Tailoring band alignment layer-by-layer using heterojunctions of two-dimensional (2D) semiconductors is an attractive prospect for producing next-generation electronic and optoelectronic devices that are ultrathin, flexible, and efficient. The 2D layers of transition metal dichalcogenides (TMDs) in laboratory devices have already shown favorable characteristics for electronic and optoelectronic applications. Despite these strides, a systematic understanding of how band alignment evolves from monolayer to multilayer structures is still lacking in experimental studies, which hinders development of novel devices based on TMDs. Here, we report on the local band alignment of monolayer, bilayer, and trilayer $\mathrm{Mo}{\mathrm{S}}_2$ on a 285-nm-thick $\mathrm{Si}{\mathrm{O}}_2$ substrate using an approach to probe the occupied electronic states based on photoemission electron microscopy and deep-ultraviolet light. Local measurements of the vacuum level and the valence band edge at the Brillouin zone center show that the addition of layers to monolayer $\mathrm{Mo}{\mathrm{S}}_2$ increases the relative work function and pushes the valence band edge toward the vacuum level. We also deduced $n$-type doping of few-layer $\mathrm{Mo}{\mathrm{S}}_2$ and type-I band alignment across monolayer-to-bilayer and bilayer-to-trilayer lateral junctions. Conducted in isolation from environmental effects owing to the vacuum condition of the measurement and an insulating $\mathrm{Si}{\mathrm{O}}_2$ substrate, this study shows a metrology to uncover electronic properties intrinsic to $\mathrm{Mo}{\mathrm{S}}_2$ semiconducting layers and emerging 2D crystals alike.

We combine space group representation theory together with the scanning of closed subdomains of the Brillouin zone with Wilson loops to algebraically determine the global band-structure topology. Considering space group No. $19$ as a case study, we show that the energy ordering of the irreducible representations at the high-symmetry points $\{\mathrm{\Gamma },S,T,U\}$ fully determines the global band topology, with all topological classes characterized through their simple and double Dirac points.

Nonlinear electric transport and switching to a negative resistance state are typical electric-field-induced phenomena in correlated electron materials, while their mechanisms are generally difficult to solve. In the present study, we apply the terahertz-radiation imaging method to an organic molecular ferroelectric, α-type bis(ethylenedithio)tetrathiafulvalene iodide salt, and investigate the nature of its negative resistance phenomenon. When the negative resistance state is produced, the ferroelectric order is melted in an elongated region with the width of ∼100 μm and that region grows along the direction inclined by about 40° from the $b$ axis with the increase of nonlinear current. A comparison of the terahertz radiation intensity with the current magnitude revealed that the melted region forms a conducting path. We interpreted the diagonal growth of the conduction path by taking into account the anisotropy of the intermolecular transfer integrals.

We present resonant x-ray emission spectroscopic data from the uranium intermetallics ${\mathrm{UPd}}_3$, USb, ${\mathrm{USn}}_3$, and ${\text{URu}}_2{\text{Si}}_2$ at the $M_{4,5}$ edges of uranium, and we compare the data to those from the well-localized $5f^2$ semiconductor ${\mathrm{UO}}_2$. The technique is especially sensitive to any oxidation of the surface, and this was found on the USb sample, thus preventing a good comparison with a material known to be $5f^3$. We have found a small energy shift between ${\mathrm{UO}}_2$ and ${\mathrm{UPd}}_3$, both known to have localized $5f^2$ configurations, which we ascribe to the effect of conduction electrons in ${\mathrm{UPd}}_3$. The spectra from ${\mathrm{UPd}}_3$ and ${\text{URu}}_2{\text{Si}}_2$ are similar, strongly suggesting a predominant $5f^2$ configuration for ${\text{URu}}_2{\text{Si}}_2$. The valence-band resonant inelastic x-ray scattering provides information on the U $P_3$ transitions (at about 18 eV) between the U $5f$ and U $6p$ states, as well as transitions of between 3 and 7 eV from the valence band into the unoccupied $5f$ states. These transitions are primarily involving mixed ligand states (O $2p$ or Pd,Ru $4d$) and U $5f$ states. Calculations are able to reproduce both of these low-energy transitions reasonably well.

The magnetoelectric coupling in possible Kitaev spin liquid $\alpha -{\mathrm{RuCl}}_3$ with the layered honeycomb structure was examined. We observed a remarkable anisotropic magnetodielectric effect in the zigzag-type antiferromagnetic phase; there is a large suppression in dielectric constant, when both electric and magnetic fields were applied parallel to the in-plane direction. A possible origin of the observed anisotropic magnetodielectric effect is discussed in terms of a magnetically induced local electric polarization with antiferroelectric correlation. Our results stimulate the model calculation including not only exchange coupling but also magnetoelectric coupling to understand the ground state of the spin system in $\alpha -{\mathrm{RuCl}}_3$.

Current-induced spin polarization in a two-dimensional electron gas with Rashba spin-orbit interaction is considered theoretically in terms of the Matsubara Green functions. This formalism allows us to describe the temperature dependence of the induced spin polarization. The electron gas is assumed to be coupled to a magnetic substrate via exchange interaction. Analytical and numerical results on the temperature dependence of spin polarization are obtained in the linear response regime. The spin polarization is presented as the sum of two terms: one proportional to the relaxation time and the other related to the Berry phase corresponding to the electronic bands of the magnetized Rashba gas. The spin-orbit torque due to Rashba interaction is also discussed. This torque appears as a result of the exchange coupling between the nonequilibrium spin polarization and the magnetic moment of the underlayer.

The sheet resistance $R_s$ of $\mathrm{A}{\mathrm{r}}^{+}$ irradiated $\mathrm{SrTi}{\mathrm{O}}_3$ in patterns with a length scale of several microns increases significantly below ∼40 K in connection with driving currents exceeding a certain threshold. The initial lower $R_s$ is recovered upon warming with accelerated recovery around 70 and 160 K. Scanning superconducting quantum interference device microscopy shows local irreversible changes in the spatial distribution of the current with a length scale of several microns. We attribute the observed nonuniform enhancement of $R_s$ to the attraction of the charged single-oxygen and dioxygen vacancies by the crystallographic domain boundaries in $\mathrm{SrTi}{\mathrm{O}}_3$. The boundaries, which are nearly ferroelectric below 40 K, are polarized by the local electrical field associated with the driven current and the clustered vacancies which suppress conductivity in their vicinity and yield a noticeable enhancement in the device resistance when the current path width is on the order of the boundary extension. The temperatures of accelerated conductivity recovery are associated with the energy barriers for the diffusion of the two types of vacancies.

We investigate the shifts of the core-level binding energies in small gold nanoclusters by using ab initio density-functional-theory calculations. The shift of the $4f$ states is calculated for magic-number nanoclusters in a wide range of sizes and morphologies. We find a nonmonotonous behavior of the core-level shift in nanoclusters depending on the size. We demonstrate that there are three main contributions to the Au $4f$ shifts, which depend sensitively on the interatomic distances, coordination, and quantum confinement. They are identified and explained by the change of the on-site electrostatic potential.

In this paper we demonstrate the necessity of including the generally omitted collective-mode contributions in calculations of the Meissner effect for nonuniform superconductors. We consider superconducting pairing with nonzero center-of-mass momentum, as is possibly relevant to high transition temperature cuprates, cold atoms, and color superconductors in quantum chromodynamics. For the concrete example of the Fulde-Ferrell phase we present a quantitative calculation of the superfluid density, showing not only that the collective-mode contributions are appreciable but also that they derive from the amplitude mode of the order parameter. This latter mode is generally viewed as being invisible in conventional superconductors. However, our analysis shows that it is extremely important in pair-density-wave-type superconductors, where it destroys stable superfluidity well before the mean-field order parameter vanishes.

The pair density wave (PDW) superconducting state has been proposed to explain the layer-decoupling effect observed in the ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$ compound at $x=1/8$ [E. Berg, E. Fradkin, E.-A. Kim, S. A. Kivelson, V. Oganesyan, J. M. Tranquada, and S. C. Zhang, Phys. Rev. Lett. 99, 127003 (2007)]. In this state the superconducting order parameter is spatially modulated, in contrast with the usual superconducting (SC) state where the order parameter is uniform. In this paper, we study the properties of the amplitude (Higgs) modes in a unidirectional PDW state. To this end we consider a phenomenological model of PDW-type states coupled to a Fermi surface of fermionic quasiparticles. In contrast to conventional superconductors that have a single Higgs mode, unidirectional PDW superconductors have two Higgs modes. While in the PDW state the Fermi surface largely remains gapless, we find that the damping of the PDW Higgs modes into fermionic quasiparticles requires exceeding an energy threshold. We show that this suppression of damping in the PDW state is due to kinematics. As a result, only one of the two Higgs modes is significantly damped. In addition, motivated by the experimental phase diagram, we discuss the mixing of Higgs modes in the coexistence regime of the PDW and uniform SC states. These results should be observable directly in a Raman spectroscopy, in momentum resolved electron energy-loss spectroscopy, and in resonant inelastic x-ray scattering, thus providing evidence of the PDW states.

We present a fully three-dimensional Bloch mode expansion technique and a photon Green function formalism to compute the quality factors, mode volumes, and Purcell enhancement distributions of a disordered W1 photonic crystal slab waveguide in the slow-light Anderson-localization regime. By considering fabrication (intrinsic) and intentional (extrinsic) disorder we find that the Purcell enhancement statistics are well described by log-normal distributions without any fitting parameters. We also compare directly the effects of hole size fluctuations as well as fluctuations in the hole position. The functional dependence of the mean and standard deviation of the quality factor and Purcell enhancement distributions is found to decrease exponentially with the square root of the extrinsic disorder parameter. The strong coupling probability between a single quantum dot and an Anderson-localized mode is numerically computed and found to exponentially decrease with the squared extrinsic disorder parameter, where low disordered systems give rise to larger probabilities when state-of-the-art quantum dots are considered. The optimal spatial regions to position quantum dots in the W1 waveguide are also discussed. These theoretical results are fundamentally interesting for disordered photonics and connect to recent experimental works on photonic crystal slab waveguides in the slow-light regime. Our three-dimensional slab results also contradict some previous findings that use simpler two-dimensional models to understand these complex planar systems.

We study a dimerized spin chain with biaxial magnetic interacting ions in the presence of externally induced three-site interactions out of equilibrium. In the general case, the three-site interactions play a role in renormalizing the effective uniform magnetic field. We find that the existence of zero-energy Majorana modes is intricately related to the sign of Pfaffian of the Bogoliubov–de Gennes Hamiltonian and the relevant $Z_2$ topological invariant. In contrast, we show that an exotic spin-liquid phase can emerge in the compass limit through a Berezinskii-Kosterlitz-Thouless (BKT) quantum phase transition. Such a BKT transition is characterized by a large dynamic exponent $z=4$, and the spin-liquid phase is robust under a uniform magnetic field. We find the relative entropy and the quantum discord can signal the BKT transitions. We also uncover a few differences in deriving the correlation functions for the systems with broken reflection symmetry.

We present a comprehensive study of how superconducting fluctuations in the normal state contribute to the conductivity tensor in a thin (119 Å) film of NbN. It is shown how these fluctuations drive a sign change in the Hall coefficient $R_{\mathrm{H}}$ for low magnetic fields near the superconducting transition. The scaling behaviors as a function of distance to the transition $\epsilon =ln(T/T_{\mathrm{c}})$ of the longitudinal (${\sigma }_{\mathrm{xx}}$) and transverse (${\sigma }_{\mathrm{xy}}$) conductivity are found to be consistent with Gaussian fluctuation theory. Moreover, excellent quantitative agreement between theory and experiment is obtained without any adjustable parameters. Our experimental results thus provide a case study of the conductivity tensor originating from short-lived Cooper pairs.

We have found Dirac nodal lines (DNLs) in the band structures of metallic rutile oxides ${\mathrm{IrO}}_2, {\mathrm{OsO}}_2$, and ${\mathrm{RuO}}_2$ and have revealed a large spin Hall conductivity contributed by these nodal lines, which explains a strong spin Hall effect (SHE) of ${\mathrm{IrO}}_2$ discovered recently. Two types of DNLs exist. The first type forms DNL networks that extend in the whole Brillouin zone and appears only in the absence of spin-orbit coupling (SOC), which induces surface states on the boundary. Because of SOC-induced band anticrossing, a large intrinsic SHE can be realized in these compounds. The second type appears at the Brillouin zone edges and is stable against SOC because of the protection of nonsymmorphic symmetry. Besides reporting these DNL materials, our work reveals the general relationship between DNLs and the SHE, indicating a way to apply Dirac nodal materials for spintronics.

We study the impact of dynamical electron-phonon (el-ph) effects on the electronic band gap of ice and liquid water by accounting for frequency-dependent Fan contributions in the el-ph mediated self-energy within the many-body perturbation theory (MBPT). We find that the dynamical el-ph coupling effects greatly reduce the static el-ph band-gap correction of the hydrogen-rich molecular ice crystal from$-2.46$ to $-0.23$ eV in great contrast to the result of Monserrat et al. [Phys. Rev. B 92, 140302 (2015)]. This is of particular importance as otherwise the static el-ph gap correction would considerably reduce the electronic band gap, leading to considerable underestimation of the intense peaks of optical absorption spectra of ice which would be in great disagreement to experimental references. By contrast, the static el-ph gap correction of liquid water is very moderate ($-0.32$ eV), and inclusion of dynamical effects slightly reduces the gap correction to $-0.19$ eV. Further, we determine the diverse sensitivity of ice and liquid water to the $GW$ self-consistency and show that the energy-only self-consistent approach ($G_n{W}_n$) exhibits large implicit vertex character in comparison to the quasiparticle self-consistent approach, for which an explicit calculation of vertex corrections is necessary for good agreement with experiment.

We derive the quasiclassical nonequilibrium Eilenberger and Usadel equations to first order in quantities small compared to the Fermi energy, valid for Dirac edge and surface electrons with spin-momentum locking $\mathbf{p}·\overline{\mathbf{\sigma }}$, as relevant for topological insulators. We discuss in detail several of the key technical points and assumptions of the derivation, and provide a Riccati parametrization of the equations. Solving first the equilibrium equations for S/N and S/F bilayers and Josephson junctions, we study the superconducting proximity effect in Dirac materials. Similarly to related works, we find that the effect of an exchange field depends strongly on the direction of the field. Only components normal to the transport direction lead to attenuation of the Cooper pair wave function inside the F. Fields parallel to the transport direction lead to phase shifts in the dependence on the superconducting phase difference for both the charge current and density of states in an S/F/S junction. Moreover, we compute the differential conductance in S/N and S/F bilayers with an applied voltage bias and determine the dependence on the length of the N and F regions and the exchange field.

The role of energy exchange between a quantum system and its environment is investigated from the perspective of the Onsager conductance matrix. We consider the thermoelectric linear transport of an interacting quantum dot coupled to two terminals under the influence of an electrical potential and a thermal bias. We implement in our model the effect of coupling to electromagnetic environmental modes created by nearby electrons within the $P\left(E\right)$ theory of dynamical Coulomb blockade. Our findings relate the lack of some symmetries among the Onsager matrix coefficients to an enhancement of the efficiency at maximum power and the occurrence of the heat rectification phenomenon.

Motivated by photocurrent spectroscopy experiments with $\mathrm{Mo}{\mathrm{S}}_2$, the direct band-to-band transition contribution to the photocurrent of transition-metal dichalcogenides is computed starting from the microscopic Hamiltonian in the Keldysh-Schwinger formalism. It turns out that the band tilting induced by source to drain voltage is necessary for a nonvanishing photocurrent. The photon energy dependence of the normalized photocurrent is found to be consistent with experimental data. For circular polarization the helicity-dependent component of the photocurrent perpendicular to the band-tilting direction is predicted, which essentially originates from the valley Hall effect.

In periodic systems, electronic wave functions of the eigenstates exhibit the periodically modulated Bloch phases and are characterized by their wave numbers $\mathbf{k}$. We theoretically address the effects of the Bloch phase in general layered materials with a stacking shift. When the interlayer shift and the Bloch wave vector $\mathbf{k}$ satisfy certain conditions, interlayer transitions of electrons are prohibited by the interference of the Bloch phase. We specify the manifolds in the $\mathbf{k}$ space where the hybridization of the Bloch states between the layers is suppressed in accord with the stacking shift. These manifolds, named stacking-adapted interference manifolds (SAIM), are obviously applicable to general layered materials regardless of a detailed atomic configuration within the unit cell. We demonstrate the robustness and usefulness of the SAIM with first-principles calculations for layered boron nitride, transition-metal dichalcogenide, graphite, and black phosphorus. We also apply the SAIM to general three-dimensional crystals to derive special k-point paths for the respective Bravais lattices, along which the Bloch-phase interference strongly suppresses the band dispersion. Our theory provides a general view on the anisotropic electronic motion intrinsic to the periodic-lattice structure.

Materials with the morphotropic phase boundary (MPB) exhibit an ultrahigh mechanical response to electrical inputs, which has been widely used in applications such as sensors and actuators. Recently, the rare-earth element doped $\mathrm{BiFe}{\mathrm{O}}_3$ (BFO) was found to possess a MPB between a rhombohedral polar phase and an orthorhombic antipolar phase with enhanced piezoelectric response, enabling it to be an attractive alternative to toxic Pb-based piezoelectric materials. Despite theoretical and experimental efforts, the phase transition behavior under electric fields has not been directly confirmed, leaving a gap in the understanding of the origin of enhanced piezoelectricity. Here, we have demonstrated an irreversible electric-field induced phase transition from the antipolar phase to the polar phase in Sm-doped BFO with the pre-MPB composition, and a reversible phase transition between the polar phase and the antipolar/nonpolar phase in Sm-doped BFO with the MPB composition. In situ transmission electron microscopy technique combined with thermodynamic calculation based on the Ginzburg-Landau-Devonshire theory indicates that the electric-field induced reversible phase transition leads to enhanced piezoelectric response and double P-E hysteresis loops. These results provide us a deep insight into the mechanism of exotic electromechanical response in the rare-earth element doped BFO system with the composition near the MPB.