We study the bulk photovoltaic effect (BPVE) in Dirac and Weyl semimetals under two-frequency light irradiation. We show that the BPVE emerges for centrosymmetric Dirac and Weyl semimetals in the presence of light fields with frequencies $\mathrm{\Omega }$ and $2\mathrm{\Omega }$. The BPVE under the two-frequency drive involves both contributions independent of the carriers lifetime $\tau$ and contributions proportional to $\tau$. Our calculations indicate that the photocurrent's direction, magnitude, and type can be dynamically controlled by tuning parameters of the driving fields. Furthermore, we find that the tilt of the Dirac cone significantly affects the photocurrent, particularly in mirror symmetry-lacking Weyl semimetals, leading to an anisotropic optical response. These findings provide new insights into the dynamic control of photocurrents in topological semimetals, offering promising applications for optoelectronic devices.

We present an efficient method of calculating the vibrational spectrum of a magnetic molecule adsorbed on a superconductor, directly related to the first derivative of the tunneling $IV$ curve. The work is motivated by a recent scanning-tunneling spectroscopy of lead phthalocyanine on superconducting Pb(100), showing a wealth of vibrational excitations, the number of which highly exceeds molecular vibrations typically encountered on normal metals. We design a minimal model, which represents the inelastic transitions by the spectral function of a frontier orbital of the molecule in isolation. The model allows for an exact solution; otherwise the full correlated superconducting problem would be hard to treat. The model parameters are supplied from an ab initio calculation, where the presence of the surface on the deformation of molecular geometry can be taken into account. The spectral function of the highest-occupied molecular orbital of the anionic ${\mathrm{PbPc}}^{1-}$ shows the best agreement with the experimental reference among other molecular charge states and orbitals. The method allows us to include multiple vibrational transitions straightforwardly.

We introduce regular series expansion for weakly and moderately correlated fermionic systems based on the fluctuating local field approach. The method relies on explicitly accounting for the leading fluctuating mode(s) and is therefore suitable for medium-sized lattices. It can also be used as a solver for the cluster approximations for infinite-size lattices. We introduce a classical fluctuating field coupled to a fermionic collective mode(s). In this way, fluctuations in selected modes are treated in a nonperturbative way. Other degrees of freedom are accounted for in the diagram expansion performed for each value of the fluctuating field. The method is benchmarked for the $U/t=1$ and $U/t=2$ Hubbard lattices at half filling. Results for susceptibility in the antiferromagnetic channel along with the spectral function are compared with the numerically exact quantum Monte Carlo data. Calculations up to the third order of the series expansion are performed and show a uniform convergence to the reference result for the susceptibility. This convergence is observed well below the dynamical mean-field theory (DMFT) Neél temperature and makes our practically simple method applicable in a much wider temperature range than DMFT-based diagrammatic schemes.

The geometric properties of parameter space are mostly described by the Berry curvature and quantum metric, which are the imaginary and real parts of a quantum geometric tensor, respectively. In this paper, we calculate the dressed Berry curvature and quantum metric containing eight Feynman diagrams, which are proportional to the leading order of the concentration of impurities. For a two-band gapped graphene model, we find the disorder does not break the original symmetry but decreases (increases) the absolute value of Berry curvature and quantum metric in conduction (valence) bands. We show how impurities affect the Berry curvature and quantum metric, deepening our understanding of the impurity effect on the electron transport properties in two-band gapped graphene.

The technique of conventional ferromagnet/heavy-metal spin-orbit torque (SOT) offers significant potential for enhancing the efficiency of magnetic memories. However, it faces fundamental physical limitations, including shunting effects from the metallic layer, broken symmetry for enabling antidamping switching, spin scattering caused by interfacial defects, and sensitivity to stray magnetic fields. To address these issues, we here propose a van der Waals (vdW) field-free SOT antiferromagnetic memory using a vdW bilayer ${\mathrm{LaBr}}_2$ (an antiferromagnet with perpendicular magnetic anisotropy) and a monolayer ${\mathrm{T}}_d$ phase ${\mathrm{WTe}}_2$ (a Weyl semimetal with broken inversion symmetry). By systematically employing density functional theory in conjunction with nonequilibrium Green's function methods and macrospin simulations, we demonstrate that the proposed vdW SOT devices exhibit remarkably low critical current density approximately $10\mathrm{MA}/{\mathrm{cm}}^2$ and rapid field-free magnetization switching in 250 ps. This facilitates excellent write performance with extremely low energy consumption. Furthermore, the device shows a significantly low read error rate, as evidenced by a high tunnel magnetoresistance ratio of up to 4250%. The superior write and read performance originates from the unique strong on-site (insulating phase) and off-site (magnetic phase) Coulomb interactions in electride ${\mathrm{LaBr}}_2$, a large nonzero $z$-component polarization in ${\mathrm{WTe}}_2$, and the proximity effect between them.

Chiral electromagnetic near fields, which are vital for chiral sensing, photochemistry, and photoemitters, are typically derived from the chiral configurations of plasmonic materials. This often necessitates complex structural engineering. In this study, we illustrate that polaritons in two-dimensional (2D) anisotropic materials can generate chiral near fields, thus eliminating the need for intricate structural designs. The near field on either side of the 2D material is single handed, with both sides exhibiting contrasting optical chirality. The scattering of the 2D material's edges to the polaritons enhances the dissymmetry factor that could be three orders of magnitude higher than that of circularly polarized light. Our research offers an innovative strategy to creating chiral near fields.

The spin-1/2 Heisenberg antiferromagnet on the diamond-decorated square lattice in the presence of a magnetic field displays various quantum phases including the Lieb-Mattis ferrimagnetic, dimer-tetramer, monomer-dimer, and spin-canted phases, in addition to the trivial fully saturated state. Thermodynamic properties of this model are investigated using several complementary analytical and numerical methods such as exact diagonalization up to the systems of 40 spins, an effective monomer-dimer description, sign-problem-free quantum Monte Carlo simulations for up to 180 spins, and a decoupling approximation. Our particular attention is focused on the parameter region favoring the dimer-tetramer phase. This ground state can be represented by a classical hard-dimer model on the square lattice and retains a macroscopic degeneracy even under a magnetic field. However, the description of the low-temperature thermodynamics close to the boundary between the macroscopically degenerate dimer-tetramer and the nondegenerate monomer-dimer phases requires an extended classical monomer-dimer lattice-gas model. Anomalous thermodynamic properties emerging in the vicinity of the dimer-tetramer phase are studied in detail. Under the adiabatic demagnetization we detect an enhanced magnetocaloric effect promoting an efficient cooling to absolute zero temperature.

Under high pressure, methane and hydrogen form ${\left({\mathrm{CH}}_4\right)}_2{\mathrm{H}}_2, {\mathrm{CH}}_4{\left({\mathrm{H}}_2\right)}_2$, and ${\left({\mathrm{CH}}_4\right)}_3{\left({\mathrm{H}}_2\right)}_{25}$ van der Waals compounds with stoichiometries determined by the ${\mathrm{H}}_2$ concentration. This study investigates the effect of compression rates (up to 183 GPa/s) on the formation and morphology of these compounds at pressures up to 60 GPa utilizing dynamic diamond anvil cells combined with time-resolved Raman spectroscopy. Compression rates exceeding 100 GPa/s inhibit the formation of ${\left({\mathrm{CH}}_4\right)}_3{\left({\mathrm{H}}_2\right)}_{25}$, while ${\mathrm{CH}}_4{\left({\mathrm{H}}_2\right)}_2$ and ${\left({\mathrm{CH}}_4\right)}_2{\mathrm{H}}_2$ remain unaffected. Furthermore, lower compression rates yield distinct leaflike and globular sample morphologies depending on the ${\mathrm{H}}_2$ concentration, while rates above 20 GPa/s consistently result in textureless mixtures, regardless of ${\mathrm{H}}_2$ concentration. These combined results demonstrate the competition between transition time and kinetic barriers, together with the critical role of the compression rate during dynamic-pressure nonequilibrium processes.

The many-body localized phase of quantum systems is an unusual dynamical phase wherein the system fails to thermalize and yet, entanglement grows unboundedly albeit very slowly in time. We present a microscopic theory of this ultraslow growth of entanglement in terms of dynamical eigenstate correlations of strongly disordered, interacting quantum systems in the many-body localized regime. These correlations involve sets of four or more eigenstates and hence, go beyond correlations involving pairs of eigenstates which are usually studied in the context of eigenstate thermalization or lack thereof. We consider the minimal case, namely, the second Rényi entropy of entanglement, of an initial product state as well as that of the time-evolution operator, wherein the correlations involve quartets of four eigenstates. We identify that the dynamics of the entanglement entropy is dominated by the spectral correlations within certain special quartets of eigenstates. We uncover the spatial structure of these special quartets and the ensuing statistics of the spectral correlations amongst the eigenstates therein, which reveals a hierarchy of timescales or equivalently, energy scales. We show that the hierarchy of these timescales along with their nontrivial distributions conspire to produce the logarithmic in time growth of entanglement, characteristic of the many-body localized regime. The underlying spatial structures in the set of special quartets also provides a microscopic understanding of the spacetime picture of the entanglement growth. The theory therefore provides a much richer perspective on entanglement growth in strongly disordered systems compared to the commonly employed phenomenological approach based on the $\ell$-bit picture.

We scrutinized the structural, electronic, elastic, mechanical, phononic, and electron-phonon coupling properties of orthorhombic NbS by executing first-principles calculations based on the density functional theory with its generalized gradient approximation. The Nb $4d$ electron states mainly contribute to the energy bands near the Fermi level. This result suggests that the electrons in the Nb $4d$ states mostly dominate the charge conduction and superconducting state properties in the studied compound. An analysis of elastic constants reveals the mechanical stability of NbS in its orthorhombic MnP-type crystal structure. In contrast, an analysis of elastic moduli indicates that this intermetallic compound has intrinsic ductility. Our phonon calculations prove the dynamical stability of NbS in its orthorhombic MnP-type crystal structure. A comparison of the phonon density of states and Eliashberg electron-phonon interaction spectral function underscores the superconducting trait of orthorhombic NbS is attributed to the vibrational modes of Nb atoms, which strongly mediate the formation of Cooper pairs due to the dominance of Nb $4d$ electrons at the Fermi level. Furthermore, our electron-phonon interaction calculations reveal that the orthorhombic NbS is a medium-coupling electron-phonon-mediated superconductor with an electron-phonon coupling parameter of 0.83 and has a superconducting transition temperature of 6.9 K, which is comparable with its recently measured value of 6.0 K.

Intensive investigation of elemental two-dimensional (2D) materials with anisotropic structures holds promise for unraveling novel physical phenomena and clarifying the relationship between dimensionality and superconductivity. Here, we successfully observed unprecedented double superconducting domes in a 2D elemental superconductor multilayer tellurene under high pressure. Upon compression, multilayer tellurene undergoes multiple structural phase transitions dissimilar with bulk tellurium, progressing from an initial hexagonal twofold $P{3}_{1}21$ phase to a quasi-2D monoclinic fourfold $P{2}_1$ phase and ultimately to the three-dimensional (3D) rhombohedral sixfold $R\overline{3}m$ and cubic eightfold $\mathit{Im}\overline{3}m$ structures. Moreover, low-temperature electrical resistance measurements demonstrate the emergence of superconductivity at 0.5 GPa with superconducting transition temperature ($T_{\mathrm{c}}$) ∼2.9 K. The pressure-dependent $T_{\mathrm{c}}$ reveals an unexpected double-dome superconducting phase diagram alongside notably different upper critical fields that benefit from the combined effect of dimensional change and structural modulation. This work observed pressure-induced double-dome superconductivity in the elemental 2D materials, offering a promising pathway for achieving uncommon superconductivity behavior in 2D systems under extreme conditions.

For spinful systems with spin 1/2, it is generally believed that $\mathcal{P}$ and $\mathcal{T}$ invariant strong and second-order topologies exist in four band and eight band systems, respectively. Here, by using periodic driving, we find it is possible to have a strong topological insulator, a second-order topological insulator, and a hybrid-order topological insulator in a single four band system. Furthermore, we established a direct connection between topology and dynamics. More convenient experimental detection for these topological phases has also been proposed. This study provides the theoretical basis for a topological insulator that possess hybrid-order boundary states beyond the conventional regimes.

Quantum materials featuring both itinerant and localized degrees of freedom exhibit numerous exotic phases and transitions that deviate from the Ginzburg-Landau paradigm. This paper uses the composite operator formalism to examine the bilayer strongly correlated Hubbard model. We observe the spontaneous breaking of layer symmetry, where the electron density in one of the layer reaches half-filling, resulting in a layer selective Mott phase (LSMP). This broken symmetry phase becomes unstable at a critical average electronic density away from half-filling. Furthermore, significant layer differentiation persists up to a moderate interlayer hopping, beyond which the system abruptly transitions to a layer uniform phase. In the LSMP phase, the electrons in the two layers are weakly hybridized, resulting in a small Fermi surface. The volume of the Fermi surface jumps at the transition from the LSMP to the uniform phase. We also discuss the physical mechanisms leading to the collapse of the LSMP phase under different perturbations.

Low-energy excitations of the excitonic states in the semimetal and semiconducting materials described by the extended Falicov-Kimball model with phonon correlation are investigated. In the framework of the unrestricted Hartree-Fock approximation, we derive a set of self-consistent equations to find the quasiparticle energies and excitonic condensate order parameters. The real part of the optical conductivity and then the dynamical excitonic susceptibility function are analytically evaluated by means of the linear response theory. Numerical results reveal the significant expansion of the excitonic condensation window in the temperature–Coulomb interaction plane by lowering the mass imbalance. Under the influence of the mass imbalance and thermal fluctuations, the BCS-BEC crossover of the excitonic condensation states has been also addressed in the collective mode signatures of the optical conductivity. Out of the condensation state, the collective excitation of the excitonic bound state is examined by analyzing the imaginary part of the dynamical excitonic susceptibility function. At large mass imbalance and low thermal fluctuations, we find an extremely sharp peak with high intensity at low frequency in the imaginary part of the dynamical excitonic susceptibility spectrum, specifying the long-lifetime exciton existence before it becomes condensed.

Correlated bilayer systems, fundamental to various transition metal compounds, exhibit intriguing phenomena due to the intrinsic competition between interlayer and intralayer couplings mediated by strong Coulomb interactions. Utilizing dynamical mean-field theory, we present a comprehensive investigation of the bilayer Hubbard model on a kagome lattice. By integrating multiple estimators, including double occupancy ($D_{\text{occ}}$), $-\beta G(\beta /2)$, the local Green's function $\mathrm{Im}{G}_{\text{loc}}\left(i{\omega }_n\right)$, the local self-energy $\mathrm{Im}{\mathrm{\Sigma }}_{\text{loc}}\left(i{\omega }_n\right)$, the dimer Green's function $\mathrm{Im}{G}_d\left(i{\omega }_n\right)$, and the dimer self-energy $\mathrm{Im}{\mathrm{\Sigma }}_d\left(i{\omega }_n\right)$, we classify various phases and their transitions or crossovers. In addition to the expected Mott state at large $U/t$ and small $t^{\prime }/t$ and the correlated band insulating state at small $U/t$ but large $t^{\prime }/t$, we identify an emergent phase where electrons lose long-range transport capability but retain mobility between the two layers. The bilayer kagome Hubbard model, with its rich phase diagram, provides valuable insights into numerous material compounds. Our theoretical engineering of Mott states offers significant implications for interpreting the diverse electronic and magnetic phenomena observed in these systems.

We investigate the migration properties of hole polarons in rutile and anatase ${\mathrm{TiO}}_2$ using Hubbard $U$ semilocal and hybrid functionals with parameters set to enforce the piecewise linearity condition. The polarons in anatase and in rutile are found to be stable with both functionals, leading to similar atomic structures, charge densities, and formation energies. To address migration properties, we determine the minimum energy paths for all nonequivalent hopping transitions through a cost-effective scheme of hybrid-functional accuracy, which combines nudged elastic band optimization at the Hubbard $U$ semilocal level with hybrid-functional single-point calculations. For anatase, the migration occurs through hopping transitions preserving the localized nature of the polaron charge density. A kinetic Monte Carlo simulation yields a mobility of $0.04 {\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at room temperature and indicates that the effective activation energy closely corresponds to the lowest transition barrier. At variance, in rutile, we find that the minimum energy path involves a delocalized intermediate state for all transitions. The low excitation energies calculated in this process suggest that experimental observations for rutile concerning the activated behavior of positive charge carriers should not be associated with polaron states.