Based on a combined charge and vector spin transport theory capable of imaging noncollinear magnetic textures on surfaces with spin-polarized scanning tunneling microscopy (SP-STM), the high-resolution tunneling electron charge and coupled spin transport properties of a variety of Néel- and Bloch-type skyrmions are investigated. Axially symmetric skyrmions are considered within the same topology class characterized by a vorticity value of $m=1$, and their helicities are varied by taking $\gamma =0$ and $\pi$ values for the Néel skyrmions and $\gamma =-\pi /2$ and $\pi /2$ values for the Bloch skyrmions. Depending on the orientation of the magnetization of the STM tip as well as on the helicity and the time reversal of the skyrmionic spin structures, several relationships between their spin transport vector components, the in-plane and out-of-plane spin transfer torque and the longitudinal spin current, are identified. The magnitudes of the spin transport vector quantities show close relation to standard charge current SP-STM images. It is also demonstrated that the SP-STM images can be used to determine the helicity of the skyrmions. Moreover, the modified spin-polarization vectors of the conduction electrons due to the local chirality of the complex spin texture are incorporated into the tunneling model. It is found that this effect modifies the apparent size of the skyrmions. These results contribute to the proper identification of topological surface magnetic objects imaged by SP-STM, and deliver important parameters for current-induced spin dynamics.

First-principles calculations of the electronic structure, magnetism, and structural stability of inverse Heusler compounds with the chemical formula $X_{2}YZ$ are presented and discussed with a goal of identifying compounds of interest for spintronics. Compounds for which the number of electrons per atom for $Y$ exceed that for $X$ and for which $X$ and $Y$ are each one of the $3d$ elements, Sc-Zn, and $Z$ is one of the group IIIA-VA elements: Al, Ga, In, Si, Ge, Sn, P, As, or Sb were considered. The formation energy per atom of each compound was calculated. By comparing our calculated formation energies to those calculated for phases in the inorganic crystal structure database of observed phases, we estimate that inverse Heuslers with formation energies within 0.052 eV/atom of the calculated convex hull are reasonably likely to be synthesizable in equilibrium. The observed trends in the formation energy and relative structural stability as the $X$, $Y$, and $Z$ elements vary are described. In addition to the Slater-Pauling gap after 12 states per formula unit in one of the spin channels, inverse Heusler phases often have gaps after 9 states or 14 states. We describe the origin and occurrence of these gaps. We identify 14 inverse Heusler semiconductors, 51 half-metals, and 50 near-half-metals with negative formation energy. In addition, our calculations predict 4 half-metals and 6 near-half-metals to lie close to the respective convex hull of stable phases, and thus may be experimentally realized under suitable synthesis conditions, resulting in potential candidates for future spintronics applications.

Within a gauge-invariant microscopic kinetic theory, we study the electromagnetic response in the superconducting states. Both superfluid and normal-fluid dynamics are involved. We predict that the normal fluid is present only when the excited superconducting velocity $v_s$ is larger than a threshold $v_L=\left|\mathrm{\Delta }\right|/k_F$. Interestingly, with the normal fluid, we find that there exists friction between the normal-fluid and superfluid currents. Due to this friction, part of the superfluid becomes viscous. Therefore a three-fluid model, normal fluid and nonviscous and viscous superfluids, is proposed. For the stationary magnetic response, at $v_s<v_L$ with only the nonviscous superfluid, the Meissner supercurrent is excited and the gap equation can be reduced to the Ginzburg-Landau equation. At $v_s\ge v_L$, with the normal fluid and nonviscous and viscous superfluids, in addition to the directly excited Meissner supercurrent in the superfluid, a normal-fluid current is also induced through the friction drag with the viscous superfluid current. Due to the normal-fluid and viscous-superfluid currents, the penetration depth is influenced by the scattering effect. In addition, a modified Ginzburg-Landau equation is proposed. We predict an exotic phase in which both the resistivity and superconducting gap are finite. As for the optical response, the excited $v_s$ oscillates with time. When $v_s<v_L$, only the nonviscous superfluid is present, whereas at $v_s\ge v_L$, normal fluid and nonviscous and viscous superfluids are present. We show that the excited normal-fluid current exhibits the Drude-model behavior, while the superfluid current consists of the Meissner supercurrent and Bogoliubov quasiparticle current. Due to the friction between the superfluid and normal-fluid currents, the optical conductivity is captured by the three-fluid model. Finally, we also study the optical excitation of the Higgs mode. By comparing the contributions from the drive and Anderson-pseudospin pump effects, we find that the drive effect is dominant at finite temperature whereas at zero temperature, both effects contribute.

We study composite $d$-wave superconductors consisting of randomly oriented and randomly distributed superconducting droplets embedded in a matrix. In a certain range of parameters the application of a small magnetic field enhances the superconductivity in these materials, while larger fields suppress superconductivity as usual in conventional superconductors. We investigate the magnetic field dependence of the superfluid density and the critical temperature of such superconductors.

We show that the stability (existence/absence) and interaction (repulsion/attraction) of chiral solitons in monoaxial chiral magnets can be varied by tilting the direction of the magnetic field. We thereby elucidate that the condensation of attractive chiral solitons causes the discontinuous phase transition predicted by a mean-field calculation. Furthermore, we theoretically demonstrate that the metastable field-polarized state destabilizes through the surface instability, which is equivalent to the vanishing surface barrier for penetration of the solitons. We experimentally measure the magnetoresistance (MR) of micrometer-sized samples in the tilted fields in a demagnetization-free configuration. We corroborate the scenario that hysteresis in MR is a sign of the existence of solitons, through agreement between our theory and experiments.

The evolution of Fermi surface (FS) states of ${\mathrm{NdFeAs}}_{1-x}{\mathrm{P}}_x{\mathrm{O}}_{0.9}{\mathrm{F}}_{0.1}$ single crystals with As/P substitution has been investigated. The critical temperature $T_c$ and the power-law exponent ($n$) of temperature-dependent resistivity $[\rho \left(T\right)={\rho }_0+A{T}^n]$ show a clear correlation above $x=0.2$, suggesting that $T_c$ is enhanced with increasing bosonic fluctuations in the same type of FS state. Around $x=0.2$, all the transport properties show anomalies, indicating that $x\sim 0.2$ is the critical composition of a drastic FS change. Angle-resolved photoemission spectroscopy has more directly revealed the distinct change of FS around $x=0.2$, that one hole FS disappears at the Brillouin zone center and the other FS with a propellerlike shape appears at the zone corner with decreasing $x$. These results are indicative of the existence of two types of FS states with different nesting conditions that are related with two $T_c$-raising mechanisms in this system.

The properties of a macroscopic assembly of weakly repulsive bosons at zero temperature are well described by Gross-Pitaevskii mean-field theory. According to this formalism the system exhibits a quantum transition from superfluid to cluster supersolid as a function of pressure. We develop a thermodynamically rigorous treatment of the different phases of the system by adopting a variational formulation of the condensate wave function—represented as a sum of Gaussians—that is amenable to exact manipulations. Not only is this description quantitatively accurate, but it is also capable to predict the order (and sometimes even the location) of the transition. We consider a number of crystal structures in two and three dimensions and determine the phase diagram. Depending on the lattice, the transition from fluid to solid can be first order or continuous, a lower coordination entailing a milder transition. In two dimensions, crystallization would occur at the same pressure on three distinct lattices (square, honeycomb, and stripes), all providing metastable phases with respect to the triangular crystal. A similar scenario holds in three dimensions, where the simple-cubic and diamond crystals also share a common melting point; however, the stable crystal at low pressure is typically fcc. Upon compression and depending on the shape of the potential, the fcc crystal may transform into hcp. We conclude by sketching a theory of the solid-fluid interface and of quantum nucleation of the solid from the fluid.

We study one-dimensional tight-binding models with a slowly varying, incommensurate off-diagonal modulation on the hopping amplitude. Interestingly, we find that the mobility edges can appear only when the off-diagonal (hopping) disorder is included, which is different from the known results induced by the diagonal disorder. We further study the situation where the off-diagonal and diagonal disorder terms (the incommensurate potential) are both included and find that the locations of mobility edges change significantly and the variation of the mobility edge becomes nonsmooth. To obtain these results, we first identify the exact expressions of mobility edges of both models by using asymptotic heuristic arguments, and then verify the conclusions by utilizing several numerical diagnostic techniques, including the inverse participation ratio, the density of states, and the Lyapunov exponent. These results will provide perspectives for future investigations on the mobility edge in low dimensional correlated disordered systems.

The magnetotransport of freestanding, vacuum filtered, thin films of $\mathrm{M}{\mathrm{o}}_2\mathrm{C}{\mathrm{T}}_z$, $\mathrm{M}{\mathrm{o}}_{1.33}\mathrm{C}{\mathrm{T}}_z$, $\mathrm{M}{\mathrm{o}}_2\mathrm{Ti}{\mathrm{C}}_2{\mathrm{T}}_z$, and $\mathrm{M}{\mathrm{o}}_2\mathrm{T}{\mathrm{i}}_2{\mathrm{C}}_3{\mathrm{T}}_z$ was measured in the 10–300-K temperature (T) range. Some of the films were annealed before measuring their transport properties. Analysis of the results suggest that—with the exception of the heavily defective $\mathrm{M}{\mathrm{o}}_{1.33}\mathrm{C}{\mathrm{T}}_z$ composition—in the 10- to 200-K temperature regime, variable range hopping between individual MXene sheets is the operative conduction mechanism. For $\mathrm{M}{\mathrm{o}}_{1.33}\mathrm{C}{\mathrm{T}}_z$ it is more likely that variable range hopping within individual flakes is rate limiting. At higher temperatures, a thermally activated process emerges in all cases. It follows that improved fabrication processes should lead to considerable improvements in the electrical transport of Mo-based MXenes.

It is proved that the energy absorption in a periodically driven classical spin system is exponentially slow in frequency, which results in a two-step relaxation called the Floquet prethermalization. This result is shown by establishing the classical limit of the quantum spin dynamics. The Floquet prethermal state is well described by the Gibbs ensemble with respect to the static effective Hamiltonian obtained by a truncation of the Floquet-Magnus expansion. On the other hand, the same effective Hamiltonian does not reproduce the local dynamics for an exponentially long time. This is due to the chaoticity of classical dynamics and in stark contrast to quantum spin systems, in which the effective Hamiltonian well reproduces the exact quantum dynamics up to an exponentially long time.

Recent experiments on electron- or hole-doped ${\mathrm{SrTiO}}_3$ have revealed a hitherto unknown form of superconductivity in which the Fermi energy of the paired electrons is much lower than the energies of the bosonic excitations thought to be responsible for the attractive interaction. We show that this situation requires a fresh look at the problem, calling for (i) a systematic modeling of the dynamical screening of the Coulomb interaction by ionic and electronic charges, (ii) a transverse optical phonon mediated pair interaction, and (iii) a determination of the energy range over which the pairing takes place. We argue that the latter is essentially given by the limiting energy beyond which quasiparticles cease to be well-defined. The model allows us to find the transition temperature as a function of both the doping concentration and the dielectric properties of the host system, in good agreement with experimental data. The additional interaction mediated by the transverse optical soft phonon is shown to be essential in explaining the observed anomalous isotope effect. The model allows us to capture the effect of the incipient (or real) ferroelectric phase in pure or oxygen isotope substituted ${\mathrm{SrTiO}}_3$.

The noncentrosymmetric superconductor, ${\mathrm{Re}}_3\mathrm{Ta}$, has been characterized in detail with a combination of magnetization, heat capacity, and electrical resistivity measurements, as well as a microscopic investigation of the internal magnetic fields using muon spin spectroscopy ($\mu \mathrm{SR}$). In low applied fields, we observe $100\%$ flux expulsion at a temperature of $T_{\mathrm{c}}=$ $4.68\mathrm{K}$, which is concomitant with a sudden decrease of the electrical resistivity to zero and a sharp discontinuity in the heat capacity, confirming bulk superconductivity in this material. We find that ${\mathrm{Re}}_3\mathrm{Ta}$ is a poor metal, with superconductivity occurring in the dirty limit, and in which the disorder in the structure dominates the physical properties. Zero-field $\mu \mathrm{SR}$ shows that the superconducting state preserves time-reversal symmetry, and transverse-field measurements of the superfluid density are well described by an isotropic $s$-wave model. A careful analysis of the internal field distribution reveals a high level of disorder in the vortex lattice. Furthermore, we have combined the experimental data and calculated the effective mass, carrier density, and electronic mean-free path in this material, and ultimately show that ${\mathrm{Re}}_3\mathrm{Ta}$ lies close to the unconventional region of the Uemura plot.

Perovskite $\mathrm{SrIr}{\mathrm{O}}_3$ (SIO) films epitaxially deposited with a thickness of about 60 nm on various substrate materials display a nearly strain-relieved state. Films grown on orthorhombic (110) $\mathrm{DySc}{\mathrm{O}}_3$ (DSO) are found to display an untwinned bulklike orthorhombic structure. However, film deposition on cubic (001) $\mathrm{SrTi}{\mathrm{O}}_3$ (STO) induces a twinned growth of SIO. Resistance measurements on the SIO films reveal only weak temperature dependence, where the resistance R increases with decreasing temperature T. Hall measurements show dominant electronlike transport throughout the temperature range from 2 to 300 K. At 2 K, the electron concentration and resistivity for SIO on STO amounts to $n_{\mathrm{e}}=1.4\times {10}^{20}\mathrm{c}{\mathrm{m}}^{-3}$ and 1 mΩ cm. Interestingly, the film resistance of untwinned SIO on DSO along the [1-10] and the [001] direction differs by up to 25%, indicating pronounced anisotropic electronic transport. The anisotropy of the resistance increases with decreasing T and displays a distinct maximum at around 86 K. The specific T dependence is similar to that of the structural anisotropy $\surd \left(a^2+b^2\right)/c$ of bulk SIO. Therefore, anisotropic electronic transport in SIO is very likely induced by the orthorhombic distortion. Consequently, for twinned SIO films on STO anisotropy vanishes nearly completely. The experimental results show that structural changes are very likely responsible for the observed anisotropic electronic transport. The strong sensitivity of the electronic transport in SIO films may be explained in terms of the narrow electronlike bands in SIO caused by spin-orbit coupling and orthorhombic distortion.

Semimetals, in which conduction and valence bands touch but do not form Fermi surfaces, have attracted considerable interest for their anomalous properties starting with the discovery of Dirac matter in graphene and other two-dimensional honeycomb materials. Here we introduce a family of three-dimensional honeycomb systems whose electronic band structures exhibit a variety of topological semimetals with Dirac nodal lines. We show that these nodal lines appear in varying numbers and mutual geometries, depending on the underlying lattice structure. They are stabilized, in most cases, by a combination of time-reversal and inversion symmetries and are accompanied by topologically protected “drumhead” surface states. In the bulk, these nodal line systems exhibit Landau level quantization and flat bands upon applying a magnetic field. In the presence of spin-orbit coupling, these topological semimetals are found to generically form (strong) topological insulators. This comprehensive classification of the electronic band structures of three-dimensional honeycomb systems might serve as guidance for future material synthesis.

Bosonic condensation of microcavity polaritons is accompanied by their relaxation from the ensemble of excited states into a single quantum state. The excess of energy is transferred to the crystal lattice that eventually involves heating of the structure. Creation of the condensate results in the local increase of the temperature, which leads to the red shift of the exciton energy providing the mechanism for polariton self-trapping. By employing the driven-dissipative Gross-Pitaevskii model, we predict a new type of a stable localized solution supported by the thermally induced self-trapping in a one-dimensional microcavity structure. The predicted solution is of a sink-type, i.e., it is characterized by the presence of converging density currents. We examine the spontaneous formation of these states from the white noise under spatially localized pumping and analyze the criteria for their stability.

We show that the moiré pattern of graphene on Ir(111) acts as efficient template for the self-assembly of well-ordered superlattices of single rare-earth adatoms. Using Sm and Dy as representative of early and late lanthanides, we observe that the array quality is determined by the deposition temperature and by a large direct capture area, leading to partial dimer formation. These superlattices result from the combination of the inhomogeneous substrate potential for adatom diffusion generated by the moiré and the interatomic electrostatic repulsion originating from electron transfer to graphene. Deposition temperature- and coverage-dependent experiments quantify the energy barriers for adatom diffusion between adjacent graphene lattice sites and between moiré unit cells, as well as the amount of charge transferred to the substrate by each adatom.

Defect-tolerant perovskite nanocrystals of the general formula Cs-Pb-$X_3$ (where $X=\mathrm{Cl}$, Br, and I) have shown exceptional potential in fundamental physics as well as in novel optoelectronic applications as the next generation of solar cells. Although exciton many-body interactions such as biexciton Stark shift, state filling, and Auger recombination are studied extensively, other important correlated effects, such as band gap renormalization (BGR) and hot-phonon bottleneck, are not explored in these nanocrystals. Here we experimentally demonstrate the carrier density dependence of the BGR and an effective hot-phonon bottleneck in $\mathrm{CsPb}{\left(\mathrm{C}{\mathrm{l}}_{0.20}\mathrm{B}{\mathrm{r}}_{0.80}\right)}_3$ mixed-halide nanocrystals. The results are compared with two other halide compositions, namely, ${\mathrm{CsPbBr}}_3$ and $\mathrm{CsPb}{\left(\mathrm{B}{\mathrm{r}}_{0.55}{\mathrm{I}}_{0.45}\right)}_3$ nanocrystals with varying band gaps. The optical response of the nanocrystals changes dramatically across the spectral range of many hundreds of meV at high carrier density due to large BGR. We have calculated the BGR constant $\approx \left(6.0\pm 0.3\right)\times {10}^{-8}\mathrm{eV}$ cm for $\mathrm{CsPb}{\left(\mathrm{C}{\mathrm{l}}_{0.20}\mathrm{B}{\mathrm{r}}_{0.80}\right)}_3$ nanocrystals that provides the amount of band gap shift as a function of carrier density. In these nanocrystals, an efficient hot-phonon bottleneck is observed at a carrier density of $3.1\times {10}^{17}\mathrm{c}{\mathrm{m}}^{-3}$ that slows down the thermalization by 1 order of magnitude. Our findings reveal that the complex kinetic profile of the exciton dynamics can be analyzed by the global target analysis using the sequential model with increasing lifetimes.

Recently, Russell et al. [Phys. Rev. Lett. 120, 047401 (2018)] have reported a clear signal of many-particle contributions to cyclotron resonance in high-mobility hexagonal boron nitride (hBN)-encapsulated graphene, observing significant variations of resonance energies as a function of the filling factor $\nu$ for a series of interband channels. To elucidate their results, Coulombic contributions to the Landau-level spectra and cyclotron resonance in graphene are examined with a possible band gap taken into account and with emphasis on revealing electron-hole ($e-h$) conjugation symmetry underlying such level and resonance spectra. Theory, based on the single-mode approximation, gives a practically good account of the experimental data; the data suggest a band gap of $\sim 10$ meV and show a profile that apparently reflects $e-h$ conjugation symmetry.