The spin configuration in the ferromagnetic part during the magnetization reversal plays a crucial role in the exchange bias effect. Through Monte Carlo simulation, the exchange bias effect in ferromagnetic-antiferromagnetic core-shell nanoparticles is investigated. Magnetization reversals in the ferromagnetic core were controlled between the coherent rotation and the domain wall motion by modulating the ferromagnetic domain wall width with parameters of uniaxial anisotropy constant and exchange coupling strength. An anomalous monotonic dependence of exchange bias on the uniaxial anisotropy constant is found in systems with small exchange coupling, showing an obvious violation of classic Meiklejohn-Bean model, while domain walls are found to form close to the interface and propagate in the ferromagnetic core with larger uniaxial anisotropy in both branches of the hysteresis. The asymmetric magnetization reversal with the formation of a spherical domain wall dramatically reduces the coercive field in the ascending branch, leading to the enhancement of the exchange bias. The results provide another degree of freedom to optimize the magnetic properties of magnetic nanoparticles for applications.

Mechanical control of magnetic order through strain is an important and promising approach in the development of advanced spintronics devices. Here, using a real-space phase-field model based on Ginzburg-Landau theory, we explore the uniaxial strain modulation of topological phase transition in ferromagnetic thin films. In the phase-field model, a magnetoelastic coupling between magnetization and strain, rather than the strain-induced anisotropic Dzyaloshinskii-Moriya interaction, is introduced to describe the strain-modulated anisotropic deformation and topological phase transition of skyrmions. The phase-field simulations show that a uniaxial tensile strain in the ferromagnetic thin films can not only change the wavelength and propagation direction of the spin spiral in helical phase but also induce topological phase transitions of skyrmions. Under specific magnetic fields, the ferromagnetic thin film exhibits the ferromagnetic-to-skyrmion and skyrmion-to-helical phase transitions serially when the uniaxial tensile strain increases, which are different from those induced by uniaxial stress in bulk ferromagnets. The present work not only provides a magnetoelastic coupling mechanism on the uniaxial strain control of topological magnetic structures but also suggests an effective way to tune the electromagnetic properties of ferromagnetic thin films for a new generation of spintronic devices based on skyrmions.

We present experimental evidence for (a) multiphase superconductivity and (b) coexistence of magnetism and superconductivity in a single structural phase of lithiated iron selenide hydroxide $\left[\right(\mathrm{L}{\mathrm{i}}_{1-x}\mathrm{F}{\mathrm{e}}_x)\text{OH]FeSe}$. Magnetic field modulated microwave spectroscopy data confirms superconductivity with at least two distinct transition temperatures attributed to well-defined superconducting phases at $T_{\mathrm{SC}1}=40\pm 2$ K and $T_{\mathrm{SC}2}=35\pm 2$ K. Magnetometry data for the upper critical fields reveal a change in the magnetic order $(T_M=12$ K) below $T_{\mathrm{SC}1}$ and $T_{\mathrm{SC}2}$ that is consistent with ferromagnetism. This occurs because the superconducting coherence length is much smaller than the structural coherence length, allowing for several different electronic and magnetic states on a single crystallite. The results give insight into the physics of complex multinary materials, where several phenomena governed by different characteristic length scales coexist.

We report on the interplay of $3d$- and $5d$-sublattice magnetism in polycrystalline samples of the double perovskite substitution series ${\mathrm{La}}_2{\mathrm{Zn}}_{1-x}{\mathrm{Co}}_x{\mathrm{IrO}}_6$. Powder x-ray diffraction reveals no major structural changes within the series. In magnetization measurements, a gradual shift of the transition temperature from ${\mathrm{T}}_N \approx 91$ K for the Co parent compound to ${\mathrm{T}}_N \approx 8.7$ K for the Zn parent compound is observed. The data on the Zn-rich members of the substitution series indicate that this is accompanied by changing roles of the $3d$ sublattice of ${\mathrm{Co}}^{2+}$ and the strongly spin-orbit coupled $5d$-sublattice of ${\mathrm{Ir}}^{4+}$ with its ${\mathrm{j}}_{\mathrm{eff}}=1/2$ ground state, as a function of the Co/Zn ratio. Temperature-dependent specific-heat studies revealed a reduced magnetic entropy, pointing towards a large spin-orbit coupling and orbital contribution in the system.

We demonstrate analytically and numerically that the dispersive Dirac cone emulating an epsilon-near-zero (ENZ) behavior is a universal property within a family of plasmonic crystals consisting of two-dimensional (2D) metals. Our starting point is a periodic array of 2D metallic sheets embedded in an inhomogeneous and anisotropic dielectric host that allows for propagation of transverse-magnetic (TM) polarized waves. By invoking a systematic bifurcation argument for arbitrary dielectric profiles in one spatial dimension, we show how TM Bloch waves experience an effective dielectric function that averages out microscopic details of the host medium. The corresponding effective dispersion relation reduces to a Dirac cone when the conductivity of the metallic sheet and the period of the array satisfy a critical condition for ENZ behavior. Our analytical findings are in excellent agreement with numerical simulations.

We theoretically discuss properties of intersubband surface plasmon polaritons (ISPPs) supported by the system consisting of a multiple quantum well (MQW) slab embedded into planar resonator with highly doped semiconducting claddings playing the role of cavity mirrors. Symmetric structures, where the MQW slab occupies the whole space between the claddings and asymmetric structures, where the MQW occupy only half of the space between mirrors, are considered. We focus mainly on the nearly degenerate structures where intersubband frequency is close to frequency of the surface plasmon of the mirrors. The ISPP characteristics are calculated numerically using a semiclassical approach based on the transfer matrix formalism and the effective-medium approximation. The claddings are described by the lossless Drude model. The possibility of engineering the dispersion of the ISPP branches is demonstrated. In particular, for certain parameters of the asymmetric structures we observe the formation of the multimode ISPP branches with two zero group velocity points. We show that the properties of the ISPP branches are reasonably well interpreted employing quasiparticle picture provided that the concept of the mode overlap factor is generalized, taking into account the dispersive character of the mirrors. In addition to this, we demonstrate that the lossless dispersion characteristics of the ISPP branches obtained in the paper are consistent with the angle-resolved reflection-absorption spectra of the GaAlAs-based realistic plasmonic resonators.

We obtained highly $n$-type doped graphene by intercalating terbium atoms between graphene and SiC(0001) through appropriate annealing in ultrahigh vacuum. After terbium intercalation angle-resolved-photoelectron spectroscopy (ARPES) showed a drastic change in the band structure around the $K$ points of the Brillouin zone: the well-known conical dispersion band of a graphene monolayer was superposed by a second conical dispersion band of a graphene monolayer with an electron density reaching ${10}^{15}{\mathrm{cm}}^{-2}$. In addition, we demonstrate that atom intercalation proceeds either below the buffer layer or between the buffer layer and the monolayer graphene. The intercalation of terbium below a pure buffer layer led to the formation of a highly $n$-doped graphene monolayer decoupled from the SiC substrate, as evidenced by ARPES and x-ray photoelectron spectroscopy measurements. The band structure of this highly $n$-doped monolayer graphene showed a kink (a deviation from the linear dispersion of the Dirac cone), which has been associated with an electron-phonon coupling constant one order of magnitude larger than those usually obtained for graphene with intercalated alkali metals.

We report the observation of bunching of monoatomic steps on vicinal W(110) surfaces induced by step-up or step-down currents across the steps. Measurements reveal that the size-scaling exponent γ, connecting the maximal slope of a bunch with its height, differs depending on the current direction. We provide a numerical perspective by using an atomistic-scale model with a conserved surface flux to mimic experimental conditions, and also show that there is an interval of parameters in which the vicinal surface is unstable against step bunching for both directions of the adatom drift.

We simultaneously measure photoresistance with electrical transport and coupled plasmon-cyclotron resonance using microwave reflection spectroscopy in high-mobility GaAs/AlGaAs quantum wells under a perpendicular magnetic field. Multiphoton transitions are revealed as sharp peaks in the resistance and the cyclotron reflection on samples with various carrier densities. Our main finding is that plasmon coupling is relevant in the cyclotron reflection spectrum but has not been observed in the electrical conductivity signal. We discuss possible mechanisms relevant to reflection or dc conductivity signal to explain this discrepancy. We further confirm a trend that more multiphoton features can be observed using higher carrier density samples.

Oxide growth with semiconductorlike accuracy has led to atomically precise thin films and interfaces that exhibit a plethora of phases and functionalities not found in the oxide bulk material. This has yielded spectacular discoveries such as the conducting, magnetic, and even superconducting ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interfaces separating two prototypical insulating perovskite materials. All these investigations, however, consider the static state at the interface, although studies on fast oxide interface dynamics would introduce a powerful degree of freedom to understanding the nature of the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface state. Here, we show that the polarization state at the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface can be optically enhanced or attenuated within picoseconds. Our observations are explained by a model based on charge propagation effects in the interfacial vicinity and transient polarization buildup at the interface.

We study the differential conductance of a time-reversal symmetric Weyl semimetal-superconductor junction with an $s$-wave superconducting state. We find that there exists an extended regime where the zero-bias differential conductance acquires the universal value of $e^2/h$ per channel, independent of the pairing and chemical potentials on each side of the junction, due to a perfect cancellation of Andreev and normal reflection contributions. This universal conductance can be attributed to the interplay of the unique spin/orbital-momentum locking and $s$-wave pairing that couples Weyl nodes of the same chirality. We expect that the universal conductance can serve as a robust and distinct signature for time-reversal symmetric Weyl fermions and be observed in the recently discovered time-reversal symmetric Weyl semimetals.

We calculate the width $2{\mathrm{\Delta }}_{\text{CT}}$ and intensity of the charge-transfer peak (the one lying at the on-site energy $E_d$) in the impurity spectral density of states as a function of $E_d$ in the SU($N$) impurity Anderson model (IAM). We use the dynamical density-matrix renormalization group (DDMRG) and the noncrossing approximation (NCA) for $N=4$ and a 1/$N$ variational approximation in the general case. In particular, while for $E_d\gg \mathrm{\Delta }$, where $\mathrm{\Delta }$ is the resonant level half-width, ${\mathrm{\Delta }}_{\text{CT}}=\mathrm{\Delta }$ as expected in the noninteracting case, for $-E_d\gg N\mathrm{\Delta }$ one has ${\mathrm{\Delta }}_{\text{CT}}=N\mathrm{\Delta }$. In the $N=2$ case, some effects of the variation of ${\mathrm{\Delta }}_{\text{CT}}$ with $E_d$ were observed in the conductance through a quantum dot connected asymmetrically to conducting leads at finite bias [J. Könemann et al., Phys. Rev. B 73, 033313 (2006)]. More dramatic effects are expected in similar experiments that can be carried out in systems of two quantum dots, carbon nanotubes or other, realizing the SU(4) IAM.

We combine the density matrix renormalization group (DMRG) with matrix product state tangent space concepts to construct a variational algorithm for finding ground states of one-dimensional quantum lattices in the thermodynamic limit. A careful comparison of this variational uniform matrix product state algorithm (VUMPS) with infinite density matrix renormalization group (IDMRG) and with infinite time evolving block decimation (ITEBD) reveals substantial gains in convergence speed and precision. We also demonstrate that VUMPS works very efficiently for Hamiltonians with long-range interactions and also for the simulation of two-dimensional models on infinite cylinders. The new algorithm can be conveniently implemented as an extension of an already existing DMRG implementation.

Amorphous semiconductors are usually characterized by a low charge carrier mobility, essentially related to their lack of long-range order. The development of such material with higher charge carrier mobility is hence challenging. Part of the issue comes from the difficulty encountered by first-principles simulations to evaluate concepts such as the electron effective mass for disordered systems since the absence of periodicity induced by the disorder precludes the use of common concepts derived from condensed matter physics. In this paper, we propose a methodology based on first-principles simulations that partially solves this problem, by quantifying the degree of delocalization of a wave function and of the connectivity between the atomic sites within this electronic state. We validate the robustness of the proposed formalism on crystalline and molecular systems and extend the insights gained to disordered/amorphous ${\mathrm{InGaZnO}}_4$ and Si. We also explore the properties of $p$-type oxide semiconductor candidates recently reported to have a low effective mass in their crystalline phases [G. Hautier et al., Nat. Commun. 4, 2292 (2013)]. Although in their amorphous phase none of the candidates present a valence band with delocalization properties matching those found in the conduction band of amorphous ${\mathrm{InGaZnO}}_4$, three of the seven analyzed materials show some potential. The most promising candidate, ${\mathrm{K}}_2{\mathrm{Sn}}_2{\mathrm{O}}_3$, is expected to possess in its amorphous phase a slightly higher hole mobility than the electron mobility in amorphous silicon.

We investigate the character of the van der Waals (vdW) torque and force between two coplanar and dielectrically anisotropic topological insulator (TI) slabs separated by a vacuum gap in the nonretardation regime, where the optic axes of the slabs are each perpendicular to the normal direction to the slab-gap interface and also generally differently oriented from each other. We find that in addition to the magnetoelectric coupling strength, the anisotropy can also influence the sign of the vdW force, viz., a repulsive vdW force can become attractive if the anisotropy is increased sufficiently. In addition, the vdW force oscillates as a function of the angular difference between the optic axes of the TI slabs, being most repulsive/least attractive (least repulsive/most attractive) for angular differences that are integer (half-integer) multiples of $\pi$. Our third finding is that the vdW torque for TI slabs is generally weaker than that for ordinary dielectric slabs. Our work provides an instance in which the vector potential appears in a calculation of the vdW interaction for which the limit is nonretarded or static.

For a thin ferromagnetic film with the Dzyaloshinskii-Moriya interaction (DMI), we derive an expression of the extrinsic ferromagnetic resonance (FMR) linewidth in a quantum-mechanical way, taking into account scatterings from structural inhomogeneity. In the presence of the DMI, the magnon dispersion exhibits rich resonant states, especially in small external magnetic fields and strong DMI strength. It is found that the FMR linewidth shows several characteristic features such as a finite linewidth at zero frequency and peaks in the low-frequency range.