The interrelationship between a material's structure and its properties lies at the heart of materials-related research. Finding how the changes of one affect the other is of primary importance in theoretical and computational materials studies. In this Letter, based on Hershfield nonequilibrium quantum statistics and the mean-field approach with steady-state density functional theory, we derive a first-principles method to calculate forces acting on the atoms induced by nonequilibrium effects, enabling structure optimizations and molecular dynamics simulations for molecular junctions under external biases. By applying the method to a few molecular devices, we found that in general, the external bias can induce profound nonequilibrium effects on both electronic and transport properties and the geometric structure of these devices, and consequent changes in electronic properties and geometric structure are closely interrelated. Particularly, when the bias voltage is above 1.0 V, significant structure phase changes could occur, causing dramatic changes in $I\text{−}V$ characteristics and vibrational spectra. These findings greatly broaden our understanding of quantum electronic devices and pave the way for discovering unconventional transport phenomena at the molecular scale.

In this manuscript, after discussing in detail the internals of our recently developed method, the dynamical projective operatorial approach (DPOA), we provide the framework to apply this method to pumped semiconductor lattice systems and, in particular, to study and analyze their electronic excitations and TR-ARPES signal. The expressions for relevant out-of-equilibrium Green's functions and TR-ARPES signal are given within the DPOA framework and, defining a retarded TR-ARPES signal, it is shown that it is possible to obtain an out-of-equilibrium version of the fluctuation-dissipation theorem. We clarify how single- and multiphoton resonances, rigid shifts, band dressings, and different types of sidebands emerge in the TR-ARPES signal. We also propose protocols for evaluating the strength of single- and multiphoton resonances and for assigning the residual excited electronic population at each crystal momentum and band to a specific excitation process. Hamiltonians, where intra- and interband transitions are selectively inhibited, are defined and used to analyze the effects on the TR-ARPES signal and the residual electronic excited population. Three relevant cases of light-matter coupling are examined within the dipole gauge: only a local dipole, only the Peierls substitution in the hopping term, and both terms at once. The transient and residual pump effects are studied in detail, including the consequences of the lattice symmetries at different crystal momenta on the TR-ARPES signal. A detailed study of the dependence of the TR-ARPES signal on the probe-pulse characteristics is also reported. To provide a guideline for understanding the complex effects and interplays and the variety of possible physical phenomena without being limited by the characteristics of a single particular real material, we have chosen to study a prototypical pumped two-band semiconductor lattice system.

Usual superconductors fall into two categories, type I, expelling magnetic fields, and type II, into which magnetic fields exceeding a lower critical field $H_{\mathrm{c}1}$ penetrate in a form of vortices characterized by two scales, the size of the normal core, $\xi$, and the London penetration depth $\lambda$. Here we demonstrate that a type-III superconductivity, realized in granular media in any dimension, hosts vortex physics in which vortices have no cores, are logarithmically confined, and carry only a gauge scale $\lambda$. Accordingly, in type-III superconductors $H_{\mathrm{c}1}=0$ at zero temperature and the Ginzburg-Landau theory must be replaced by a topological gauge theory. Type-III superconductivity is destroyed not by Cooper pair breaking but by vortex proliferation generalizing the Berezinskii-Kosterlitz-Thouless mechanism to any dimension.

We propose altermagnet-superconductor junctions as a way to achieve a thermoelectric response in metals free of external or stray magnetic fields. We combine qualitative analysis in a simplified model with a more rigorous approach based on the inverse proximity effect in the functional-integral formulation. We show that coupling an altermagnet to a superconductor in a bilayer induces a momentum-dependent spin splitting in the superconductor. When tunneling occurs between this bilayer and a different altermagnet, a spin-dependent particle-hole symmetry breakdown arises in the transport, which leads to a thermoelectric response. Our results show that the altermagnet-superconductor junctions may achieve comparable thermoelectric performance to ferromagnet-superconductor junctions, featuring a nonmonotonic dependence of the figure of merit on the strength of the altermagnetic splitting. We also point out an often overlooked fact regarding the inverse proximity effect in superconductors, namely that even in a normal metal-superconductor junction there is a minigap in the superconductor, which gives rise to a four-peak structure in the DOS reminiscent of spin-split superconductors. Our results show that altermagnetic metals, unlike conventional antiferromagnets, can be used for efficient cryogenic thermoelectricity.

The discovery of high-$T_c$ superconductivity (SC) in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ (LNO) has aroused a great deal of interest. Previously, it was proposed that the Ni-$3d_{z^2}$ orbital is crucial to realize the high-$T_c$ SC in LNO. The preformed Cooper pairs therein acquire coherence via hybridization with the $3d_{x^2-y^2}$ orbital to form the SC. However, we held a different viewpoint that the interlayer pairing $s$-wave SC is induced by the $3d_{x^2-y^2}$ orbital, driven by the strong interlayer superexchange interaction. To include effects from both $E_g$ orbitals, we establish a two-orbital bilayer $t\text{−}J$ model. Our calculations reveal that due to the no-double-occupancy constraint, the $3d_{x^2-y^2}$ band and the $3d_{z^2}$ bonding band are flattened by a factor of about 2 and 10, respectively, which is consistent with recent angle-resolved photoemission spectroscopy measurements. Consequently, a high-temperature SC can be hardly induced in the $3d_{z^2}$ orbital due to the difficulty to develop phase coherence. However, it can be easily achieved by the $3d_{x^2-y^2}$ orbital under realistic interaction strength. With electron doping, the $3d_{z^2}$-band gradually dives below the Fermi level, but $T_c$ continues to enhance, suggesting that it is not necessary for the high-$T_c$ SC in LNO. With hole doping, $T_c$ initially drops and then rises, accompanied by the crossover from the BCS to BEC-type superconducting transitions.

The application of phase-field modeling has been expanded into smaller and smaller-scale phenomena. However, phase-field models are typically based on the phase transition theory of continuum mechanics and their application to atomic systems has been limited. Here, we have developed phase-field modeling for atoms, in which atom fluctuations are represented by Gaussian distributions, and their motions are calculated by the conservative type phase-field equation. We found that our modeling successfully reproduced the stabilization of ${\mathrm{SrTiO}}_3$ lattices under $NVT$ and $NPT$ conditions. Moreover, we found that our modeling may enable us to obtain nonequilibrium thermodynamics properties such as the time evolution of entropy and local free energy distributions for atomic systems, which are difficult for the conventional molecular dynamics simulation. This research will expand the application of the phase-field concept into the single atom length scale, suggesting that this methodology can be applied to more diverse systems and purposes.

Various strategies have been explored to manipulate the valley degrees of freedom in transition metal dichalcogenides, given their potential applications in quantum information. However, the influence of defects on valley polarization in these materials has received relatively little attention. Here we present findings on the significantly enhanced valley polarization and magnetic response of excitons in $\mathrm{Mo}{\mathrm{S}}_2$ monolayers surrounded by multilayers, achieved through an argon-etched assisted chemical vapor deposition. Low-temperature optical spectroscopy measurements support the identification of sulfur vacancies and their impact on the excitonic properties of the $\mathrm{Mo}{\mathrm{S}}_2$ monolayers. In addition, circular polarization resolved magnetophotoluminescence measurements reveal enhanced valley polarization and the asymmetric magnetic responses of both band-edge free and in-gap bound excitons, which can be attributed to the formation of charged dark excitons. Our study opens avenues for controlling valley polarization and investigating novel quantum optical phenomena through defect engineering in two-dimensional semiconductors.

We theoretically investigate the influence of the normal state quantum geometry on the superconducting phase in (111)-oriented oxide interfaces and discuss some of the implications for the ${\text{LaAlO}}_3\text{−}{\text{SrTiO}}_3$ (LAO-STO) heterostructure. From a tight-binding modeling of the interface, we derive a two-band low-energy model, allowing us to analytically compute the quantum geometry and giving us access to the superfluid weight, as well as to showcase the role of two particular relevant energy scales. One is given by the trigonal crystal field which stems from the local trigonal symmetry at the interface, and the other one is due to orbital mixing at the interface. Our calculations indicate that the variation of the superfluid weight with the chemical potential $\mu$ is controlled by the quantum geometry in the low-$\mu$ limit where it presents a dome. At higher values of $\mu$ the conventional contribution dominates. To make quantitative comparisons between our results and experimental findings, we rely on an experimentally observed global reduction of the superfluid weight that we apply to both the conventional and geometric contributions. Furthermore, an experimentally measured nonmonotonic variation of $\mu$ with the gate voltage $V_g$ is taken into account and yields a two-dome scenario for the superconducting critical temperature as a function of $V_g$. The observed dome in the low-$V_g$ regime is explained by the nonmonotonic evolution of a dominant conventional part of the superfluid density. In contrast, the expected second dome at larger values of $V_g$ limit would be due to a dominant quantum-geometric contribution.

We study the nonequilibrium electronic structure of a model Dirac semimetal ${\mathrm{ZrTe}}_5$ by using time-and-angle resolved photoemission spectroscopy and density functional theory–based electron and phonon calculations. By measuring the electronic dispersion near the $\mathrm{\Gamma }$ point at time delays up to 10 picoseconds, we discovered that the band spectral weight does not recover during the measured temporal window, revealing the existence of a light-induced metastable state in the electronic structure of this material. Our calculations find that the photoexcited $A_{1g}$ phonon mode leads to a band renormalization that both supports our experimental observations at the zone center and predicts changes to the band structure outside of our experimental window, ultimately showing the evolution from a direct to an indirect gap semimetal; such band renormalization dramatically reduces the electron-hole recombination rate giving rise to the metastability in this system.

We present first-principles results on monolayer (ML) and bulk titanium carbide (TiC) exhibiting dual topological characteristics: a quantum spin Hall insulator state in two-dimensional ML and a type-II Dirac semimetal state in its three-dimensional bulk form. The nontrivial nature of ML TiC is confirmed through the calculation of a ${\mathrm{Z}}_2$ invariant, spin Hall conductivity, and edge states. The edge band structure of ML TiC displays a single pair of gapless edge states at the $M$ point. The insulating topological phase in ML TiC is driven by a band inversion around the $M$ point involving Ti-$d$ orbitals, with a nontrivial band gap of 0.47 eV. Our findings also indicate that ML TiC possesses excellent dynamical and thermal stability. Moreover, the bulk, ML hollow sphere arrays, and a 4-nm-thick film of TiC have already been synthesized, suggesting the high feasibility of the experimental synthesizing of ML TiC. On the other hand, we demonstrate that the bulk TiC hosts both a type-II Dirac cone and a topological nodal surface without spin-orbit coupling, protected by a combined symmetry of inversion and time reversal. The presence of spin-orbit coupling removes the nodal surface while preserving the type-II Dirac cone. Surface states connecting bulk Dirac nodes in bulk TiC can be readily observed, making the surface characteristics easily detectable in experiments.

We perform sign-problem-free determinant quantum Monte Carlo simulations of the optical Su-Schrieffer-Heeger model on a half-filled honeycomb lattice. In particular, we investigate the model's semimetal (SM) to Kekulé valence bond solid (KVBS) phase transition at zero and finite temperatures as a function of phonon energy and interaction strength. Using hybrid Monte Carlo sampling methods, we can simulate the model near the adiabatic regime, allowing us to access regions of parameter space relevant to graphene. Our simulations suggest that the SM-KVBS transition is weakly first-order at all temperatures, with graphene situated close to the phase boundary in the SM region of the phase diagram. Our results highlight the important role bond-stretching phonon modes play in the formation of KVBS order in strained graphene-derived systems.

The surface of a three-dimensional topological insulator (TI) hosts surface states whose properties are determined by a Dirac-like equation. The electronic system on the surface of TI nanowires with a polygonal cross-sectional shape adopts the corresponding polygonal shape. In a constant transverse magnetic field, such an electronic system exhibits rich properties because different facets of the polygon experience different values of the magnetic field due to the changing magnetic field projection between facets. We investigate the energy spectrum and transport properties of nanowires, where we consider three different polygonal shapes, all showing distinct properties visible in the energy spectrum and transport properties. Here we propose that the wire conductance can be used to establish the presence of nanowire surface states by rotating the magnetic field and observing a variation in agreement with the cross-sectional shape of the nanowire. Distinguishing between the different shapes also works in the presence of impurities as long as conductance steps are discernible, thus revealing the subband structure.

We perform a diffusion Monte Carlo study on the excitonic complexes in the electron-hole (e-h) bilayer systems realized in multivalley semiconductors. Our focus is on polyexcitons with $n$ e-h pairs and charged polyexcitons consisting of $n$ e-h pairs plus one excess electron. By introducing the spin and two-valley degrees of freedom for each carrier, one can relax Pauli-blocking effects. Consequently, we find stable polyexcitons up to $n=4$ and charged polyexcitons up to $n=3$ for small interlayer distances and for any e-h mass ratio. It turns out that the separation energies required to remove an exciton from these excitonic complexes increase linearly with $n$, suggesting that each pair of excitons is bound by an identical “chemical bond,” irrespective of the number $n$ or the type of excitonic complex. In charged polyexcitons, each exciton exhibits an “affinity” with the excess electron. We posit excitons and excess electron as the fundamental building blocks of such complexes by providing a compelling picture of their internal structures.

Plasmon resonances in metal nanoparticles provide a powerful way to amplify molecular circular dichroism (CD). In this paper, we use time-dependent density-functional theory to systematically investigate the CD enhancement of a molecule on a silver nanoparticle or sandwiched between two silver nanoparticles. We find that in both systems the enhancement increases roughly as the square root of the number of Ag atoms up to the considered 923-atom Ag particle. The enhancement in dimers is more significant. In addition, we analyze the influences of the molecular orientation, molecule-nanoparticle distance, and the molecular coverage on the CD enhancement.

The complex phase diagram of the Fe(Se,S) system poses new challenges to understanding the relationship of the nematic phase, spin fluctuations and orderings, and superconductivity in this system and other iron-based superconductors. Combining density functional theory and dynamical mean field theory $(\mathrm{DFT}+\mathrm{DMFT})$, we carry out a comprehensive first-principles study of the crystal structures, electronic structures, topological properties, and spin dynamics of stoichiometric FeSe and FeS and doped ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$ $(x=0.25, 0.5, \text{and} 0.75)$. $\mathrm{DFT}+\mathrm{DMFT}$ optimized crystal structures of FeSe and FeS agree very well with experimental results and significantly improve DFT predictions. We find that at $x=0$ and 0.25, ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$ is topologically nontrivial and has topological Dirac surface states on the (001) surface. The system undergoes a topological phase transition between $x=0.25$ and 0.5 and becomes a topologically trivial compound at $x\ge$ 0.5. Meanwhile, with increasing S doping from $x=0$ to 1, both the electronic correlation strength, measured by the mass enhancement of Fe $3d$ electrons, and the low-energy spin fluctuations are suppressed monotonically. Moreover, when there is negligible nematicity at $x>$ 0.25, the evolution of the fluctuating total spin of the Fe $3d$ electrons exhibits the same trend as the experimentally determined superconducting transition temperature with increasing S doping. It can be concluded that both electronic correlation and spin fluctuations play important roles in the superconductivity of Fe(Se,S), whereas nematicity is detrimental to superconductivity.

Proximity coupling of ferromagnetic insulator EuS to the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ has been proposed to break time-reversal symmetry near the surface of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, introducing an energy gap or a tilt in the surface Dirac cone. As an inverse proximity effect, strong spin-orbit coupling available in the topological surface states can enhance the Curie temperature of ferromagnetism in EuS largely beyond its bulk value, and also generate a magnetic anisotropy. This can result in a canting of the magnetic moment of Eu ions in a plane perpendicular to the interface. Here, we investigate theoretically electronic transport properties arising from the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$/EuS interfaces in the planar Hall geometry. Our analysis, based on a realistic model Hamiltonian and a semiclassical formalism for the Boltzmann transport equation, reveals distinct intriguing features of anisotropic planar Hall conductivity, depending on different scenarios for the canting of the Eu moments: fixed Eu moment canting, and freely-orientable Eu moment in response to the external in-plane magnetic field. The anisotropy in the planar Hall conductivity arises from the asymmetric Berry curvature of the gapped topological surface states. We also explore topological Hall effect of the Dirac surface states, coupled to a skyrmion crystal which can emerge in the EuS due to the interplay of ferromagnetic Heisenberg exchange, interfacial Dzyaloshinskii-Moriya interaction, and perpendicular alignment of the Eu moment. Our study provides new impetus for probing complex interplay between magnetic exchange interactions and topological surface states via anisotropic planar Hall effects.

The physics of doped Mott insulators is at the heart of strongly correlated materials and is believed to constitute an essential ingredient for high-temperature superconductivity. In systems with higher $\mathrm{SU}\left(N\right)$ spin symmetries, even richer magnetic ground states appear at a filling of one particle per site compared to the case of $\mathrm{SU}\left(2\right)$ spins, but their fate upon doping remains largely unexplored. Here we address this question by studying a single hole in the SU(3) $t\text{−}J$ model whose undoped ground state features long-range, diagonal spin stripes. By analyzing both ground-state and dynamical properties utilizing the density matrix renormalization group, we establish the appearence of magnetic polarons consisting of chargons and flavor defects whose dynamics is constrained to a single effective dimension along the ordered diagonal. We semi-analytically describe the system using geometric string theory, where paths of hole motion are the fundamental degrees of freedom. With recent advances in the realization and control of $\mathrm{SU}\left(N\right)$ Fermi-Hubbard models with ultracold atoms in optical lattices, our results can directly be observed in quantum gas microscopes with single-site resolution. Our work suggests the appearance of intricate ground states at finite doping constituted by emergent, coupled Luttinger liquids along diagonals, and is a first step towards exploring a wealth of physics in doped $\mathrm{SU}\left(N\right)$ Fermi-Hubbard models on various geometries.