The successful synthesis of the carbon-boron clathrate ${\mathrm{SrB}}_3{\mathrm{C}}_3$ under high-pressure conditions of 50 GPa opens up a new possibility for exploring high-temperature superconductors at ambient pressure. Based on the first-principle calculation, we designed a class of ${\mathrm{LaH}}_{10}$-like clathrate compounds $\mathit{Fm}\overline{3}m-X{\mathrm{B}}_2{\mathrm{C}}_8$ ($X$ = K, Rb, Cs, Sr, Ba, Ga, In, Tl, Sn, Pb, and Bi) and investigate their physical properties and potential superconductivities. Our calculations reveal that the dynamic stability of $\mathit{Fm}\overline{3}m-X{\mathrm{B}}_2{\mathrm{C}}_8$ at ambient pressure is determined by the degree of compatibility between the host metal $X$ and the carbon-boron sublattices. Especially, $p$ orbitals of the $p$-region metals can enhance the interaction of the guest atoms with B-C cages, which results in maintaining these clathrates as dynamically stable. Moreover, altering the oxidation states of the guest atoms can adjust the electronic density of states near the Fermi surface, which in turn affects the superconducting transition temperatures ($T_c$'s) of these compounds. Herein, when filled with $+1$ oxidation state metals, the $T_c$'s of $X{\mathrm{B}}_2{\mathrm{C}}_8$ ($X$ = K, Rb, Cs, Ga, In, and Tl) all exceed the liquid-nitrogen boiling point of 77 K and the $T_c$ of ${\mathrm{TlB}}_2{\mathrm{C}}_8$ is expected to reach 96 K at ambient pressure, which is the highest among the studied carbon-boron compounds.

Superconducting quantum interference devices (SQUIDs) show great promise as quantum bits (qubits) but continue to be hindered by flux noise. The flux noise power spectra of SQUIDs go as $1/f^{\alpha }$, where $\alpha$ is the temperature-dependent noise exponent. Experiments find $0.5\lesssim \alpha \lesssim 1$. Furthermore, experiments find that the noise power spectra versus frequency at different temperatures pivot about or cross at a common point for each SQUID. To try to better understand the results and motivated by experimental evidence that magnetic moments on the surface of SQUIDS produce flux noise, we present the results of our Monte Carlo simulations of various spin systems on 2D lattices. We find that only spin glasses produce $\alpha \sim 1$ at low temperature. We find that aliasing of the noise power spectra at high frequencies can lead to spectral pivoting if it is in proximity to a knee at a slightly lower frequency. We show that the pivot frequency depends on the method of site selection and how often the magnetization is recorded. The spectral pivoting that occurs in our simulations is due to aliasing and does not explain the spectral pivoting of experiments.

We investigate the role of temperature on the topological insulating state of metal dichalcogenide monolayers, $1{\mathrm{T}}^{\prime }\text{−}{MX}_2$ ($M=\text{W}$, Mo and $X=\text{S}$, Se). Using first principles calculations based on density functional theory, we consider three temperature-related contributions to the topological band gap: electrons coupling with short-wavelength phonons, with long-wavelength phonons via Fröhlich coupling, and thermal expansion. We find that electron-phonon coupling promotes the topology of the electronic structures of all $1{\mathrm{T}}^{\prime }\text{−}{MX}_2$ monolayers, while thermal expansion acts as a counteracting effect. Additionally, we derive the band renormalization from Fröhlich coupling in the two-dimensional context and observe its relatively modest contribution to $1{\mathrm{T}}^{\prime }\text{−}{MX}_2$ monolayers. Finally, we present a simplified physical picture to understand the “inverse Varshni” effect driven by band inversion in topological insulators. Our work reveals that, among the four $1{\mathrm{T}}^{\prime }\text{−}{MX}_2$ studied monolayers, ${\mathrm{MoSe}}_2$ is a promising candidate for room-temperature applications because its band gap displays remarkable resilience against thermal expansion, while the topological order of ${\mathrm{WS}}_2$ can be tuned under the combined influence of strain and temperature. Both materials represent novel examples of temperature-promoted topological insulators.

Signatures of dynamical quantum phase transitions and chaos can be found in the time evolution of generalized partition functions such as spectral form factors (SFF) and Loschmidt echoes. While a lot of work has focused on the nature of such systems in a variety of strongly interacting quantum theories, in this work, we study their behavior in short-range entangled topological phases, particularly focusing on the role of symmetry-protected topological zero modes. We show, using both analytical and numerical methods, how the existence of such zero modes in any representative system can mask the SFF with large period (akin to generalized Rabi) oscillations, hiding any behavior arising from the bulk of the spectrum. Moreover, in a quenched disordered system, these zero modes fundamentally change the late-time universal behavior reflecting the chaotic signatures of the zero-energy manifold. Our study uncovers the rich physics underlying the interplay of chaotic signatures and topological characteristics in a quantum system.

We have studied the Kondo effect of a spin-1/2 impurity coupled to a two-dimensional altermagnet host material. To attain the low-temperature many-body Kondo physics of the system, we have performed a numerical renormalization group calculations that allows us to access the spectral properties of the system at zero temperature. The impurity spectral function and the Kondo temperature were calculated for different set of parameters, including Rashba spin-orbit coupling (RSOC) and an external magnetic field. Interestingly, in the RSOC and altermagnetic fields, the hybridization function is spin independent, despite the characteristic broken time-reversal symmetry of the altermagnet. This is because the alternating sign of the spin splitting of the bands in the momentum space renders equal contributions for both spin components of the hybridization function. Our results demonstrate that, although the hybridization function is time-reversal symmetric, the Kondo temperature is substantially suppressed by the altermagnet coupling. Moreover, we have investigated the effect of an external magnetic field applied in the altermagnet along different directions. Interestingly, we observe an important restraining of the Kondo peak which depends strongly on the direction of the field. This anisotropic effect is, however, masked if strong Zeeman splitting takes place at the impurity, as it shatters the Kondo-singlet state.

We present three different neural network (NN) algorithms to calculate thermodynamic properties as well as dynamic correlation functions at finite temperatures for quantum lattice models. The first method is based on purification, which allows for the exact calculation of the operator trace. The second one is based on a sampling of the trace using minimally entangled states, whereas the third one makes use of quantum typicality. In the latter case, we approximate a typical infinite-temperature state by wave functions which are given by a product of a projected pair and a NN part and evolve this typical state in imaginary time.

Ultrathin two-dimensional materials have shown a complete paradigm shift for understanding of physical and electronic properties because of quantum confinement effects and symmetry breaking at nanoscale. Bulk $2H\text{−}\mathrm{Ta}{\mathrm{S}}_2$ undergoes an incommensurate charge density wave (I-CDW) at temperature $T_{\mathrm{I}-\mathrm{CDW}}\sim 76$ K, however, the onset of CDW in atomically thin layers is not clear. We have explored the CDW instability in exfoliated $2H\text{−}\mathrm{Ta}{\mathrm{S}}_2$ using low temperature polarized Raman spectroscopy and first principles calculation. We have emphasized CDW associated modes, $M_1\sim 125 \mathrm{c}{\mathrm{m}}^{-1}$ ($E_1$), $M_2\sim 158 \mathrm{c}{\mathrm{m}}^{-1}$ ($A_1$), and $M_3\sim 334 \mathrm{c}{\mathrm{m}}^{-1}$ ($E_2$), in the ultrathin layer of a single unit cell with thickness $d\sim 3$ nm. The asymmetric Fano line shape of $M_2$ and its anomalous behavior suggests strong electron-phonon coupling, which mainly drives the CDW instability in $2H\text{−}\mathrm{Ta}{\mathrm{S}}_2$. Our observations provide key evidence that CDW can persist even in one unit cell with a $T_{\mathrm{I}-\mathrm{CDW}}\sim 200$ K, which is much higher than bulk $2H\text{−}\mathrm{Ta}{\mathrm{S}}_2$, and the results are supported by density functional theory.

In most cases, as the strength of electron-phonon coupling increases, the effective mass of polarons typically increases. However, in this paper, we uncover a fascinating phenomenon: the presence of light polarons even within the strong coupling regime, where electron-phonon coupling includes both Holstein and bond types (electron-phonon coupling occurring on the hopping bonds). To investigate this, we employ a diagrammatic Monte Carlo method based on the path-integral formulation of the particle sector and the Fock-state representation of the phonon sector. Our study centers on analyzing the impact of bond electron-phonon coupling on the Holstein polaron's essential properties, including its effective mass, ground-state energy, and the average number of phonons. We examine two distinct scenarios: one where the phonon frequency of both Holstein and bond types is the same, and another where the phonon frequency of the Holstein type is twice as large as that of the bond type. In both of these cases, when the Holstein coupling falls within the light mass regime, we observe minimal changes in the effective mass as a function of bond coupling, compared to the case of the bare bond polaron. However, the behavior of the effective mass undergoes a significant change in the heavy mass regime. Most intriguingly, we discover a nonmonotonic dependence of the effective mass on the bond coupling $g_B$ when the Holstein coupling $g_H$ falls within the heavy mass regime. This finding holds promise, particularly in the context of bipolarons, where achieving a light effective mass and a large binding energy to get a compact size is crucial.

We present a study of the unoccupied electronic states of one monolayer (ML) Tl epitaxially grown on Ag(111) in a moiré superstructure. This two-dimensional atomic-layer material is investigated by scanning tunneling microscopy/spectroscopy, spin-resolved inverse photoemission, and calculations based on density functional theory. The unoccupied band structure exhibits characteristic spin-dependent hybridization between overlayer states influenced by the substrate. Most of the experimentally observed bands, their $E({\mathbf{k}}_{\parallel })$ behavior, and their Rashba-type spin dependence can be qualitatively described by a simple model for a Tl/Ag bilayer. A more realistic superstructure model reflecting the moiré structure provides deeper insight into the hybridization mechanisms for states of different orbital composition, further elucidated by calculations of the charge densities. Experimentally, $E({\mathbf{k}}_{\parallel })$ measurements as well as the analysis of spin-dependent spectral intensities allow us to distinguish different orbital contributions in the respective spin-up and spin-down components leading to hybridization gaps with spin-dependent size. Most interestingly, spin-dependent hybridization with overlayer states was discovered for an image-potential-induced surface band, which is mainly located in front of the sample surface.

We perform investigations about the Andreev reflections (ARs) in normal-superconducting (NS) junctions based on the few-layer black phosphorus, by considering the irradiation of the linearly polarized off-resonant light. Following the analysis on the band structures, we propose the types of ARs in such NS junctions. Specifically, it has been found that when the incident energy $E$ is less than the Fermi energy $E_F$, the intraband retro AR occurs, and otherwise the interband specular AR is achieved. A forbidden region of ARs is formed between the retro AR and the specular AR since the band gap leads to a momentum-filtering feature. By using the linearly polarized off-resonant light along $y$ direction, we are allowed to modify the size of this forbidden region. On the other hand, when the normal direction of the NS-junction interface is changed from parallel to perpendicular, by using the linearly polarized off-resonant light along $x$ direction, the retro and specular ARs can be transformed into each other, resulting in an anisotropic differential conductance, which is a unique observable signature for AR in the experiment.

We investigate polariton-phonon hybrid excitations, which describe the collective excitations of emitter-photon polaritons and vibrational phonons, in a periodic array of vibrating two-level emitters interacting with waveguide photons. These excitations present unexpected features in the subradiant regime, including the appearance of topological edge states and transitions between ergodic and multifractal excitations. We reveal that these features are attributed to the emergence of a phonon-induced quasiperiodic structure, where phonons can effectively create a periodic potential that is incommensurate with the emitter lattice. A possible realization consisting of an array of laser-cooled atoms trapped near an optical nanofiber is also proposed. Our results demonstrate the possibility of utilizing vibrations as a unique degree of freedom in the exploration of many-body physics with waveguide quantum electrodynamics systems.

Energy filter methods in combination with quantum simulation can efficiently access the properties of quantum many-body systems at finite energy densities [S. Lu et al., PRX Quantum 2, 020321 (2021)]. Classically simulating this algorithm with tensor networks can be used to investigate the microcanonical properties of large spin chains, as recently shown by Y. Yang et al. [Phys. Rev. B 106, 024307 (2022)]. Here we extend this strategy to explore the properties of off-diagonal matrix elements of observables in the energy eigenbasis, fundamentally connected to the thermalization behavior and the eigenstate thermalization hypothesis. We test the method on integrable and nonintegrable spin chains of up to 60 sites, much larger than accessible with exact diagonalization. Our results allow us to explore the scaling of the off-diagonal functions with the size and energy difference, and to establish quantitative differences between integrable and nonintegrable cases.

The ultrafast transmission electron microscope equipped with an electron energy spectrometer provides a platform to study the electron-photon interaction in both the energy and momentum dimensions. Here, we analyzed the fraction of inelastically scattered electrons and the energy-momentum exchange in the free-electron–photon interaction mediated by a Ag/$\mathrm{S}{\mathrm{i}}_3{\mathrm{N}}_4$ bilayer thin film. Taking advantage of the high transverse coherence electrons emitted by the laser-driven Schottky field emission gun, the quantized transverse momentum transfer was characterized in reciprocal space. We report experimental data on the relatively large transverse momentum transfer caused by momentum spread in free-electron–photon interactions. The momentum uncertainty of the photons guarantees momentum matching and provides an effective way to control the magnitude of the transverse momentum quanta by tilting the film, demonstrating the potential for controlling the free-electron wave function in momentum space.

A quantum kinetic equation for the density matrix is derived from nonequilibrium Green functions using the generalized Kadanoff-Baym ansatz and corrections beyond. The Markovian reconstruction of the Green function and the quasistationary approximation are used to approximate the corrections, making their implementation in numerical simulations faster. On electron tunneling under conditions when the prediction of the uncorrected kinetic equation seriously departs from the exact solution, we show that the corrections reduce the error by an order of magnitude.

The parallel stripe phase is remarkable both in its own right, and in relation to the other phases with which it coexists. Its inhomogeneous nature makes such states susceptible to random fields from quenched magnetic vacancies. We argue this is the case by introducing low concentrations of nonmagnetic Zn impurities (0%–10%) into ${\mathrm{La}}_{1.6-x}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ (Nd-LSCO) with $x=0.125$ in single-crystal form, well below the percolation threshold of $\sim 41\%$ for a two-dimensional square lattice. Elastic neutron scattering measurements on these crystals show clear magnetic quasi-Bragg peaks at all Zn dopings. While all the Zn-doped crystals display order parameters that merge into each other and the background at $\sim 68$ K, the temperature dependence of the order parameter as a function of Zn concentration is drastically different. This result is consistent with meandering charge stripes within the parallel stripe phase, which are pinned in the presence of quenched magnetic vacancies. In turn it implies vacancies that preferentially occupy sites within the charge stripes, and hence that can be very effective at disrupting superconductivity in Nd-LSCO ($x=0.125$), and, by extension, in all systems exhibiting parallel stripes.

We report on the microscopic superconducting properties of the Dirac semimetal ${\mathrm{PdTe}}_2$. In this study, we have focused on mosaic crystals of ${\mathrm{PdTe}}_2$, and used detailed zero-field and transverse-field muon-spin relaxation/rotation $\left(\mu \mathrm{SR}\right)$, ac-magnetic susceptibility, and resistivity measurements to investigate their superconducting properties. The magnetic susceptibility measurements reveal two superconducting transition temperatures at 1.8 and 1.6 K, respectively, in agreement with earlier reports. In contrary to these reports, we find that these mosaic ${\mathrm{PdTe}}_2$ crystals are not type-I, but rather type-II superconductors. In fact, we observe the clear manifestation of a flux-line lattice through a clear diamagnetic shift and Gaussian broadening of the Fourier spectra in the superconducting state. This behavior is likely caused by the disorder in the mosaic crystals of ${\mathrm{PdTe}}_2$ studied here. Our analysis of the superconducting order parameter by the means of temperature-dependent magnetic penetration depth $\lambda \left(T\right)$ reveals a fully gapped superconducting state that can be well-fitted using an $s$-wave symmetric gap. We find that ${\mathrm{PdTe}}_2$ is a promising model system for the investigation and interplay of nontrivial topology, surface superconductivity, and type-II bulk superconductivity in a van der Waals material. Moreover, our results indicate that the superconductivity in this material can be easily modified from type-I to type-II by disorder in the system.

Halide perovskites are highly promising light-harvesting materials with strong ionic character, enabling in principle the combination of a solar cell and a Li-ion battery in one integrated photobattery device. Here, we investigate Li ions inside crystals of ${\mathrm{CsPbI}}_3$ as a prototype compound by means of density-functional-theory calculations. Our findings demonstrate that the interstitial location and migration of Li ions depend strongly on the dynamic nature of the crystal structure of the perovskite compound. We consider two limiting cases for Li in ${\mathrm{CsPbI}}_3$,(i) the cubic-symmetry structure as a model for the limit of fast-ion motion and (ii) a distorted cubic structure as a model for the limit of slow-ion motion. For both limiting cases we obtain moderate energy barriers for migrating Li ions, which highlight the potential of halide perovskites like ${\mathrm{CsPbI}}_3$ for applications in photobattery devices.

As the most abundant substance on the earth's crust, the behavior of α-quartz under pressure has always been a research hotspot, which has inherent crystallographic significance and is of significance to understanding the rocky parts of the earth. This paper provides the discussion about the stability of α-quartz under pressure by analyzing its sound velocity based on first-principles calculation. It is discovered that under pressure, the mechanical instability (24.5 GPa) of α-quartz precedes the dynamic instability (40 GPa). The results show that the trend of increasing and then decreasing for compressed sound velocity in different directions occurs before the structural change, and the pressure point that starts to decrease is about 20 GPa. The mechanical instability at 24.5 GPa is caused by amorphization of α-quartz. The propagation velocities of the plastic wave in crystal after structural change under pressure are discussed. It is considered that the plastic wave first propagates along the direction $K$ to $G$ in the $a$ axis, and the velocity is about 429 m/s.