In general, shortening the laser pulse width can suppress thermal dissipation to allow for sub-picosecond phase change during amorphization, yet there is no consistent evidence for the mechanistic exploration of its ultrafast dynamics at the atomic scale. Herein, a real-time time-dependent density-functional theory molecular dynamic (rt-TDDFT-MD) in a photoexcited fashion is applied to disclose an undetected effect of the experimentally observed Y-Sb-Te amorphization process at elevated temperature. Varying the optical excitation from $2\%$ to $8\%$ of valence electrons during the amorphization process can result in a variety of structures ranging from entire crystalline to amorphied shapes, satisfying the amorphous state at 1.2 ps upon $8\%$ excitation. We conclude that the low concentration of photoexcitation fails to conquer the potential barrier height of Y-Sb-Te for fully facilitating amorphization. The $8\%$ of photoinduced Te-$p$ electrons enhances the massive occupation of Y-$d{t}_{2g}$ orbitals and creates a flatter potential energy surface, which forces Y-centered motifs to perform $d{t}_{2g}\text{−}\text{and}{e}_g$-directed transition in the signature bond angle of the rock salt lattice. Thus it can rapidly dissociate the Sb-Te bond and facilitate the ultrafast formation of the amorphous phase. The incorporation of optical interventions drives the lattice to manifest disordered states, as well as allowing for accurate manipulation of the electronic structure to further reduce the time consumed in the amorphization process. The present works pave the way for the design of PCMs-based optoelectronic and microelectronic devices with comprehensive ultrafast response.

By measuring magnetizations of pristine and Ta-doped ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ single crystals, we have carried out systematic studies on the lower critical field, critical current density, and equilibrium magnetization of this kagome system. The lower critical field has been investigated in the two typical samples, and the temperature-dependent lower critical field obtained in Ta-doped sample can be fitted by using the model with two $s$-wave superconducting gaps yielding the larger gap of $2{\mathrm{\Delta }}_{s1}/k_{\mathrm{B}}{T}_{\mathrm{c}}=7.9(\pm 1.8)$. This indicates a strong-coupling feature of the V-based superconductors. The measured magnetization hysteresis loops allow us to calculate the critical current density, which shows a very weak bulk vortex pinning. The magnetization hysteresis loops measured in these two kinds of samples can be well described by a recently proposed generalized phenomenological model, which leads to the determination of many fundamental parameters for these superconductors. Our systematic results and detailed analysis conclude that this V-based kagome system has features of strong-coupling superconductivity, relatively large Ginzburg-Landau parameter, and weak vortex coupling.

The leading superconducting instabilities of the two-dimensional extended repulsive one-band Hubbard model within spin-fluctuation pairing theory depend sensitively on electron density, band, and interaction parameters. We map out the phase diagrams within a random-phase-approximation spin- and charge-fluctuation approach, and find that while $B_{1g}$ ($d_{x^2-y^2}$) and $B_{2g}$ ($d_{xy}$) pairing dominates in the absence of repulsive longer-range Coulomb interactions $V_{\mathrm{NN}}$, the latter induces pairing in other symmetry channels, including, e.g., $A_{2g}$ ($g$-wave), nodal $A_{1g}$ (extended $s$-wave), or nodal $E_u$ ($p$-wave) spin-triplet superconductivity. At the lowest temperatures, transition boundaries in the phase diagrams between symmetry-distinct spin-singlet orders generate complex time-reversal symmetry broken superpositions. By contrast, we find that boundaries between singlet and triplet regions are characterized by first-order transitions. Finally, motivated by recent photoemission experiments, we have determined the influence of an additional explicitly attractive nearest-neighbor interaction, $V_{\mathrm{NN}}<0$, on the superconducting gap structure. Depending on the electronic filling, such an attraction boosts $E_u$ ($p$-wave) spin-triplet or $B_{1g}$ ($d_{x^2-y^2}$) spin-singlet ordering.

An advanced theoretical framework is introduced and examined. Its main idea is to extract properties of the superconducting pairing gap function $\mathrm{\Delta }\left(\omega \right)$ in conventional, nearly localized superconductors. To test the approach, we present an experimentally relevant benchmark model with defined normal and superconducting sectors. The developed reverse engineering framework consists of two logic steps. First, dismantle the superconducting density of states into the effects coming from the superconducting pairing and effects inherited from the normal state. Second, extract and reconstruct properties of $\mathrm{\Delta }\left(\omega \right)$ and compare it to the superconducting sector of the defined benchmark model. Applying this approach, we can (i) simulate extraction from the actual experimental low-temperature tunneling data and comment on their required properties, and (ii) maintain absolute control above the reconstructed Cooper-pair-influencing properties during ameliorating the individual steps of the method.

We performed angle-resolved photoemission spectroscopy (ARPES) experiments on an electron-doped ${\mathrm{Sr}}_2{\mathrm{RhO}}_4$ system ${\mathrm{Sr}}_{2-x}{\mathrm{Ce}}_x{\mathrm{RhO}}_4$ in order to investigate the electron doping-induced metal-insulator transition (MIT). We establish the universality of MIT in electron-doped ${\mathrm{Sr}}_2{\mathrm{RhO}}_4$ by comparing results from ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{RhO}}_4$ and ${\mathrm{Sr}}_{2-x}{\mathrm{Ce}}_x{\mathrm{RhO}}_4$. Via a systematic analysis of doping-dependent transport and ARPES data, we show that the correlation driven MIT with a noninteger electron number in electron-doped ${\mathrm{Sr}}_2{\mathrm{RhO}}_4$ is universal and thus independent of the dopant. Within the universality, the ARPES analysis shows that the band topology determined by the spin-orbit coupling (SOC) is likely a control parameter of the insulating gap size and critical electron number of the MIT. We present a phase diagram of the insulating phase as a function of the effective SOC and electron number.

Higher-order topological crystalline phases in low-dimensional interacting quantum systems represent a challenging and largely unexplored research topic. Here, we derive a Hamiltonian describing fermions interacting through correlated hopping processes that break chiral invariance, but preserve both inversion and time-reversal symmetries. In this way, we show that our one-dimensional model gives rise to an interacting second-order topological insulating phase that supports gapped edge states. The topological nature of such an interacting phase turns out to be revealed by both long-range order of a nonlocal string correlation function and by even degeneracy of the entanglement spectrum. For strong interactions we instead find that the topological crystalline phase is destroyed and replaced by a singlet superconducting phase. The latter, characterized by local fermionic pairing, turns out to appear both in a homogeneous and in a phase separated form. Relevantly, the derived one-dimensional model and the second-order topological insulator can be explored and investigated in atomic quantum simulators.

We study the microscopic dynamics of competing ordered phases in a two-dimensional correlated electron model, which is driven with a pulsed electric field of finite duration. In order to go beyond a mean-field treatment of the electronic interactions we adopt a large-$N$ generalization of the Hubbard model and combine it with the semiclassical fermionic truncated Wigner approximation as a time evolution method. This allows us to calculate dephasing corrections to the mean-field dynamics and to obtain stationary states, which we interpret as prethermal order. We use this framework to simulate the light-induced transition between two competing phases (bond density wave and staggered flux) and find that the postpulse stationary state order parameter values are not determined alone by the amount of absorbed energy but depend explicitly on the driving frequency and field direction. While the transition between the two prethermal phases takes place at similar total energies in the low- and high-frequency regimes, we identify an intermediate-frequency regime for which it occurs with minimal heating.

In a one-dimensional Bose gas, there is no nontrivial scattering channel involving three Bogoliubov quasiparticles that conserves both energy and momentum. Nevertheless, we show that such three-wave mixing processes (Beliaev and Landau damping) account for their decay via interactions with thermal fluctuations. Within an appropriate time window where the Fermi golden rule is expected to apply, the occupation number of the initially occupied mode decays exponentially and the rate takes a simple analytic form. The result is shown to compare favorably with simulations based on the truncated Wigner approximation. It is also shown that the same processes slow down the exponential growth of phonons induced by a parametric oscillation.

Despite advances toward improving the quality of $p$-type 4H-SiC substrates and layers, we still have no model capable of accounting for the multitude of boron-related optical, junction, and paramagnetic resonance experiments available in the literature. A conspicuous puzzle is the observation of two shallow boron defects with rather distinct axial orientations as found by electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) data. This feature is not observed in material doped with other group-III elements. Another open issue involves conflicting conclusions from photoluminescence and EPR studies of a deeper boron center, which has been linked to rather distinct models, either based on substitutional or vacancy-related boron defects. We unlock these and other problems by means of first-principles calculations, where the temperature-dependent stability, the electronic activity, and the paramagnetic response of boron defects in 4H-SiC are investigated.

Metallic ferromagnets with strongly interacting electrons often exhibit remarkable electronic phases such as ferromagnetic superconductivity, complex spin textures, and nontrivial topology. In this report, we discuss the synthesis of a layered magnetic metal $\mathrm{Ni}{\mathrm{Ta}}_4{\mathrm{Se}}_8$ (or ${\mathrm{Ni}}_{1/4}\mathrm{Ta}{\mathrm{Se}}_2$) with a Curie temperature of 58 Kelvin. Magnetization data and density functional theory calculations indicate that the nickel atoms host uniaxial ferromagnetic order of about $0.7{\mu }_B$ per atom, while an even smaller moment is generated in the itinerant tantalum conduction electrons. Strong correlations are evident in flat bands near the Fermi level, a high heat capacity coefficient, and a high Kadowaki-Woods ratio. Density functional theory calculations suggest that electron and hole Fermi surfaces in the ferromagnetic phase are associated with opposite spin polarization. When the system is diluted of magnetic ions, the samples become superconducting below about 2 Kelvin. We discuss possible mechanisms for superconductivity in this family.

At the moment, the most accurate theoretical method to describe excitons is the solution of the Bethe-Salpeter equation in the GW approximation (GW-BSE). However, because of its computation cost, time-dependent density functional theory (TDDFT) is becoming the alternative approach to GW-BSE to describe excitons in solids. Nowadays, the most efficient strategy to describe optical spectra of solids in TDDFT is to use long-range corrected exchange-correlation kernels on top of GW or scissor-corrected energies. In recent years, a different strategy based on range-separated hybrid functionals started to be developed in the framework of time-dependent generalized Kohn-Sham density functional theory. Here we compare the performance of long-range corrected kernels with range-separated hybrid functionals for the description of excitons in solids. This comparison has the purpose to weight the pros and cons of using range-separated hybrid functionals, giving new perspectives for theoretical developments of these functionals. We illustrate the comparison for the case of Si and LiF, representative of solid-state excitons.

Twisted multilayers of two-dimensional (2D) materials are an increasingly important platform for investigating quantum phases of matter, and in particular, strongly correlated electrons. The moiré pattern introduced by the relative twist between layers creates effective potentials of long wavelength, leading to electron localization. However, in contrast to the abundance of 2D materials, few twisted heterostructures have been studied until now. Here we develop a first-principles continuum theory to study the electronic bands introduced by moire patterns of twisted Janus transition metal dichalcogenide (TMD) homo- and heterobilayers. The model includes lattice relaxation, stacking-dependent effective mass, and Rashba spin-orbit coupling. We then perform a high-throughput generation and characterization of DFT-extracted continuum models for more than a hundred possible combinations of materials and stackings. Our model predicts that the moiré physics and emergent symmetries depend on chemical composition, vertical layer orientation, and twist angle, so that the miniband wave functions can form triangular, honeycomb, and kagome networks. Rashba spin-orbit effects, peculiar of these systems, can dominate the moiré bandwidth at small angles. Our work enables detailed investigation of Janus twisted heterostructures, allowing the discovery and control of novel electronic phenomena.

We calculate the absorption spectra of a conductive disk in the presence of a metal gate. In our analysis, we use the conductivity described by the Drude model. We examine the frequency and line width of the absorption resonances associated with the excited plasma waves. They are estimated both numerically and analytically for the fundamental and axisymmetric plasma modes. The line width is determined by the collisional and radiative losses. In high mobility samples, the latter may dominate even when the resonant frequencies can be described by a standard quasistatic approximation, i.e., neglecting electromagnetic retardation. Placing a metal near the disk drastically affects the radiative broadening of the line width due to the interference of electromagnetic fields created by the disk and the metal charges. Bringing the disk in close proximity to the gate makes the total field almost vanish. In that case, the line width is defined solely by the collisional broadening. As the disk is moved away from the gate, the line width first shows a reduction since the plasmon transforms into plasmon-polariton, which is dressed with undamped electromagnetic field surrounding the disk. Then it increases and exhibits decaying oscillations, reaching the asymptotic value of the ungated disk.

The ground-state properties of the Hubbard model with attractive local pairing interactions are investigated in the framework of lattice density-functional theory. A remarkable correlation is revealed between the interaction-energy functional $W\left[\mathit{\eta }\right]$ corresponding to the Bloch-state occupation-number distribution ${\eta }_{\kappa \sigma }$ and the entropy $S\left[\mathit{\eta }\right]$ of a system of noninteracting fermions having the same ${\eta }_{\mathit{k}\sigma }$. The relation between $W\left[\mathit{\eta }\right]$ and $S\left[\mathit{\eta }\right]$ is shown to be approximately linear for a wide range of ground-state representable occupation-number distributions ${\eta }_{\mathit{k}\sigma }$. Taking advantage of this statistical analogy, a simple explicit ansatz for $W\left[\mathit{\eta }\right]$ of the attractive Hubbard model is proposed, which can be applied to arbitrary periodic systems. The accuracy of this approximation is demonstrated by calculating the main ground-state properties of the model on several 1D and 2D bipartite and nonbipartite lattices and by comparing the results with exact diagonalizations.

Quantum spin Hall (QSH) insulators exhibit spin-polarized conducting edge states that are topologically protected from backscattering and offer unique opportunities to address fundamental science questions and device applications. Finding viable materials that host such topological states, however, remains a continuing challenge. Here, by using in-depth first-principles theoretical modeling, we predict large band gap QSH insulators in the recently synthesized bottom-up two-dimensional $M{\mathrm{Si}}_2{Z}_4$ ($M$ = Mo or W and $Z$ = P or As) material family with $1T^{\prime }$ structure. A structural distortion in the $2H$ phase drives a band inversion between the metal (Mo/W) $d$ and $p$ states of P/As to realize spinless Dirac states without spin-orbit coupling. When spin-orbit coupling is included, a hybridization gap as large as $\sim 204$ meV opens up at the band-crossing points, realizing spin-polarized conducting edge states with nearly quantized spin Hall conductivity. We also show that the inverted band gap can be tuned with a vertical electric field, which drives a topological phase transition from the QSH to a trivial insulator with Rashba-like edge states. Our study identifies the two-dimensional $M{\mathrm{Si}}_2{Z}_4$ material family in the $1T^{\prime }$ structure as large band gap, tunable QSH insulators with protected spin-polarized edge states and large spin Hall conductivity.

We develop a reliable parameter-free analytic continuation method for quantum many-body calculations. Our method is based on a kernel grid, a causal spline, a regularization using the second-derivative roughness penalty, and the L-curve criterion. We also develop the L-curve averaged deviation to estimate the precision of our analytic continuation. To deal with statistically obtained data more efficiently, we further develop a bootstrap-averaged analytic continuation method. In the test using the exact imaginary-frequency Green's function with added statistical error, our method produces the spectral function that converges systematically to the exact one as the statistical error decreases. As an application, we simulate the two-orbital Hubbard model for various electron numbers with the dynamical-mean field theory in the imaginary time and obtain the real-frequency self-energy with our analytic continuation method, clearly identifying a non-Fermi-liquid behavior as the electron number approaches the half filling from the quarter filling. Our analytic continuation can be used widely, and it will facilitate drawing clear conclusions from imaginary-time quantum many-body calculations.

Thouless pumping is a mechanism to perform the topologically protected transport of particles by adiabatically modulating the Hamiltonian. The transported current is a topological invariant that is intimately related to the integer quantum Hall effect. Most of the previous works focus on topological pumping in linear and square lattices. In this Letter, we theoretically propose a mechanism to perform topological pumping in arrays of spin chains with complex geometries. To achieve this, we consider an array where the spin chains are coupled through their edges, which allows us to split the populations to generate superpositions of spin excitations in different spin chains. We show that due to the topological protection, the quantum superpositions can be transported through the array against the effect of disorder. This approach will open another avenue to transport excitations and correlated states with potential applications in quantum technologies and information processing. Our ideas can be realized in state-of-the-art quantum simulators such as cold atoms and superconducting qubit arrays.

Like the twisting control in magic-angle twisted bilayer graphene, the stacking control is another mechanical approach to manipulate the fundamental properties of solids, especially the van der Waals materials. We explore the stacking-induced magnetic frustration and the spiral spin liquid on a multilayer triangular lattice antiferromagnet where the system is built from ABC stacking with competing intralayer and interlayers couplings. By combining the nematic bond theory and the self-consistent Gaussian approximation, we establish the phase diagram for this ABC-stacked multilayer magnet. It is shown that the system supports a wide regime of spiral spin liquid with multiple degenerate spiral lines in the reciprocal space, separating the low-temperature spiral order and the high-temperature featureless paramagnet. The transition to the spiral order from the spiral spin liquid regime is first order. We further show that the spiral-spin-liquid behavior persists even with small perturbations such as further neighbor intralayer exchanges. The connection to the ABC-stacked magnets, the effects of Ising or planar spin anisotropy, and the outlook on the stacking-engineered quantum magnets are discussed.

Phonons are an ideal platform for realizing stable spinless two-dimensional (2D) Dirac points because they have a bosonic nature and hard-to-break time-reversal symmetry. It should be noted that the twofold degenerate nodal points in the phonon dispersions of almost all reported 2D materials are misclassified as “Dirac points” owing to a historical issue. The correct name for these twofold degenerate nodal points should be “Weyl” because 2D phononic systems are essentially spinless and because each twofold degenerate point is described by a Weyl model in two dimensions. To date, reports of fourfold degenerate Dirac point phonons in 2D materials are lacking. In this paper, we searched through all 80 layer groups (LGs) and discovered that Dirac phonons can be realized in 7 of the 80 LGs. Moreover, the Dirac points in the phonon dispersions of 2D materials can be divided into essential and accidental degenerate points, which appear at high-symmetry points and on high-symmetry lines, respectively. Guided by symmetry analysis, we identified the presence of Dirac phonons in several 2D material candidates with six LGs. This work offers a method for identifying Dirac phonons in 2D and proposes 2D material candidates for realizing Dirac phonons.

Inspired by the discovery and fascinating properties of kagome metals $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A$= K, Rb, Cs), we investigate the superconductivity and topological properties of the $A{\mathrm{V}}_3{\mathrm{Sb}}_5$-prototype kagome materials $\mathrm{Cs}{M}_3{\mathrm{Te}}_5$ ($M$=Ti, Zr, Hf) using first-principles calculations. The calculated results of formation energy and phonon dispersion indicate that this family of kagome materials are stable and may be synthesized in experiments. By analytically solving the Allen-Dynes-modified McMillan formula, the superconducting critical temperatures for ${\mathrm{CsTi}}_3{\mathrm{Te}}_5, {\mathrm{CsZr}}_3{\mathrm{Te}}_5$, and ${\mathrm{CsHf}}_3{\mathrm{Te}}_5$ are predicted as 8.02, 5.47, and 3.51 K, respectively. The electron-phonon couplings are dominated by the coupling between the in-plane atomic vibrational modes and the in-plane electronic orbitals. In addition, the ${\mathrm{CsZr}}_3{\mathrm{Te}}_5$ and ${\mathrm{CsHf}}_3{\mathrm{Te}}_5$ can be categorized as ${\mathbb{Z}}_2$ topological metals according to the calculations of topological invariant and surface states. Such coexistence of superconductivity and nontrivial topological properties in ${\mathrm{CsZr}}_3{\mathrm{Te}}_5$ and ${\mathrm{CsHf}}_3{\mathrm{Te}}_5$ are beneficial to study the interaction between superconductivity and nontrivial band character.