We demonstrate that an image recognition algorithm based on a convolutional neural network provides a powerful procedure to differentiate between ergodic, nonergodic extended (fractal), and localized phases in various systems: Single-particle models, including random-matrix and random-graph models, and many-body quantum systems. We propose an efficient procedure in which the network is successfully trained on a small data set of only 500 wave functions (images) per class for a single model which exhibits these phases. The trained network is then used to classify phases in the other models. We discuss the strengths and limitations of the approach.

We calculate the charge gap $\mathrm{\Delta }{E}_C$ and the spin gap $\mathrm{\Delta }{E}_S$ of the ionic Hubbard chain including electron-hole symmetric density-dependent hopping. The vanishing of $\mathrm{\Delta }{E}_C$ $\left(\mathrm{\Delta }{E}_S\right)$ signals a quantum critical point in the charge (spin) sector. Between both critical points, the system is a fully gapped spontaneously dimerized insulator. We focus our study in this region. Including alternation in the hopping, it is possible to perform an adiabatic Thouless pump of one charge per cycle, but with a velocity limited by the size of the gaps.

The Hubbard and related models serve as a fundamental starting point in understanding the novel experimental phenomena in correlated electron materials, such as superconductivity, Mott insulator, magnetism. Recent numerical simulations demonstrate that the next-nearest-neighbor hopping $t^{\prime }$ plays a key role for the superconductivity. However, the impacts of long-range intersite interactions in such a $t^{\prime }$-Hubbard model are less explored. Using the state-of-art density-matrix renormalization group method, we investigate the $t^{\prime }$-Hubbard model on a two-leg ladder with the intersite interactions extended to the fourth neighbors. The numerical results show that the quasi-long-range superconducting correlation remains stable under the repulsive intersite interactions and these long-range repulsive interactions only change the ground state quantitatively. In addition, inspired by recent experiments on one-dimensional cuprate chain ${\mathrm{Ba}}_{2-\mathrm{x}}{\mathrm{Sr}}_{\mathrm{x}}{\mathrm{CuO}}_{3+\delta }$, we find that the nearest-neighbor attractive interaction significantly enhances the superconducting correlation when it is comparable to the nearest-neighbor hopping $t$. Stronger attraction drives the system into an electron-hole phase separation. Finally, we discuss the effects of the on-site interaction on superconductivity.

The density functional plus dynamical mean-field theory is used to study the spin excitation spectra of ${\mathrm{SrRu}}_2{\mathrm{O}}_6$. A good quantitative agreement with experimental spin excitation spectra is found. Depending on the size of the Hund's coupling $J_{\mathrm{H}}$, the system chooses either the Mott insulator or covalent insulator state when magnetic ordering is not allowed. We find that the nature of the paramagnetic state has a negligible influence on the charge and spin excitation spectra. We find that antiferromagnetic correlations hide the covalent insulator state for realistic choices of the interaction parameters.

Ferroelectricity in halide perovskites has attracted great attention, ranging from fundamental studies to technological applications. Here, we studied the structural and electrical properties of superlattice $\mathrm{CsSn}{\mathrm{I}}_3/\mathrm{CsSi}{\mathrm{I}}_3$ by first principles. The ground state of $\mathrm{CsSn}{\mathrm{I}}_3/\mathrm{CsSi}{\mathrm{I}}_3$ is a polar phase with a space group of $\mathit{Pc}$, and it possesses four trilinear couplings between polar and nonpolar modes. The $\mathit{Pc}$ state is eightfold degenerate in total energy against the simultaneous inversion of couples of modes. Under an electric field, the switching of polarization is accompanied by the switching of antiferroelectric octahedral distortion tilting. Moreover, the switching barrier of $\mathrm{CsSn}{\mathrm{I}}_3/\mathrm{CsSi}{\mathrm{I}}_3$ can be reduced by applying a compressive strain. The unique trilinear couplings and ferroelectricity in our system suggest the potential application of lead-free halide perovskites in several energy conversion mechanisms such as piezoelectric, pyroelectric, electrical energy storage, and photovoltaic.

Recently, high-order sideband polarimetry has been established as an experimental method that links the polarization of sidebands to an interference of Bloch waves. However, the robustness of sideband polarizations to increasing dephasing remains to be explored. Here, we investigate the dependence of high-order sideband generation in bulk gallium arsenide on dephasing by tuning temperature. We find that the intensities of the sidebands, but not their polarizations, depend strongly on temperature. Using our polarimetry method, we are able to isolate the contributions of electron-heavy hole (HH) and electron-light hole (LH) pairs to sideband intensities, and separately extract the nonequilibrium dephasing coefficients associated with the longitudinal optical (LO) phonons and acoustic (A) phonons for each species of electron-hole pair. We find that ${\mathrm{\Gamma }}_{\text{HH},\text{A}}=6.1\pm 1.6\mu \mathrm{eV}/\mathrm{K}$, ${\mathrm{\Gamma }}_{\text{LH},\text{A}}<1.5\mu \mathrm{eV}/\mathrm{K}$, ${\mathrm{\Gamma }}_{\text{HH},\text{LO}}=14\pm 3\mathrm{meV}$, and ${\mathrm{\Gamma }}_{\text{LH},\text{LO}}=30\pm 3\mathrm{meV}$.

We explore a hexagonal cavity that supports chiral topological whispering gallery (CTWG) modes, which are formed by a gyromagnetic photonic crystal. This mode represents a unique type of topologically protected state, displaying chirality in its propagation within photonic crystals. Finite element method simulations demonstrate the presence of discrete edge states within the topological band gap, arising from the coupling of chiral edge states and whispering gallery modes. Since the cavity exclusively supports edge state modes with group velocity in one direction, it can generate purely traveling modes and remains immune to interference modes. Furthermore, we introduced defects and disorder to assess the cavity's robustness, showcasing the effective maintenance of CTWG modes under various perturbations. Our topological cavity platform offers useful prototype of robust topological photonic devices. The existence of this mode can have important implications for applications such as topological traveling wave devices and efficient optical couplers.

We present a detailed study of correlation-induced electronic reconstruction in ${\mathrm{NbO}}_2$ bulk crystal. Using density functional plus dynamical mean-field theory $(\mathrm{DFT}+\mathrm{DMFT})$, we show the importance of multiorbital Coulomb interactions in concert with first-principles band structure calculations for a consistent understanding of emergent pseudogap and incipient Mott localization in ${\mathrm{NbO}}_2$. We explain the pseudogapped incoherent metal to a Mott-localization-like transition seen in experiment. We trace its origin to changes in orbital polarization, itself renormalized from its DFT value by sizable multiorbital electron correlations. Our results should be of interest for future studies on electronic switching in Mott memristors for neuromorphic computing devices.

We study an incommensurate XXZ spin chain coupled to a collection of local harmonic baths. At zero temperature, by varying the strength of the coupling to the bath the chain undergoes a quantum phase transition between a Luttinger liquid phase and a spin-density wave (SDW). Our results are consistent with the Berezinskii-Kosterlitz-Thouless (BKT) transition. As opposed to the standard mechanism, the SDW emerges in the absence of the opening of a gap, but it is due to “fractional excitations” induced by the bath. We also show, by computing the DC conductivity, that the system is insulating in the presence of a subohmic bath. We interpret this phenomenon as localization induced by the bath à la Caldeira and Leggett.

The precise control of Weyl physics in realistic materials offers a promising avenue to construct accessible topological quantum systems, and thus draws widespread attention in condensed-matter physics. Here, based on first-principles calculations, the maximally localized Wannier function based tight-binding model, and Floquet theorem, we study the light-manipulated evolution of Weyl physics in a carbon allotrope ${\mathrm{C}}_6$ crystallizing a face-centered orthogonal structure (fco-${\mathrm{C}}_6$), an ideal Weyl semimetal with two pairs of Weyl nodes, under irradiation of a linearly polarized light. We show that the positions of Weyl nodes and Fermi arcs can be accurately controlled by changing the light intensity. Moreover, we employ a low-energy effective $k·p$ model to understand light-controllable Weyl physics. The results indicate that the symmetry of light-irradiated fco-${\mathrm{C}}_6$ can be selectively preserved, which guarantees that the light-manipulated Weyl nodes can only move in the high-symmetry plane in momentum space. Our work not only demonstrates the efficacy of employing periodic driving light fields as an efficient approach to manipulate the Weyl physics, but also paves a reliable pathway for designing accessible topological states under light irradiation.

We report electrical transport properties of ${\mathrm{EuZn}}_2{\mathrm{As}}_2$ under pressures up to 26 GPa. At ambient pressure, ${\mathrm{EuZn}}_2{\mathrm{As}}_2$ exhibits an insulating ground state and a $\approx 200\%$ negative magnetoresistance (MR) at $B=5$ T and $T=2$ K. For pressures up to $\sim 3$ GPa, the system becomes more insulating, accompanied by a dramatic enhancement of the MR $(B=5 \mathrm{T} \text{and} T=2 \mathrm{K})$ up to $\approx 14000\%$. Further increase of pressure drives ${\mathrm{EuZn}}_2{\mathrm{As}}_2$ into a metallic state without significant MR. Resistivity measurements under field reveal ferromagnetic characteristics associated with the metallic ground state in pressurized ${\mathrm{EuZn}}_2{\mathrm{As}}_2$, distinct from the antiferromagnetic state realized at ambient pressure. We propose the pressure-induced insulator-metal transition and the colossal MR both originate from transformations of the magnetic ground state that strongly reconstruct the electronic structure. This view is supported by first-principles calculations, which further reveal that while antiferromagnetic ${\mathrm{EuZn}}_2{\mathrm{As}}_2$ is a trivial insulator, spin-polarized/ferromagnetic ${\mathrm{EuZn}}_2{\mathrm{As}}_2$ is a Weyl metal, indicting that the insulator-metal transition in ${\mathrm{EuZn}}_2{\mathrm{As}}_2$ is also a topological phase transition. These pressure- and field-induced topological phase transitions in ${\mathrm{EuZn}}_2{\mathrm{As}}_2$ are remarkably similar to behaviors reported in ${\mathrm{EuCd}}_2{\mathrm{As}}_2$, suggesting a common origin. Our findings demonstrate that spin-orbit coupling, electronic topology, and particularly the magnetic ground state play key roles in determining the physical properties of Eu-based magnetic topological materials.

We consider the critical behavior associated with incommensurate unidirectional charge-density-wave ordering in a weakly orthorhombic system subject to uniaxial strain as an experimentally significant example of $U\left(1\right)\times U\left(1\right)$ multicriticality. We show that, depending on microscopic details, the phase diagram can have qualitatively different structures which can involve a vestigial metanematic critical point, a pair of tricritical points, a decoupled tetracritical point, or (at least at mean-field level) a bicritical point. We analyze the emergent symmetries in the critical regime and find that these can—at least in some cases—involve an emergent ${\mathbb{Z}}_2$ order parameter symmetry.

We investigate theoretically the stochastic dynamics of resonance electronic energy transfer (RET) in a bidimensional overexcited ensemble of donor and acceptor molecules. We find that, after initial optical excitation of all the donors, the reaction kinetics is well-described by a nonlinear mean-field theory. The latter provides a solid way to define and compute an effective rate of RET, even for disordered samples. We predict that this effective rate scales as ${\langle R\rangle }^{\alpha }$, with $\langle R\rangle$ the average distance between individual excited donors and their nearest-neighbor acceptor molecules, and $\alpha \in [-6,-2]$ an exponent depending on the spatial distribution of molecular pairs in the sample. Using a kinetic Monte Carlo approach, we show departures from this macroscopic mean-field description arising from fluctuations and spatial correlations between several molecules involved in the RET process. We expect this prediction to be relevant for both molecular science and biology, where the control and optimization of the RET dynamics is a key issue.

Tracking the mechanism behind the unconventional superconductors has been recognized as a great challenge and remains one of the major problems in multifields. Layered transition metal dichalcogenides (TMDs) offer an important platform for exploring novel physics. Here, we have discovered that the synergetic Re doping and pressure effect on $\mathrm{V}{\mathrm{Se}}_2$ prompts the appearance of multiband superconductivity at 11.5 GPa with $T_{\mathrm{c}}\sim 3.8\mathrm{K}$. By the application of external magnetic fields, this superconducting state is signified by the unusual temperature-upper critical field relationship and the upper critical field characteristic with an upward curvature in the whole pressure range of $12.4\text{–}72.7\mathrm{GPa}$. The clear experimental evidence of a dominant carrier-type change from electron to hole strongly supports the Lifshitz transition at 37.3 GPa. The tendency of $T_{\mathrm{c}}\left(P\right)$ with a dome shape derives from the structural transition and Lifshitz transition at 20.9 and 37.3 GPa, respectively. The first-principles calculations further clarify that the $\mathrm{V}\text{−}3d, \mathrm{Re}\text{−}5d$, and $\mathrm{Se}\text{−}4p$ orbitals at the Fermi level contribute to the transformation from the single-band into multiband superconductivity. This work reports the multiband superconductivity in TMDs materials via pressure treatment.

Recent theoretical and experimental work suggests that the honeycomb cobaltates, initially proposed as candidate Kitaev quantum magnets, are in fact described by a pseudospin-$1/2$ easy-plane spin Hamiltonian with nearest-neighbor ferromagnetic (FM) exchange $J_1$ being frustrated by antiferromagnetic third-neighbor exchange $J_3$ and weaker compass anisotropies. Using exact diagonalization and density-matrix renormalization group (DMRG) calculations, we show that this model exhibits FM order at small $J_3/J_1$ and zigzag (ZZ) order at large $J_3/J_1$, separated by an intermediate phase, which we label as $\widetilde{\mathrm{SL}}$. This $\widetilde{\mathrm{SL}}$ phase is shown to exhibit spin-liquid-like correlations in DMRG, although we cannot preclude weak broken symmetries, e.g., weak Ising type Néel order, given the limits on our explored system sizes. Using a modified parton mean field theory and variational Monte Carlo on Gutzwiller projected wave functions, we show that the optimal FM and ZZ orders as well as the intermediate $\widetilde{\mathrm{SL}}$ state are proximate to a “parent” Dirac spin liquid (SL). This Dirac SL is shown to capture the broad continuum in the temperature and magnetic field dependent terahertz spectroscopy of ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$, and the reported low-temperature metallic thermal conductivity in ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$ and ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$ upon incorporating disorder-induced broadening.

Here we report a ferromagnetic compound ${\mathrm{Ba}}_9{\mathrm{V}}_3{\mathrm{Te}}_{15}$ with quasi-one-dimensional (1D) spin chains. It was synthesized at high-temperature and high-pressure conditions and systematically investigated via structural, transport, magnetic and heat capacity measurements. ${\mathrm{Ba}}_9{\mathrm{V}}_3{\mathrm{Te}}_{15}$ mainly consists of trimerized face-sharing $\mathrm{V}{\mathrm{Te}}_6$ octahedral chains running along the $c$ axis, which are separated by a distance of $10.184\left(8\right)\AA$, thus demonstrating a strong 1D structural character. ${\mathrm{Ba}}_9{\mathrm{V}}_3{\mathrm{Te}}_{15}$ is a semiconductor with a band gap $\sim 128\mathrm{meV}$ and undergoes a ferromagnetic transition with $T_{\mathrm{C}}\sim 3.6\mathrm{K}$. Far above $T_{\mathrm{C}}$, the short-range spin orders due to the intrachain spin coupling have developed, which leads to a negative magnetoresistance effect, the deviation of magnetic susceptibility from the Curie-Weiss law and $T^{1/2}$ dependence of magnetic heat capacity. Our results reveal that ${\mathrm{Ba}}_9{\mathrm{V}}_3{\mathrm{Te}}_{15}$ provides a good opportunity to study the intrinsic properties of a system with quasi-1D ferromagnetic spin chains.

We study the ground state and spin excitations in ${\mathrm{Ba}}_3{\mathrm{MnSb}}_2{\mathrm{O}}_9$, an easy-plane $S=5/2$ triangular lattice antiferromagnet. By combining single-crystal neutron scattering, electric spin resonance (ESR), and spin wave calculations, we determine the frustrated quasi-two-dimensional spin Hamiltonian parameters describing the material. While the material has a slight monoclinic structural distortion, which could allow for isosceles-triangular exchanges and biaxial anisotropy by symmetry, we observe no deviation from the behavior expected for spin waves in the in-plane ${120}^{\circ }$ state. Even the easy-plane anisotropy is so small that it can only be detected by ESR in our study. In conjunction with the quasi-two-dimensionality, our study establishes that ${\mathrm{Ba}}_3{\mathrm{MnSb}}_2{\mathrm{O}}_9$ is a nearly ideal triangular lattice antiferromagnet with the quasiclassical spin $S=5/2$, which suggests that it has the potential for an experimental study of $Z$- or $Z_2$-vortex excitations.