We show that an isotropic dipolar particle in the vicinity of a substrate made of nonreciprocal plasmonic materials can experience a lateral thermal-fluctuations-induced force and torque when the particle's temperature differs from that of the slab and the environment. We connect the existence of the lateral force to the asymmetric dispersion of nonreciprocal surface polaritons and the existence of the lateral torque to the spin-momentum locking of such surface waves. Using the formalism of fluctuational electrodynamics, we show that the features of lateral force and torque should be experimentally observable using a substrate of doped indium antimonide (InSb) placed in an external magnetic field, and for a variety of dielectric particles. Interestingly, we also find that the directions of the lateral force and the torque depend on the constituent materials of the particles, which suggests a sorting mechanism based on nonequilibrium fluctuational electrodynamics.

The coherent third-order nonlinear response of monolayer transition-metal dichalcogenide semiconductors, such as ${\mathrm{MoSe}}_2$, is dominated by the nonlinear exciton response, as well as biexciton and trion resonances. The fact that these resonances may be spectrally close together makes identification of the signatures, for example in differential transmission (DT), challenging. Instead of focusing on explaining a given set of experimental data, a systematic study aimed at elucidating the roles of intravalley and intervalley long-range electron-hole $(e-\mathrm{h})$ exchange on the DT spectra is presented. Previous works have shown that the $e-\mathrm{h}$ long-range exchange introduces a linear leading-order term in the exciton dispersion. Based on a generalized Lippmann-Schwinger equation, we show that the presence of this linear dispersion term can reduce the biexciton binding energy to zero, contrary to the conventional situation of quadratic dispersion where an arbitrarily weak (well-behaved) attractive interaction always supports bound state(s). The effects of spin scattering and the spin-orbit interaction caused by $e-\mathrm{h}$ exchange are also clarified, and the DT line shape at the exciton and trion resonance is studied as a function of $e-\mathrm{h}$ exchange strength. In particular, as the exciton line shape is determined by the interplay of linear exciton susceptibility and the bound-state two-exciton resonance in the $T$ matrix, the line shape at the trion is similarly determined by the interplay of the linear trion susceptibility and the bound-state exciton-trion resonance in the $T$ matrix.

We present a theoretical framework to describe experiments directed to controlling single-atom spin dynamics by electrical means using a scanning tunneling microscope. We propose a simple model consisting of a quantum impurity connected to electrodes where an electrical time-dependent bias is applied. We solve the problem in the limit of weak coupling between the impurity and the electrodes by means of a quantum master equation that is derived by the nonequilibrium Green's function formalism. We show results in two cases. The first case is just a single atomic orbital subjected to a time-dependent electric field, and the second case consists of a single atomic orbital coupled to a second spin-1/2. The first case reproduces the main experimental features of Ti atoms on MgO/Ag (100) while the second one directly addresses the experiments on two Ti atoms. These calculations permit us to explore the effect of different parameters on the driving of the atomic spins as well as to reproduce experimental fingerprints.

Fluctuating Cooper pairs formed by spatially separated electrons and holes are precursors of their equilibrium condensation. Their presence strongly impacts transport phenomena and interlayer tunneling in double-layer systems above the transition temperature. Here, we consider a hybrid graphene/quantum-well double-layer system and focus on the dynamics of fluctuating Cooper pairs formed by conventional electrons and Dirac holes. We demonstrate that the chiral nature of Dirac holes is manifested in the presence of two (almost) degenerate competing pairing channels, which are intertwined by effective pseudospin-orbit interactions. We argue that the spectrum of the Ginzburg-Landau Hamiltonian describing the energetics of fluctuating Cooper pairs is geometrically nontrivial and can be characterized by the half-integer topological Chern number. We derive a kinetic equation for fluctuating Cooper pairs and demonstrate that their nontrivial geometries generate two anomalous velocities of distinct geometrical origins. These velocities are intricately connected with the Berry curvature and the quantum metric for the Ginzburg-Landau Hamiltonian, respectively. The resulting anomalous contributions to conductivity are singular at the transition temperature, and we discuss possible setups for their experimental observation.

${\mathrm{CaCo}}_{2-y}{\mathrm{As}}_2$ is an unusual itinerant magnet with signatures of extreme magnetic frustration. The conditions for establishing magnetic order in such itinerant frustrated magnets, either by reducing frustration or increasing electronic correlations, is an open question. Here, we use results from inelastic neutron scattering and magnetic susceptibility measurements and density functional theory calculations to show that hole doping in $\mathrm{Ca}{\left({\mathrm{Co}}_{1-x}{\mathrm{Fe}}_x\right)}_{2-y}{\mathrm{As}}_2$ suppresses magnetic order by quenching the magnetic moment while maintaining the same level of magnetic frustration. The suppression is due to tuning the Fermi energy away from a peak in the electronic density of states originating from a flat conduction band. This results in the complete elimination of the magnetic moment by $x\approx 0.25$, providing a clear example of a Stoner-type transition.

A density-functional-theory-based relativistic scattering formalism is used to study charge transport through thin Pt films with room-temperature lattice disorder. A Fuchs-Sondheimer specularity coefficient $p\sim 0.5$ is needed to describe the suppression of the charge current at the surface even in the absence of surface roughness. The charge current drives a spin Hall current perpendicular to the surface. Analyzing the latter with a model that is universally used to interpret the spin Hall effect in thin films and layered materials, we are unable to recover values of the spin-flip diffusion length $l_{\mathrm{sf}}$ and spin Hall angle ${\mathrm{\Theta }}_{\mathrm{sH}}$ that we obtain for bulk Pt using the same approximations. We trace this to the boundary conditions used and develop a generalized model that takes surface effects into account. A reduced value of ${\mathrm{\Theta }}_{\mathrm{sH}}$ at the surface is then found to describe the first-principles transport results extremely well. The in-plane spin Hall effect is substantially enhanced at the surface.

Helical symmetry of massive Dirac fermions is broken explicitly in the presence of electric and magnetic fields. Here we present two equations for the divergence of helical and axial vector currents following the Jackiw-Johnson approach to the anomaly of the neutral axial vector current. We discover the contribution from the helical symmetry breaking is attributed to the occupancy of the two states at the top of the valence band and the bottom of the conduction band. The explicit symmetry breaking fully cancels the anomalous correction from quantum fluctuation in the band gap. The chiral anomaly can be derived from the helical symmetry breaking. It provides an alternative route to understanding the chiral anomaly from the point of view of the helical symmetry breaking. The pertinent physical consequences in condensed matter are the helical magnetic effect, which means a charge current circulating at the direction of the magnetic field, and the mass dependent positive longitudinal magnetoconductivity as a transport signature. The discovery not only reflects anomalous magnetotransport properties of massive Dirac materials, but also reveals the close relation between the helical symmetry breaking and the physics of chiral anomaly in quantum field theory and high energy physics.

We point out that fractionalized bosonic charge excitations can explain the recently discovered photoinduced superconducting-like response in ${\kappa \text{-(ET})}_2\text{Cu}\left[{\text{N(CN)}}_2\right]\text{Br}$, an organic metal close to the Mott transition. The pump laser exerts a periodic drive on the fractionalized field, creating a nonequilibrium condensate, which gives a Drude peak much narrower than the equilibrium scattering rate, hence a superconducting-like response. Our proposal illuminates alternative possibilities of detecting fractionalization and can be readily tested in spin liquid candidates and in cold atom systems.

We study an interacting quantum dot in contact with a superconducting island described by the Richardson model with a Coulomb repulsion term controlling the number of electrons on the island. This Hamiltonian admits a compact matrix-product-operator representation and can be efficiently and accurately solved using the density-matrix renormalization group. We systematically explore the effects of the charging energy $E_c$. For $E_c$ comparable to the superconducting gap $\mathrm{\Delta }$, the subgap states are stabilized by the combination of Kondo exchange coupling and charge redistribution driven by the Coulomb interaction. The subgap states exist for both even and odd superconductor ground-state occupancy, but with very distinctive excitation spectra in each case. The spectral peaks are not symmetric with respect to the chemical potential and may undergo discontinuous changes as a function of gate voltages.

We employ dual-gated 30 °-twisted bilayer graphene to demonstrate simultaneous ultrahigh mobility and conductivity (up to 40 mS at room temperature), unattainable in a single layer of graphene. We find quantitative agreement with a simple phenomenology of parallel conduction between two pristine graphene sheets, with a gate-controlled carrier distribution. Based on the parallel transport mechanism, we then introduce a method for in situ measurements of the chemical potential of the two layers. This twist-enabled approach, neither requiring a dielectric spacer, nor separate contacting, has the potential to greatly simplify the measurement of thermodynamic quantities in graphene-based systems of high current interest.

We provide a unified semiclassical theory for the conserved current of nonconserved quantities, and manifest it in two physical contexts: the spin current of Bloch electrons and the charge current of mean-field Bogoliubov quasiparticles. We reveal that the previously overlooked torque quadrupole density and Berry phase correction to the torque dipole density are essential to assure a circulating spin current with null net flow at equilibrium. The band geometric origin of spin transport is ascertained to be the momentum space Berry curvature instead of the spin Berry curvature, paving the way for material related studies. In superconductors the attained conserved charge current corresponds to the quasiparticle charge current renormalized by the condensate backflow. Its circulation at equilibrium gives an orbital magnetization, which involves the characteristics of superconductivity, such as the Berry curvature arising from unconventional pairing and an orbital magnetic moment induced by the charge dipole of moving quasiparticles.

By using the gauge-invariant kinetic equation approach [Yang and Wu, Phys. Rev. B 98, 094507 (2018); Phys. Rev. B 100, 104513 (2019)], we construct the coupled dual dynamics of macroscopic phase coherence and microscopic electronic fluids in cuprate superconductors. We prove that the developed dual dynamics provides an efficient and simplified approach to formulate the dephasing process of macroscopic superconducting phase coherence, as well as its influence on microscopic electronic fluids (including gap, densities of superfluid and normal fluid, and in particular, the transport property to determine superconducting transition temperature $T_c$). We then present a theoretical description of the preformed Cooper pairs in a pseudogap state. The key origin of a pseudogap state comes from the quantum effect of disorder, which excites the macroscopic inhomogeneous phase fluctuation through the Josephson effect. Influenced by this phase fluctuation, there exist normal fluid and viscous superfluid below $T_c$ in cuprate superconductors, in addition to conventional nonviscous superfluid. The normal fluid always emerges around nodal points even at zero temperature, whereas the viscous superfluid emerges due to the friction between superfluid and normal fluids. Particularly, the nonviscous superfluid gets suppressed when the phase fluctuation is enhanced by the increasing temperature, until vanishes at $T_c$. Then, the system enters the pseudogap state, showing the nonzero resistivity as well as the finite gap from the viscous superfluid. By further increasing temperature to $T^{\mathrm{os}}$, the viscous superfluid and hence gap vanish. An experimental scheme to distinguish the densities of normal fluid as well as viscous and nonviscous superfluids is proposed. Finally, this theory is also applied to low-dimensional disordered $s$-wave superconductors.

The polar properties of antiphase boundaries (APBs) in ${\mathrm{PbZrO}}_3$ are analyzed in detail using a recently developed layer group approach in order parameter (OP) space and compared with the results from Landau-Ginzburg free energy description. It is shown that the former approach reveals the properties of the microscopic APBs and predicts polar APB structures at particular positions inside the unit cell, which agree very well with recent experimental observations [Wei et al., Nat. Commun. 5, 3031 (2014), Wei et al., Mater. Res. Bull. 62, 101 (2015)]. The systematic usage of the method is developed. In contrast with it, the commonly used free energy description obscures the microscopic features but still can reflect the macroscopic properties of the APBs by considering the bilinear coupling of polarization and OP gradients. The relation between the layer group approach and the Landau-Ginzburg free energy description is discussed, and two mechanisms of polarization switching inside the APBs are distinguished. It is illustrated that the polar APBs observed in ${\mathrm{PbZrO}}_3$ are consistently and naturally explained by the layer group approach. This analysis is expected to have a significant impact also in other materials.

When covered by gadolinium (Gd) atoms, silicene, a freestanding monolayer of Si atoms in a honeycomb network, remains stable above the room temperature and becomes a two-dimensional (2D) ferromagnetic semiconductor, despite the antiferromagnetic ground state of three-dimensional bulk ${\mathrm{GdSi}}_2$ crystal. In thin ${\mathrm{GdSi}}_2$ multilayers, even if magnetic moments are ordered parallel in the same Gd atomic planes, they are antiparallel between nearest Gd planes; hence they exhibit a ferrimagnetic behavior. In contrast, a freestanding ${\mathrm{Gd}}_2{\mathrm{Si}}_2$ monolayer constructed by covering silicene from both sides by Gd atoms is a stable antiferromagnetic metal due to the mirror symmetry. While multilayers covered by Gd from both sides having an odd number of Gd planes have a ferrimagneticlike ground state, even-numbered ones have antiferromagnetic ground state, but none of them is ferromagnetic. Silicon atoms intervening between Gd planes are responsible for these intriguing magnetic orders conforming with the recent experiments performed on Si(111) surface. Additionally, the magnetic states of these 2D gadolinium disilicide monolayers can be monitored by applied tensile strain and by the coverage/decoration of Gd. These predictions obtained by using first-principles, spin-polarized, density functional theory calculations combined with Monte Carlo simulations herald that C, B, Si, Ge, Sn, and their compounds functionalized by rare-earth atoms can lead to novel nanostructures in 2D spintronics.

Ferrovalley materials can achieve manipulation of the valley degree of freedom with intrinsic spontaneous valley polarization introduced by their intrinsic ferromagnetism. A good ferrovalley material should possess perpendicular magnetic anisotropy (PMA), the valence band maximum (VBM) or conduction band minimum at valley points, strong ferromagnetic (FM) coupling, and proper valley splitting. In this work, the monolayer ${\mathrm{GdCl}}_2$ is proposed as a potential candidate material for valleytronic applications using the first-principles calculations. It is proved that monolayer ${\mathrm{GdCl}}_2$ is a FM semiconductor with the easy axis along the out-of-plane direction and strong FM coupling. A spontaneous valley polarization with a valley splitting of 42.3 meV is produced due to its intrinsic ferromagnetism and spin-orbit coupling. Such valley splitting is larger than the thermal energy at room temperature, suggesting the possibility of room-temperature device operations. Although the VBM of unstrained monolayer ${\mathrm{GdCl}}_2$ is away from the valley points, a very small compressive strain (about 1%) can make the VBM move to the valley points. We propose a possible way to realize the anomalous valley Hall effect in monolayer ${\mathrm{GdCl}}_2$ using the piezoelectric effect, not an external electric field, namely, the piezoelectric anomalous valley Hall effect. This phenomenon could be classified as piezovalleytronics, being similar to piezotronics and piezophototronics. The only independent piezoelectric strain coefficient $d_{11}$ is $-2.708$ pm/V, which is comparable to that of the classical bulk piezoelectric material $\alpha$-quartz ($d_{11}=2.3$ pm/V). The biaxial in-plane strain and electronic correlation effects are considered to confirm the reliability of our results. Finally, monolayer ${\mathrm{GdF}}_2$ is predicted to be a ferrovalley material with dynamic and mechanical stabilities, PMA, the VBM at the valley points, strong FM coupling, valley splitting of 47.6 meV, and $d_{11}$ of 0.584 pm/V. Our work provides a possible way to achieve the anomalous valley Hall effect using the piezoelectric effect, which may stimulate further experimental works related to valleytronics.

Using conformal field theory calculations of the energy spectrum, within the XXZ model we investigate effects of the flux insertion and the Umklapp term. We discuss two approaches to the evaluation of the Drude weight, the first corresponding to the linear response theory and the second corresponding to the twisted boson theory with the Umklapp term. Divergences obtained in the context of the former contradict the Bethe ansatz results, with the two approaches coinciding for the free fermion point only. The origin of this discrepancy is in the different order in which the Umklapp term and the flux insertion are treated, where the marginal perturbation should be considered before the irrelevant perturbation. We calculate the scaling of the conductivity with system size and temperature in the long-wave limit.

We implement a computational pipeline based on a recent machine learning technique, namely, topological data analysis (TDA), that has the capability of extracting powerful information-carrying topological features. We apply such a method to study quantum phase transitions, and to showcase its validity and potential, we exploit such a method for the investigation of two quantum systems of paramount importance: the two-dimensional periodic Anderson model and the Hubbard model on the honeycomb lattice, both cases on the half filling. To this end, we have performed unbiased auxiliary-field quantum Monte Carlo simulations, feeding the TDA with snapshots of the Hubbard-Stratonovich fields through the course of the simulations. The quantum critical points obtained from TDA agree quantitatively well with the existing literature, therefore suggesting that this technique could be used to investigate quantum systems where the analysis of the phase transitions is still a challenge.

We study the nonlinear conductance through a quantum dot, specifically its dependence on the asymmetries in the tunnel couplings and bias voltages $V$, at low energies. Extending the microscopic Fermi liquid theory for the Anderson impurity model, we obtain an exact formula for the steady current $I$ up to terms of order $V^3$ in the presence of these asymmetries. The coefficients for the nonlinear terms are described in terms of a set of the Fermi liquid parameters: the phase shift, static susceptibilities, and three-body correlation functions of electrons in the quantum dots, defined with respect to the equilibrium ground state. We calculate these correlation functions, using the numerical renormalization group approach (NRG), over a wide range of impurity-electron filling that can be controlled by a gate voltage in real systems. The NRG results show that the order $V^2$ nonlinear current is enhanced significantly in the valence fluctuation regime. It is caused by the order $V$ energy shift of the impurity level, induced in the presence of the tunneling or bias asymmetry. Furthermore, in the valence fluctuation regime, we also find that the order $V^3$ nonlinear current exhibits a shoulder structure, for which the three-body correlations that evolve for large asymmetries play an essential role.

X-ray Raman scattering spectra of ferroelectric $\mathrm{BaTi}{\mathrm{O}}_3$ were measured under excitation around the Ti $K$-absorption edge. Density of states of the unoccupied Ti $3d$ state was measured using Ti $2p$ core excitations. Anisotropy was observed in the ferroelectric phase with respect to spontaneous polarization direction. This anisotropy decreased with rising temperature and vanished in the paraelectric phase. Experimental results showed the ion displacement and electronic structure are closely related, and the hybridization between Ti $3d$ and O $2p$ states affects the ferroelectric phase transition.

Silver chloride is a material that has been investigated and used for many decades. Of particular interest are its optical properties, but only few fundamental theoretical studies exist. We present first-principles results for the optical properties of AgCl, obtained using time-dependent density functional theory and many-body perturbation theory. We show that optical properties exhibit strong excitonic effects, which are correctly captured only by solving the Bethe-Salpeter equation starting from quasiparticle self-consistent $GW$ results. Numerical simulations are made feasible by using a model screening for the electron-hole interaction in a way that avoids the calculation of the static dielectric constant. Finally, the localization of bright and dark excitons is discussed.