We performed broadband optical transmission measurements of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and In-doped ${\left({\text{Bi}}_{1-x}{\text{In}}_x\right)}_2{\text{Se}}_3$ thin films, where in the latter the spin-orbit coupling (SOC) strength can be tuned by introducing In. Drude and interband transitions exhibit In-dependent changes that are consistent with evolution from the metallic $(x=0)$ to insulating $(x=1)$ nature of the end compounds. Most notably, an optical absorption peak located at $\hslash \omega =1\mathrm{eV}$ in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is completely quenched at $x=0.06$, the critical concentration where the phase transition from TI into non-TI takes place. For this $x$, the surface state (SS) has vanished from the band structure as well. The correlation between the 1 eV optical peak and the SS in the $x$ dependencies suggests that the peak is associated with the SS. We further show that when ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is electrically gated, the 1 eV peak becomes stronger (weaker) when electron is depleted from (accumulated into) the SS. These observations combined together demonstrate that under the $\hslash \omega =1\mathrm{eV}$ illumination electron is excited from a bulk band into the topological surface band of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. The optical population of the surface band is of significant importance not only for fundamental study but also for TI-based optoelectronic device application.

Using Green's function of semi-infinite Weyl semimetals, we present the quantum theory of the collective charge dynamics of Fermi arcs. We find that the Fermi arc plasmons in undoped Weyl semimetals are linearly dispersing gapless plasmon modes. The gaplessness comes from proper consideration of the deep penetration of surface states near the end of the Fermi arcs into the interior of the Weyl semimetal. The linear dispersion—rather than square root dispersion of pure 2D electron systems with extended Fermi surface—arises from the strong anisotropy introduced by the Fermi arc itself, due to which the continuum of surface particle-hole (PH) excitations in this system will have a strong resemblance to a stack of one-dimensional (1D) electron systems. This places the Fermi arc electron liquid in between the 1D and 2D electron liquids. Contamination with the bulk PH excitations in the doped case gives rise to the damping of the Fermi arc sounds.

We study the formation of bound states in a one-dimensional, single-component Fermi chain with attractive interactions. The phase diagram, computed from DMRG (density matrix renormalization group), shows not only a superfluid of paired fermions (pair phase) and a liquid of three-fermion bound states (trion phase), but also a phase with two gapless modes. We show that the latter phase is described by a two-component Tomonaga-Luttinger liquid (TLL) theory, consisting of one charged and one neutral mode. We argue based on our numerical data, that the single, pair, and trion phases are descendants of the two-component TLL theory. We speculate on the nature of the phase transitions amongst these phases.

From the dynamics of a broad class of classical mean-field glass models, one may obtain a quantum model with finite zero-temperature entropy, a quantum transition at zero temperature, and a time-reparametrization (quasi)invariance in the dynamical equations for correlations. The low-eigenvalue spectrum of the resulting quantum model is directly related to the structure and exploration of metastable states in the landscape of the original classical glass model. This mapping reveals deep connections between classical glasses and the properties of SYK-like models.

We propose a realization of a two-dimensional higher-order topological insulator with ultracold atoms loaded into orbital angular momentum (OAM) states of an optical lattice. The symmetries of the OAM states induce relative phases in the tunneling amplitudes that allow to describe the system in terms of two decoupled lattice models. Each of these models displays one-dimensional edge states and zero-dimensional corner states that are correlated with the topological properties of the bulk. We show that the topologically nontrivial regime can be explored in a wide range of experimentally feasible values of the parameters of the physical system. Furthermore, we propose an alternative way to characterize the second-order topological corner states based on the computation of the Zak's phases of the bands of first-order edge states.

Recent realizations of intrinsic, long-range magnetic orders in two-dimensional (2D) van der Waals materials have ignited tremendous research interest. In this work, we employ the XXZ Heisenberg model and Monte Carlo simulations to study a fundamental property of these emerging 2D magnetic materials, the Curie temperature ($T_c$). By including both on-site and neighbor couplings extracted from first-principles simulations, we have calculated $T_c$ of monolayer chromium trihalides and $\mathrm{C}{\mathrm{r}}_2\mathrm{G}{\mathrm{e}}_2\mathrm{T}{\mathrm{e}}_6$, which are of broad interest currently, and the simulation results agree with available measurements. We also clarify the roles played by anisotropic and isotropic interactions in deciding $T_c$ of magnetic orders. Particularly, we find a universal, linear dependence between $T_c$ and magnetic interactions within the parameter space of realistic materials. With this linear dependence, we can predict $T_c$ of general 2D lattice structures, omitting the Monte Carlo simulations. Compared with the widely used Ising model, mean-field theory, and spin-wave theory, this work provides a convenient and quantitative estimation of $T_c$, giving hope to speeding up the search for novel 2D materials with higher Curie temperatures.

We investigated the self-induced inverse spin Hall effect in ferromagnets. Temperature ($T$), thickness ($t$), and angular-dependent measurements of transverse voltage in spin pumping experiments were performed with permalloy films. Results revealed nonmonotonous $T$ dependence of the self-induced transverse voltage. Qualitative agreement was found with first-principles calculations unravelling the skew scattering, side-jump, and intrinsic contributions to the $T$-dependent spin Hall conductivity. Experimental data were similar whatever the material in contact with permalloy (oxides or metals), and revealed an increase of produced current with $t$, demonstrating a bulk origin of the effect.

We report magnetotransport measurements of the critical field behavior of thin Al films deposited onto multiply connected substrates. The substrates were fabricated via a standard electrochemical process that produced a triangular array of 66-nm-diameter holes having a lattice constant of 100 nm. The critical field transition of the Al films was measured near $T_c$ as a function of field orientation relative to the substrate normal. With the field oriented along the normal ($\theta =0$), we observe reentrant superconductivity at a characteristic matching field $H_m=0.22$ T, corresponding to one flux quantum per hole. In tilted fields, the position $H^{*}$ of the reentrance feature increases as $sec\left(\theta \right)$, but the resistivity traces are somewhat more complex than those of a continuous superconducting film. We show that when the tilt angle is tuned such that $H^{*}$ is of the order of the upper critical field $H_c$, the entire critical region is dominated by the enhanced dissipation associated with a submatching perpendicular component of the applied field. At higher tilt angles a local maximum in the critical field is observed when the perpendicular component of the field is equal to the matching field.

We present a materials informatics approach to search for superconducting hydrogen compounds, which is based on a genetic algorithm and a genetic programing. This method consists of five stages: (i) collection of physical and chemical property data, (ii) development of superconductivity predictor based on the collected data by a genetic programing, (iii) prediction of potential candidates for high temperature superconductivity by regression analysis, (iv) crystal structure search of the candidates by a genetic algorithm, and (v) validation of the superconductivity by first-principles calculations. By repeatedly performing the process as (i) $\to$ (ii) $\to$ (iii) $\to$ (iv) $\to$ (v) $\to$ (i) $\to \cdots$, the database and predictor are further improved, which leads to an efficient search for superconducting materials. Using the first-principles data of binary hydrogen compounds, many of which have not been experimentally realized yet, we applied this method to hypothetical ternary ones and predicted ${\mathrm{KScH}}_{12}$ with a modulated hydrogen cage showing the superconducting critical temperature of 122 K at 300 GPa and ${\mathrm{GaAsH}}_6$ showing 98 K at 180 GPa.

We carried out a phase-sensitive experiment on Josephson trijunctions constructed on the surface of three-dimensional topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. Through local flux contol and contact resistance measurement, we observed that the minigap at the ends of the trijunctions approaches zero linearly with varying phase difference and closes completely at $\pi$ phase difference. The minigap at the center of the trijunction is protected to close over extended regions in phase space. These results agree with the theory by Fu and Kane [Phys. Rev. Lett. 100, 096407 (2008)].

Quantum dynamics of strongly correlated systems is a challenging problem. Although the low energy fractional excitations of one-dimensional integrable models are often well understood, exploring quantum dynamics in these systems remains challenging in the gapless regime, especially at intermediate and high energies. Based on the algebraic Bethe ansatz formalism, we study spin dynamics in a representative one-dimensional strongly correlated model, i.e., the antiferromagnetic spin-$\frac{1}{2}$ XXZ chain with the Ising anisotropy, via the form-factor formulas. Various excitations at different energy scales are identified crucial to the dynamic spin structure factors under the guidance of sum rules. At small magnetic polarizations, gapless excitations dominate the low energy spin dynamics arising from the magnetic-field-induced incommensurability. In contrast, spin dynamics at intermediate and high energies is characterized by the two- and three-string states, which are multiparticle excitations based on the commensurate Néel ordered background. Our work is helpful for experimental studies on spin dynamics in both condensed matter and cold atom systems beyond the low energy effective Luttinger liquid theory. Based on an intuitive physical picture, we speculate that the dynamic feature at high energies due to the multiparticle antibound state excitations can be generalized to nonintegrable spin systems.

Features of low-dimensional magnetism resulting from a square-net arrangement of Co atoms in trirutile ${\mathrm{CoTa}}_2{\mathrm{O}}_6$ are studied in the present work by means of density functional theory and are compared with the experimental results of specific heat and neutron diffraction. The small total energy differences between the ferromagnetic (FM) and antiferromagnetic (AFM) configuration of ${\mathrm{CoTa}}_2{\mathrm{O}}_6$ shows that competing magnetic ground states exist, with the possibility of transition from FM to AFM phase at low temperature. Our calculation further suggests the semiconducting behavior for ${\mathrm{CoTa}}_2{\mathrm{O}}_6$ with a band gap of $\sim 0.41$ eV. The calculated magnetic anisotropy energy is $\sim 2.5$ meV with its easy axis along the [100] (in-plane) direction. Studying the evolution of magnetism in ${\mathrm{Co}}_{1-x}{\mathrm{Mg}}_x{\mathrm{Ta}}_2{\mathrm{O}}_6$ ($x=0$, 0.1, 0.3, 0.5, 0.7, and 1), it is found that the sharp AFM transition exhibited by ${\mathrm{CoTa}}_2{\mathrm{O}}_6$ at $T_N=6.2$ K in its heat capacity vanishes with Mg dilution, indicating the obvious effect of weakening the superexchange pathways of Co. The current specific heat study reveals the robust nature of $T_N$ for ${\mathrm{CoTa}}_2{\mathrm{O}}_6$ in applied magnetic fields. Clear indication of short-range magnetism is obtained from the magnetic entropy, however, diffuse components are absent in neutron diffraction data. At $T_N, {\mathrm{CoTa}}_2{\mathrm{O}}_6$ enters a long-range ordered magnetic state which can be described using a propagation vector, ($\frac{1}{4}\frac{1}{4}0$). Upon Mg dilution at $x\ge 0.1$, the long-range ordered magnetism is destroyed. The present results should motivate an investigation of magnetic excitations in this low-dimensional anisotropic magnet.

Magneto-optical imaging of Nb rings, open rings, and strips reveals that the topology affects the dendritic flux avalanches in the samples. In particular, dendrites crossing the entire width of the sample appear only in the rings. Such dendrites appear when the difference between the applied field and the average field inside the central hole reaches a certain threshold level $\mathrm{\Delta }{H}_{\mathrm{th}}$. With increasing applied field, this condition is reached quasiperiodically as a crossing dendrite creates momentarily a hot channel through which flux flows into the central hole, balancing the field inside the hole with that outside the ring. The threshold $\mathrm{\Delta }{H}_{\mathrm{th}}$ differs in magnitude from the onset field $H_{\mathrm{th}}$ of magnetic instability, and exhibits qualitatively different dependence on the rim width.

We compute the partition function of a massive free boson in a square lattice using a tensor network algorithm. We introduce a singular value decomposition of continuous matrices that leads to very accurate numerical results. It shows the emergence of a corner double line fixed-point structure. In the massless limit, we reproduce the results of conformal field theory including a precise value of the central charge.

The out-of-time-ordered correlator (OTOC) is central to the understanding of information scrambling in quantum many-body systems. In this work, we show that the OTOC in a quantum many-body system close to its critical point obeys dynamical scaling laws which are specified by a few universal critical exponents of the quantum critical point. Such scaling laws of the OTOC imply a universal form for the butterfly velocity of a chaotic system in the quantum critical region and allow one to locate the quantum critical point and extract all universal critical exponents of the quantum phase transitions. We numerically confirm the universality of the butterfly velocity in a chaotic model, namely, the transverse axial next-nearest-neighbor Ising model, and show the feasibility of extracting the critical properties of quantum phase transitions from OTOC using the Lipkin-Meshkov-Glick model.

The optical response of (InGa)(AsSb)/GaAs quantum dots (QDs) grown on GaP (001) substrates is studied by means of excitation and temperature-dependent photoluminescence (PL), and it is related to their complex electronic structure. Such QDs exhibit concurrently direct and indirect transitions, which allows the swapping of $\mathrm{\Gamma }$ and $L$ quantum confined states in energy, depending on details of their stoichiometry. Based on realistic data on QD structure and composition, derived from high-resolution transmission electron microscopy (HRTEM) measurements, simulations by means of $\mathbf{k}·\mathbf{p}$ theory are performed. The theoretical prediction of both momentum direct and indirect type-I optical transitions are confirmed by the experiments presented here. Additional investigations by a combination of Raman and photoreflectance spectroscopy show modifications of the hydrostatic strain in the QD layer, depending on the sequential addition of QDs and capping layer. A variation of the excitation density across four orders of magnitude reveals a 50-meV energy blueshift of the QD emission. Our findings suggest that the assignment of the type of transition, based solely by the observation of a blueshift with increased pumping, is insufficient. We propose therefore a more consistent approach based on the analysis of the character of the blueshift evolution with optical pumping, which employs a numerical model based on a semi-self-consistent configuration interaction method.

Experimental and theoretical studies are performed on ${\mathrm{La}}_3{\mathrm{Ir}}_3{\mathrm{O}}_{11}$, an iridate hosting a $+4.33$ fractional valence state for Ir ions and a three-dimensional frustrated structure composed of edge-shared ${\mathrm{Ir}}_2{\mathrm{O}}_{10}$ dimers. These features are expected to enhance intersite hoppings and reduce magnetic moments of Ir ions. However, a spin-orbit driven Mott insulating transport is observed, which is supported by our first-principles calculations. Most importantly, geometrical frustration and competing interactions result in a non-Curie-Weiss paramagnetic ground state, revealing no magnetic order down to 2 K. This unusual state is further demonstrated by a theoretical modeling process, suggesting a possible candidate for the spin-liquid state.

In this ab initio study, we investigate the initial stages of small polaron formation across four metal peroxides. By separating out electronic and lattice energy contributions to the formation of electron polarons, we find both large and small formation barriers directly correlated with an electronic relaxation delay. Further analysis of the electronic structure evolution during polaron formation, within the constraints set by the generalized form of Koopmans' theorem, reveals that hybridization between the polaron anchoring orbital and the conduction band minimum determines the electronic relaxation delay. This hybridization physics is shown to play a dominant role in the magnitude of a polaron formation barrier and the adiabaticity of polaron charge localization. Weaker hybridization is correlated with larger more diabatic formation barriers, while stronger hybridization is correlated with smaller more adiabatic formation barriers. These ab initio insights may lead to new approaches towards tailoring the formation of small polarons in energy and electronic materials.

As platforms of Majorana modes, topological insulator (quantum anomalous Hall insulator)/superconductor heterostructures have attracted tremendous attention over the past decade. Here we substitute the topological insulator with its higher-order counterparts. Concretely, we consider second-order topological insulators (SOTIs) without time-reversal symmetry and investigate SOTI/superconductor heterostructures in both two and three dimensions. Remarkably, we find that such novel heterostructures provide natural realizations of second-order topological superconductors (SOTSCs) which host Majorana corner modes in two dimensions and chiral Majorana hinge modes in three dimensions. As here the realization of SOTSCs requires neither special pairings nor magnetic fields, such SOTI/superconductor heterostructures are outstanding platforms of Majorana modes and may have wide application in the future.