample-to-sample fluctuations in both studied microscopic models.

By deriving a general framework and analyzing concrete examples, we demonstrate a class of dynamical quantum phase transitions (DQPTs) in one-dimensional two-band systems going through double-quench processes. When this type of DQPT occurs, the Loschmidt amplitude vanishes and the rate function remains singular after the second quench, meaning the final state continually has no overlap with the initial state. This type of DQPT is named metamorphic DQPT to differentiate it from ordinary DQPTs that only exhibit zero Loschmidt amplitude and singular rate function at discrete time points. The metamorphic DQPTs occur at zero as well as finite temperatures. Our examples of the Su-Schrieffer-Heeger (SSH) model and Kitaev chain illustrate the conditions and behavior of the metamorphic DQPT. Since ordinary DQPTs have been experimentally realized in many systems, similar setups with double quenches will demonstrate the metamorphic DQPT. Our findings thus provide additional controls of dynamical evolution of quantum systems.

Starting from a tight-binding model on the kagome lattice near the van Hove filling, the superconducting (SC) properties are investigated self-consistently by using the Bogoliubov–de Gennes equations with the consideration of the inequivalent third-neighbor (TN) bonds. Near the van Hove filling, the most favorable SC pairings are found to derive from the electrons belonging to the same sublattice sites, including the on-site $s$-wave and the spin-singlet/spin-triplet TN pairings. The inequivalent TN bonds will result in multiple SC components with different orbital angular momenta for the TN SC pairings. Whereas the density of states and the temperature ($T$) dependence of the spin-lattice relaxation rate ($T_1^{-1}$) exhibit distinct line shapes in the SC state for the three cases, a peak structure in the $T$ dependence of $T_1^{-1}$ can be found for all of them just below $T_c$ as a result of the van Hove singularity, even though the SC gap has nodes. The effects of magnetic vortices on the low-energy excitations and on the $T$ dependence of $T_1^{-1}$ with the implications of the results are also discussed for all the cases.

In this study, we have prepared single crystals of ytterbium-based layered compounds ${\mathrm{Yb}}_{4}T{\mathrm{Ge}}_8$ ($T=\mathrm{Cr}$, Mn, Fe, Co, and Ni) and investigated their magnetic, thermal, and transport properties systematically. The magnetic properties for all the five compounds show a typical behavior for the Kondo-lattice system at high temperatures and are highly anisotropic due to orbital moments in the degenerate ground state. At low temperatures, an additional magnetic correlation enhances the magnetic susceptibility, and both the heat capacity and the electrical resistivity deviate from the Fermi-liquid-like behavior. Such unconventional behaviors at low temperatures are discussed considering a contribution of strongly correlated characters in $T$ atoms to the conduction band.

In superconductors, vortices induced by a magnetic field are nucleated where some random fluctuations determine the nucleation position, and then may be pinned by impurities or boundaries, impeding the development of vortex-based quantum devices. Here, we propose a superconducting structure, which allows to nucleate and control vortices on-demand by controlling magnetic fields and currents. Using time-dependent Ginzburg-Landau theory, we study a driven vortex motion in two-dimensional Corbino geometries of superconductor-normal metal-superconductor Josephson junctions. We remedy the randomness of nucleation by introducing normal conducting rails to the Corbino disk to guide the nucleation process and motion of vortices towards the junction. We elaborate on the consequences of rail-vortex and vortex-vortex interactions to the quantization of resistance across the junction. Finally, we simulate the nucleations and manipulations of two and four vortices in Corbino networks, and discuss its application to Majorana zero mode braiding operations. Our study provides a potential route towards quantum computation with non-Abelian anyons.

Finding suitable indicators for characterizing quantum phase transitions plays an important role in understanding different phases of matter. It is especially important for fracton phases where a combination of topology and fractionalization leads to exotic features not seen in other known quantum phases. In this paper, we consider the above problem by studying phase transition in the X-cube model in the presence of a nonlinear perturbation. Using an analysis of the ground state fidelity and identifying a discontinuity in the global entanglement, we show there is a first-order quantum phase transition from a type-I fracton phase with a highly entangled nature to a magnetized phase. Accordingly, we conclude that the global entanglement, as a measure of the total quantum correlations in the ground state, can well capture certain features of fracton phase transitions. Then, we introduce a nonlocal order parameter in the form of a foliated operator which can characterize the above phase transition. We particularly show that such an order parameter has a geometric nature which captures specific differences of fracton phases with topological phases. Our study is specifically based on a well-known dual mapping to the classical plaquette Ising model where it shows the importance of such dualities in studying different quantum phases of matter.

Nonlinear optical (NLO) effects have attracted great interest recently. However, most of the computational studies on NLO use the independent particle approximation and ignore many-body effects. Here we develop a generic Green's function framework to calculate the NLO response functions, which can incorporate various many-body interactions. We focus on the electron-phonon coupling and reveal that phonon dressing can make significant impacts on nonlinear photocurrent, such as the bulk photovoltaic (BPV) and bulk spin photovoltaic (BSPV) currents. BPV and BSPV should be zero for centrosymmetric crystals, but when phonons are driven out of equilibrium by, for example, a temperature gradient $\nabla T$, the optical selection rules are altered and phonon-pumped BPV and BSPV currents can be nonzero in nominally centrosymmetric crystal. Moreover, we show that such NLO responses under nonequilibrium phonon dressing can be nonreciprocal, as the direction of the current does not necessarily get reversed when the direction of the temperature gradient is reversed.

Chiral helimagnetic materials have recently attracted much attention for spintronic applications due to their long-range helical magnetic order, topological spin textures, and potential for hosting skyrmions. Their robust spin texture would provide a new concept of ultrafast magnetic memory if it can be controlled by an ultrafast optical pulse. Using time-resolved magneto-optical Kerr spectroscopy, we show that magnetism in the single-crystalline chiral helimagnet ${\text{Cr}}_{1/3}{\text{NbS}}_2$ can be induced by an ultrafast optical pulse. At low temperatures and in the absence of magnetic fields, ${\text{Cr}}_{1/3}{\text{NbS}}_2$ exhibits a chiral helical magnetic phase with a long-range helical spin order, but it contains zero net magnetization. However, after the laser pulse excitation we observe magnetization forming over tens of picoseconds, far exceeding the duration of the laser pulse. We attribute this peculiar behavior to a laser-induced phase transition from the chiral helimagnetic phase, with zero net magnetization, to a chiral conical helimagnetic phase, with finite magnetization. Ab initio density functional calculations provide a detailed microscopic picture of the mechanism behind the observation. The resonant magnon-phonon coupling with specific phonons leads to an elastic deformation of the chiral helimagnetic phase. Finally, at finite magnetic fields, the net magnetization is excited by the laser pulse and precesses around the equilibrium position, leading to anomalous magnetization precession as a function of the external magnetic field at specific magnetic phases.

The coupling of electrons and phonons is governed wisely by the symmetry properties of the crystal structures. In particular, for two-dimensional (2D) systems, it has been suggested that the electrons do not couple to phonons with pure out-of-plane distortion, as long as there is a ${\sigma }_h$ symmetry. We show that such a statement is correct when constituents of the unit-cell layer are only located in the ${\sigma }_h$ symmetric plane; a prominent example of such a system is graphene. For those 2D crystals in which atoms are vertically located away from the horizontal symmetric plane (e.g., 1H transition metal dichalcogenides), acoustic flexural modes do not couple to the electrons up to linear order, while optical flexural phonons, which preserve ${\sigma }_h$ symmetry, do couple with the electrons. Our conclusions are supported by an analytic argument together with numerical calculations using density functional perturbation theory.

In analogy to the Su-Schrieffer-Heeger (SSH) model, one-dimensional (1D) electromagnetic (EM) crystals can exhibit nontrivial topological properties. When a nontrivial EM crystal is in contact with its trivial counterpart, a topologically protected interface state is formed. While much attention has been focused on a single interface state, multiple interface states can interact collectively when suitable conditions are met, giving rise to nontrivial band topologies resembling the standard SSH model. Here, we study the topological properties of 1D metallic superlattices that support multiple interface states. We first demonstrate that a single interface state exists at the boundary between two topologically distinct metallic arrays that carry Bloch-like surface plasmon polaritons. Such an interface state is then used as the building block for further constructing the superlattices. By exploiting the separation between two dimerized interface states and the distinct inter- and intrainterface configurations to facilitate different intracell and intercell interactions, we vary the band topology of the superlattices. More importantly, a new superlattice interface state is formed when trivial and nontrivial superlattices are brought together. The superlattice interface state is found to have smaller angular divergence and longer localization length than its single counterpart, thus is desired for robust signal transmission and a high finesse cavity.

We investigate the potential of supervised machine learning to propagate a quantum system in time. While Markovian dynamics can be learned easily, given a sufficient amount of data, non-Markovian systems are nontrivial and their description requires the memory knowledge of past states. Here we analyze the feature of such memory by taking a simple 1D Heisenberg model as many-body Hamiltonian, and construct a non-Markovian description by representing the system over the single-particle reduced density matrix. The number of past states required for this representation to reproduce the time-dependent dynamics is found to grow exponentially with the number of spins and with the density of the system spectrum. Most importantly, we demonstrate that neural networks can work as time propagators at any time in the future and that they can be concatenated in time forming an autoregression. Such neural-network autoregression can be used to generate long-time and arbitrary dense time trajectories. Finally, we investigate the time resolution needed to represent the system memory. We find two regimes: For fine memory samplings the memory needed remains constant, while longer memories are required for coarse samplings, although the total number of time steps remains constant. The boundary between these two regimes is set by the period corresponding to the highest frequency in the system spectrum, demonstrating that neural network can overcome the limitation set by the Shannon-Nyquist sampling theorem.

The weak ergodicity breaking induced by quantum many-body scars (QMBSs) represents an intriguing concept that has received great attention in recent years due to its relation to unusual nonequilibrium behavior. Here, we reveal that this phenomenon can occur in a particular regime of a lattice gauge theory, where QMBSs emerge due to the presence of an extensive number of local constraints. In particular, by analyzing the gauged Kitaev model, we provide an example where QMBSs appear in a regime where charges are deconfined. By means of both numerical and analytical approaches, we find a variety of scarred states far away from the regime where the model is integrable. The presence of these states is revealed both by tracing them directly from the analytically reachable limit, as well as by quantum quenches showing persistent oscillations for specific initial states.

Thermal expansion, lattice dynamics, heat capacity, compressibility, and pressure stability of the intermetallic clathrate ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$ have been investigated by a combination of first-principles calculations and experimentation. Direct comparison of the properties of ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$ with those of the low-density elemental modification ${\mathrm{Si}}_{136}$ provide insight into the effects of filling the silicon clathrate framework cages with Na on these properties. Calculations of the phonon dispersion only yield sensible results if the Na atoms in the large cages of the structure are displaced from the cage centers, but the exact nature of off-centering is difficult to elucidate conclusively. Pronounced peaks in the calculated phonon density of states for ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$, absent for ${\mathrm{Si}}_{136}$, reflect the presence of low-energy vibrational modes associated with the guest atoms, in agreement with prior inelastic neutron-scattering experiments and reflected in marked temperature dependence of the guest atom atomic displacement parameters determined by single-crystal x-ray diffraction. The bulk modulus is only weakly influenced by filling the Si framework cages with Na, whereas the phase stability under pressure is significantly enhanced. The room-temperature linear coefficient of thermal expansion (CTE) is nearly a factor of 3 greater for ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$ compared to ${\mathrm{Si}}_{136}$. Negative thermal expansion (NTE), observed in ${\mathrm{Si}}_{136}$ below 100 K, is noticeably absent in ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$. In contrast to ${\mathrm{Si}}_{136}$, the thermal expansion behavior in ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$ is relatively well described by the conventional Grüneisen-Debye model in the temperature range of 10–700 K. First-principles calculations in the quasiharmonic approximation correctly predict an increase in high-temperature CTE with Na loading, although the increase is less than observed in experiment. The calculations also fail to capture the absence of NTE in ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$, perhaps due to anharmonic effects and/or inadequateness of the ordered structural model.

We introduce a translational and rotational invariant local representation for vector fields, which can be employed in the construction of machine learning energy models of solids and molecules. This allows us to describe, on the same footing, the energy fluctuations due to the atomic motion, the longitudinal and transverse excitations of the vector field, and their mutual interplay. The formalism can then be applied to physical systems where the total energy is determined by a vector density, as in the case of magnetism. Our representation is constructed over the power spectrum of the combined angular momentum describing the local atomic positions and the vector field, and it can be used in conjunction with different machine learning schemes and data taken from accurate ab initio electronic structure theories. We demonstrate the descriptive power of our representation for a range of classical spin Hamiltonian and machine learning algorithms. In particular, we construct energy models based on both linear Ridge regression, as in conventional spectral neighbor analysis potentials, and the Gaussian approximation. These are both built to represent a Heisenberg-type Hamiltonian including a longitudinal energy term and spin-lattice coupling.

The electronic structure, phonons, electron-phonon coupling, and superconductivity are theoretically studied in the noncentrosymmetric superconductor $\mathrm{Th}\mathrm{Co}{\mathrm{C}}_2$ as a function of pressure in the pressure range 0 to 20 GPa. We find that the electronic band splitting induced by the spin-orbit coupling is enhanced under pressure. In spite of the overall stiffening of the crystal lattice, the electron-phonon coupling constant $\lambda$ increases with pressure, from 0.583 at 0 GPa to 0.652 at 20 GPa. If the isotropic Eliashberg electron-phonon coupling theory is used to simulate the effect on the critical temperature $T_c$, such an increase in $\lambda$ results in a substantial increase of $T_c$ from 2.5 K at 0 GPa to 4 K at 20 GPa. This shows that examining the effect of pressure offers a chance to resolve the pairing mechanism in $\mathrm{Th}\mathrm{Co}{\mathrm{C}}_2$.

We report a high-pressure study on the antiperovskite phosphide superconductor $\mathrm{Sr}{\left({\mathrm{Pt}}_{0.9}{\mathrm{Pd}}_{0.1}\right)}_3\mathrm{P}$. The superconducting transition temperature $T_{\mathrm{c}}$ decreases with a negative differential pressure coefficient, $d\ln T_{\mathrm{c}}/dP\approx \text{–}0.052\mathrm{G}{\mathrm{Pa}}^{\text{–}1}$ $(d{T}_{\mathrm{c}}/dP\approx \text{–}0.33\mathrm{K}/\mathrm{GPa}$ for $P<10\mathrm{GPa})$, which is a very high value for antiperovskite superconductors. We find that the upper critical field ${\mu }_0{H}_{\mathrm{c}2}\left(0\right)$ also decreases with increasing pressure, but the ${\mu }_0{H}_{\mathrm{c}2}\left(0\right)/k_{\mathrm{B}}{T}_c$, e.g., $2{\mathrm{\Delta }}_0{\left[\pi N\left(E_F\right)\right]}^{1/2}/k_{\mathrm{B}}{T}_{\mathrm{c}}$, is doubled from ∼0.5–0.7 T/K at ambient pressure to ∼1.2 T/K at 31.7 GPa. Our comparative studies on the dependences of $T_{\mathrm{c}}$ as functions of physical pressure versus chemical doping support that $\mathrm{Sr}{\left({\mathrm{Pt}}_{0.9}{\mathrm{Pd}}_{0.1}\right)}_3\mathrm{P}$ belongs to a phonon-mediated Bardeen-Cooper-Schrieffer-type superconductor with enhanced superconducting coupling strengths. The reduction of $T_{\mathrm{c}}$ with pressure should mainly originate from the decrease of the density of states at the Fermi level and the phonon-hardening effect. Our results shed some light on understanding the superconducting mechanisms of Pt-based antiperovskite superconductors.

We study numerically and analytically the dynamics of a single directed elastic string driven through a three-dimensional disordered medium. In the quasistatic limit the string is super-rough in the direction of the driving force, with roughness exponent ${\zeta }_{\parallel }=1.25\pm 0.01$, dynamic exponent $z_{\parallel }=1.43\pm 0.01$, correlation-length exponent $\nu =1.33\pm 0.02$, depinning exponent $\beta =0.24\pm 0.01$, and avalanche-size exponent ${\tau }_{\parallel }=1.09\pm 0.03$. In the transverse direction we find ${\zeta }_{\perp }=0.5\pm 0.01, z_{\perp }=2.27\pm 0.05$, and ${\tau }_{\perp }=1.17\pm 0.06$. Our results show that transverse fluctuations do not alter the critical exponents in the driving direction, as predicted by the planar approximation (PA) proposed by Ertas and Kardar (EK) [Phys. Rev. B 53, 3520 (1996)]. We check the PA for the measured force-force correlator, comparing to the functional renormalization-group and numerical simulations. Both random-bond (RB) and random-field (RF) disorder yield a single universality class, indistinguishable from the one of an elastic string in a two-dimensional random medium. While relations $z_{\perp }=z_{\parallel }+1/\nu$ and $\nu =1/(2-{\zeta }_{\parallel })$ of EK are satisfied, the transversal movement is that of a Brownian, with a clock set locally by the forward movement. This implies ${\zeta }_{\perp }=(2-d)/2$, distinct from EK. Finally, at small driving velocities the distribution of local parallel displacements has a negative skewness, while in the transverse direction it is a Gaussian. For large scales, the system can be described by anisotropic effective temperatures defined from generalized fluctuation-dissipation relations. In the fast-flow regime the local displacement distributions become Gaussian in both directions and the effective temperatures vanish as $T_{\mathtt{eff}}^{\perp }\sim 1/v$ and $T_{\mathtt{eff}}^{\parallel }\sim 1/v^3$ for RB disorder and as $T_{\mathtt{eff}}^{\perp }\approx T_{\mathtt{eff}}^{\parallel }\sim 1/v$ for RF disorder.