We consider current-driven motion of magnetic skyrmions in granular magnetic films. The study uses micromagnetic modeling and phenomenological analysis based on the Thiele formalism. Remarkably, the disorder enhances the effective skyrmion Hall effect that depends on the magnitude of the driving force (the cur- rent density and nonadiabaticity parameter). The origin is the sliding motion of the skyrmion along the grain boundaries, followed by pinning and depinning at the grain junctions. A side jump can occur during this depinning process. In addition, the critical current that triggers the skyrmion motion depends on the relative size of the crystallites with respect to the skyrmion size. Finally, when the skyrmion trajectory is confined along an edge by the nonadiabatic Magnus force, the critical current density can be significantly reduced. Our results imply that narrow nanowires exhibit higher skyrmion mobilities.

Spin-polarized normal-metal wires coupled to a superconductor can host Majorana states at their ends. These decay into the bulk and are protected by a minigap in the spectrum. Previous studies have found that strong coupling between the superconductor and the normal-metal wire renormalizes the properties of this low-energy phase. Here, we develop a semiclassical scattering approach to explain these renormalization effects. We show that a renormalization of the propagation velocity in the normal wire originates from double Andreev reflection processes at the superconductor interface and that it continues to exist in the absence of a proximity-induced minigap in the normal-metal wire. We also show that the renormalization effects exist for arbitrary transparency of the normal-metal–superconductor interface, provided the superconductor coherence length is sufficiently long in comparison to the thickness of the normal metal.

The $d$-wave superconductivity (SC) is analyzed within the three-band $d\text{−}p$ model with the use of the diagrammatic expansion of the Gutzwiller wave function method (DE-GWF). The determined stability regime of the superconducting state appears in the range of hole doping $\delta \lesssim 0.35$, with the optimal doping close to $\delta \approx 0.19$. The pairing amplitudes between the $d$ orbitals due to copper and $p_x/p_y$ orbitals due to oxygen are analyzed together with the hybrid $d\text{−}p$ pairing. The $d\text{−}d$ pairing between the nearest-neighboring atomic sites constitutes the dominant contribution to the SC phase. Moreover, it is shown that the decrease of both the Coulomb repulsion on the copper atomic sites ($U_d$) and the charge-transfer energy between the oxygen and copper atomic sites (${\epsilon }_{dp}$) increases the pairing strength as it moves the system from the strong- to the intermediate-correlation regime, where the pairing is maximized. This result is consistent with our analysis of the ratio of changes in the hole content at the $d$ and $p$ orbitals due to doping, which, according to an experimental study, increases with the increasing maximal critical temperature [cf. D. Rybicki et al., Nat. Commun. 7, 11413 (2016)]. Furthermore, the results for the three-band model are compared to those for the effective single-band picture and similarities between the two approaches are discussed. For the sake of completeness, the normal-state characteristics determined from the DE-GWF approach are compared with those resulting from the variational quantum Monte Carlo method with intersite correlations included through the appropriate Jastrow factors.

We explore the unconventional propagation of light in a three-dimensional (3D) superlattice of coupled resonant cavities in a 3D photonic band-gap crystal. Such a 3D cavity superlattice is the photonic analog of the Anderson model for spins and electrons in the limit of zero disorder. Using the plane-wave expansion method, we calculate the dispersion relations of the 3D cavity superlattice with the cubic inverse woodpile structure that reveal five coupled-cavity bands, typical of quadrupole-like resonances. For three out of five bands, we observe that the dispersion bandwidth is significantly larger in the $(k_x,k_z)$-diagonal directions than in other directions. To explain the directionality of the dispersion bandwidth, we employ the tight-binding method from which we derive coupling coefficients in three dimensions. For all converged coupled-cavity bands, we find that light hops predominantly in a few high-symmetry directions including the Cartesian $(x,y,z)$ directions, therefore we propose the name “Cartesian light.” Such 3D Cartesian hopping of light in a band gap yields propagation as superlattice Bloch modes that differ fundamentally from the conventional 3D spatially extended Bloch wave propagation in crystals, from light tunneling through a band gap, from coupled-resonator optical waveguiding, and also from light diffusing at the edge of a gap.

In the $\mathrm{GO}/\mathrm{MoS}{\mathrm{e}}_2$ semiconductor heterostructure, we have demonstrated a subtle control on the doping dynamics by modulating interlayer coupling through the combination of strain-reducing relative rotation of the constituting layers and variation of ligand type and concentration. By first-principles calculations incorporating spin-orbit coupling, we have investigated the impact of variable interlayer coupling in introducing noncollinear magnetic behavior in the heterostructure. The outcome of varying carrier type and their respective concentrations are investigated by static as well as time-dependent density functional calculations, which indicate the presence of optical anisotropy and time-dependent optical phenomena such as exciton quenching and band-gap renormalization. The performance of such heterostructures as channel material in devices with top and edge metal contacts is analyzed. Our self-consistent quantum transport calculations have evinced that the interface-induced variation in doping pattern is extrapolated only for devices with top contacts. The edge contact, although it exhibits a better transmission, is inefficient in sensing the ligand-induced doping modulation introduced via vertical interlayer charge transfer.

Scanning tunneling microscopy and spectroscopy are used to image the charge density wave at the surface of cleaved ${\mathrm{VSe}}_2$ and to probe its local density of states at 5 K. The main features in the spectrum are linked to the contributions of the $p$-like and $d$-like bands of ${\mathrm{VSe}}_2$ found in angle-resolved photoemission spectroscopy and tight-binding calculations. Different from previous tunneling spectroscopy work, we find a narrow partial gap at the Fermi level that we associate with the charge density wave phase. The energy scale of the gap found in the experiment is in good agreement with the charge density wave transition temperature of ${\mathrm{VSe}}_2$, under the assumption of weak electron-phonon coupling, consistent with the Peierls model of Fermi surface nesting. The role of defects is investigated, which reveals that the partial gap in the density of states and hence the charge density wave itself is extremely stable, though the order, phase, and amplitude of the charge density waves on the surface are strongly perturbed by defects.

We investigate the electronic structure of the highly anisotropic $\beta$ phase of metallic plutonium, within the combination of density functional theory and dynamical mean field theory. Its crystal structure gives rise to site and orbital selective electronic correlations, with coherent $\mathrm{Pu}\text{−}5f_{5/2}$ states and very incoherent $\mathrm{Pu}\text{−}5f_{7/2}$ states. The Hund's coupling is essential for determining the level of correlations of electrons in $\mathrm{Pu}\text{−}5f$ states, and for the quasiparticle multiplets features in the $\mathrm{Pu}\text{−}5f$ spectral function.

An interacting lattice model describing the subspace spanned by a set of strongly correlated bands is rigorously coupled to density functional theory to enable ab initio calculations of geometric and topological material properties. The strongly correlated subspace is identified from the occupation number band structure as opposed to a mean-field energy band structure. The self-consistent solution of the many-body model Hamiltonian and a generalized Kohn-Sham equation exactly incorporates momentum-dependent and crystal-symmetric correlations into electronic structure calculations in a way that does not rely on a separation of energy scales. Calculations for a multiorbital Hubbard model demonstrate that the theory accurately reproduces the many-body macroscopic polarization.

We have investigated the electronic structures of Ce Kondo systems, CeNiSn, CeRhAs, and CeRhSb, which have attracted recent attention as promising candidates of “Möbius Kondo insulators.” In the conventional density functional theory calculations, all three systems are obtained to be semimetallic, while only CeRhAs opens a full gap in the band calculation incorporating the more elaborate (modified Becke-Jones) exchange-correlation potential. Intriguingly, for all three systems, we have obtained the hourglass-type bulk band crossings around $\mathbf{k}=S$, which produce Dirac nodal-loop structures enclosing S in the [100] Brillouin zone boundary. Such topologically protected hourglass-type band structures arise from the spin-orbit coupling on top of the nonsymmorphic crystal symmetry $\left(Pnma\right)$ of these systems. Our finding suggests that Ce Kondo insulators would display temperature $\left(T\right)$ -dependent topological phase transitions from a potential Möbius Kondo insulator at low $T$ to a Dirac nodal-loop semimetal at high $T$.

We investigate and experimentally observe the existence of topologically protected interface modes in a one-dimensional mechanical lattice and we report on the effect of nonlinearities on topological protection. The lattice consists of a one-dimensional array of spinners with nearest-neighbor coupling resulting from magnetic interactions. The distance between the spinners is spatially modulated to obtain a diatomic configuration, and to produce a nontrivial interface by breaking spatial inversion symmetry. For small amplitudes of motion, the interactions are approximately linear, and the system supports topologically protected interface modes at frequencies inside the bulk band gap of the lattice. Nonlinearities induced by increasing amplitude of motion cause the interface modes to shift and merge with the bulk bands. The resulting edge-to-bulk transition causes the extinction of the topologically protected interface mode and extends it to the entire length of the chain. Such transition is predicted by analytical calculations and verified by experimental observations. The paper thus investigates topologically protected interface modes obtained through broken spatial inversion symmetry, and documents the lack of robustness in the presence of nonlinearities.

We present a microscopic theory for electron-phonon energy exchange in Anderson insulators at low temperatures. The major contribution to the cooling power $J_{e\text{−}ph}\left(T_{el}\right)$ as a function of electron temperature $T_{el}$ is shown to be directly related to the correlation function of the local density of electron states $K\left(\omega \right)$. In Anderson insulators not far from localization transition, the correlation function $K\left(\omega \right)$ is enhanced at small $\omega$ by the wave function's multifractality and by the presence of Mott's resonant pairs of states. The theory we develop explains a huge enhancement of the cooling power observed in insulating indium oxide films as compared to predictions of the theory previously developed for disordered metals.

The crystal structures of a perovskite-related layered compound ${\mathrm{Sr}}_3{\mathrm{W}}_2{\mathrm{O}}_9$ are investigated within a broad range of temperatures using x-ray diffraction and transmission electron microscopy. We observe successive structural phase transitions from the high-temperature ($R\overline{3}m$) to the middle-temperature ($R\overline{3}c$), and then to the low-temperature ($C2/c$) phases. The order parameter between the high- and middle-temperature phases can be regarded as the tilt angle of ${\mathrm{WO}}_6$ octahedra. In the middle-temperature phase, with increasing temperature, the tilt angle of ${\mathrm{WO}}_6$ octahedra decreases and approaches that of the high-temperature phase (zero tilting angle). In the low-temperature phase, an additional monoclinic distortion, which breaks the threefold symmetry that is observed in the high- and middle-temperature phases, appears. The successive structural phase transitions in ${\mathrm{Sr}}_3{\mathrm{W}}_2{\mathrm{O}}_9$ are attributed to the tilting of ${\mathrm{WO}}_6$ octahedra and subsequent octahedral distortion related to the layered structure.

We provide a theoretical framework for the prediction and interpretation of momentum-dependent phonon spectra due to coherent inelastic scattering of electrons. We complete the approach with first-principles lattice dynamics using periodic density functional theory and compare to recent electron energy-loss measurements on cubic and hexagonal boron nitride performed within a scanning transmission electron microscope. The combination of theory and experiment provides the ability to interpret momentum-dependent phonon spectra obtained at nanometer spatial resolution in the electron microscope.

We measure the transient strain profile in a nanoscale multilayer system composed of yttrium, holmium, and niobium after laser excitation using ultrafast x-ray diffraction. The strain propagation through each layer is determined by transient changes in the material-specific Bragg angles. We experimentally derive the exponentially decreasing stress profile driving the strain wave and show that it closely matches the optical penetration depth. Below the Néel temperature of Ho, the optical excitation triggers negative thermal expansion, which is induced by a quasi-instantaneous contractive stress and a second contractive stress contribution increasing on a 12-ps timescale. These two timescales were recently measured for the spin disordering in Ho [Rettig et al., Phys. Rev. Lett. 116, 257202 (2016)]. As a consequence, we observe an unconventional bipolar strain pulse with an inverted sign traveling through the heterostructure.

Time-resolved spontaneous Raman spectroscopy serves as a probe for incoherent quasiparticle and collective excitation dynamics, and allows to distinguish symmetry changes across a photoinduced phase transition through the inelastic light scattering selection rules. Largely unexplored is the role of the Raman resonance enhancement in the time domain, and the transient interaction between scattering from quasiparticles and collective excitations, with the latter interaction leading to a Fano interference. In this work we report on the observation of a phonon Raman tensor quench and Fano interference after strong photoexcitation of an intrinsic semiconductor. We observed a dynamic phonon scattering rate asymmetry and spectral asymmetry through simultaneous detection of both the anti-Stokes and Stokes response. The asymmetric phonon scattering rate is ascribed to the combined effect of the transient phonon population and the reduction of the phonon Raman tensor resulting from the photoexcited hole population. This same hole population results in a strong enhancement of the Fano spectral asymmetry.

Motivated by a novel cyclic compound ${\text{Fe}}_{10}{\text{Gd}}_{10}$ with record ground state spin in which the arrangement of magnetic ions with $s=\frac{5}{2}$ and $s=\frac{7}{2}$ corresponds to a sawtooth chain, we investigate the thermodynamics of the delta chain with competing ferro- and antiferromagnetic interactions. We study both classical and quantum versions of the model. The classical model is exactly solved and quantum effects are studied using full diagonalization and a finite temperature Lanczos technique for finite delta chains as well as modified spin wave theory. It is shown that the main features of the magnetic susceptibility of the quantum spin delta chain are correctly described by the classical spin model, while quantum effects significantly change the low-temperature behavior of the specific heat. The relation of the obtained results to the ${\text{Fe}}_{10}{\text{Gd}}_{10}$ system is discussed.

Fascinating phenomena observed under applied pressure and magnetic field are attracting much research attention. Recent experiments have shown that application of the pressure or magnetic field to the ${\mathrm{USb}}_2$ compound induce the transformations of the ground-state antiferromagnetic (AFM) structure $(+--+)$ to, respectively, ferromagnetic (FM) or ferrimagnetic structures. Remarkably, the magnetic critical temperature of the FM state, induced by pressure, is more than two times smaller than the Néel temperature of the AFM ground state. We performed density-functional theory (DFT) and DFT+$U$ studies to reveal the origin of the unusual magnetic ground state of the system and the driving mechanisms of the phase transitions. We investigate both the magnetic anisotropy properties and the parameters of the interatomic exchange interactions. To study pressure-induced effects we carry out calculations for reduced volume and demonstrate that the existence of the AFM-FM phase transformation depends on the peculiar features of the magnetic anisotropy. We discuss why the magnetic field that couples directly to the magnetic moments of atoms leads to the phase transition to the ferrimagnetic state whereas the pressure that does not couple directly to magnetic moments results in the FM structure. Our work demonstrates how the competition of different physical factors leads to variety of unusual properties of the antiferromagnetic ${\mathrm{USb}}_2$.

Modifying the conventional spin-wave theory in a manner based on the Wick decomposition, we present an elaborate thermodynamics of square-lattice quantum antiferromagnets. Our scheme is no longer accompanied by the notorious problem of an artificial transition to the paramagnetic state inherent in modified spin waves in the Hartree-Fock approximation. In the cases of spin $\frac{1}{2}$ and spin 1, various modified-spin-wave findings for the internal energy, specific heat, static uniform susceptibility, and dynamic structure factor are not only numerically compared with quantum Monte Carlo calculations and Lanczos exact diagonalizations but also analytically expanded into low-temperature series. Modified spin waves interacting via the Wick decomposition provide reliable thermodynamics over the whole temperature range of absolute zero to infinity. Adding higher-order spin couplings such as ring exchange interaction to the naivest Heisenberg Hamiltonian, we precisely reproduce inelastic-neutron-scattering measurements of the high-temperature-superconductor-parent antiferromagnet ${\mathrm{La}}_2{\mathrm{CuO}}_4$. Modifying Dyson-Maleev bosons combined with auxiliary pseudofermions also yields thermodynamics of square-lattice antiferromagnets free from thermal breakdown, but it is less precise unless temperature is sufficiently low. Applying all the schemes to layered antiferromagnets as well, we discuss the advantages and disadvantages of modified spin-wave and combined boson-pseudofermion representations.

In two-dimensional (2D) superconductors, an insulating state can be induced either by increasing disorder or by applying a magnetic field $H$. Many scenarios have been put forth to explain the superconductor to insulator transition (SIT). One of the main difficulties in discerning between the various scenarios is to elucidate the nature of the emergent insulating state. This complicatedness stems from the lack of a continuous mapping of the superconducting to insulating phase diagram in a single sample. Here we use the 2D electron liquid formed at the interface between (111) ${\mathrm{SrTiO}}_3$ and ${\mathrm{LaAlO}}_3$ to study the SIT as a function of electrostatic gate and magnetic field. This crystalline interface exhibits very prominent features: (a) a very large and anisotropic magnetoresistance (MR) peak emerging from the superconducting state and persisting even when the sample is totally insulating, (b) hysteresis in the MR peak, and (c) linear MR for low perpendicular magnetic fields. We identify a new magnetic field scale, where the superconducting fluctuations are muted, and find a length scale ${\xi }_{\text{ins}}$ for the vortex fluctuation in the insulating state. Our findings suggest that vortex excitations and Cooper pair localization are responsible for the observed SIT. Surprisingly, these excitations exist even when the sample is tuned into the zero-field insulating state.

We report magnetic torque measurements on iron-pnictide superconductors ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($x\approx$ 0.69 and 0.76) to an applied field of $B_a=45$ T. The peak effect is observed in torque-vs-field curves below the irreversibility field. It is enhanced and becomes asymmetric as the field is tilted from the $c$ axis. For field directions close to the $ab$ plane, increasing- and decreasing-field curves peak at markedly different fields and exhibit a sharp jump, suggestive of a first-order phase transition, on the high- and the low-field sides of the peak, respectively. Complicated history dependence of the torque is observed in the peak-effect region. We construct and discuss the temperature– ($T$) applied-magnetic-field ($B_a$) phase diagram. Since the upper critical field for the $ab$-plane direction is comparable to the Pauli limit, we also consider possible influence of the spin paramagnetic effect on the anomalous peak effect.