We consider a hybrid structure consisting of superconducting or normal leads with a combined ferromagnet–three-dimensional topological insulator interlayer. We compare responses of a Josephson junction and a normal junction to magnetic texture dynamics. In both cases the electromotive force resulting from the magnetization dynamics generates a voltage between the junction leads. For an open circuit this voltage is the same for normal and superconducting leads and allows for electrical detection of magnetization dynamics and a structure of a given magnetic texture. However, under the applied current the electrical response of the Josephson junction is essentially different due to the strong dependence of the critical Josephson current on the magnetization direction and can be used for experimental probing of this dependence. We propose a setup which is able to detect a defect motion and to provide detailed information about the structure of magnetic inhomogeneity. The discussed effect could be of interest for spintronics applications.

Two-dimensional (2D) boron has been predicted to exhibit various structural polymorphs and interesting physics, including superconductivity, ideal strength, negative Possion's ratio, and a higher thermal conductivity than graphene, etc. However, the difficulty in experimental synthesis of 2D boron has further reduced its exploration and applications. In this work we have used a crystal structure search method and first-principles calculations. We have proposed a scheme of “synthesize under high pressure and peel off at ambient conditions” to first predict a bulk phase of $\mathrm{K}{\mathrm{B}}_4$, and then peel off free standing 2D boron from it. Based on electron-phonon coupling calculations, we estimated that 2D boron is an intrinsic superconductor with $T_c=17.9\mathrm{K}$. In addition we found a strong anisotropy of the electron-phonon coupling strength in 2D boron, thus the superconductivity in 2D boron possesses a quasi-1D feature. Our proposed scheme for the synthesis of free standing 2D boron will stimulate further studies of superconductivity in low dimensional materials, both theoretically and experimentally.

We have investigated a series of superconducting bridges based on homogeneous amorphous WSi and MoSi films, with bridge widths $w$ ranging from 2 to $1000\mu \mathrm{m}$ and film thicknesses $d\sim 4-6$ and 100 nm. Upon decreasing the bridge widths below the respective Pearl lengths, we observe in all cases distinct changes in the characteristics of the resistive transitions to superconductivity. For each of the films, the resistivity curves $R(B,T)$ separate at a well-defined and field-dependent temperature $T^{*}\left(B\right)$ with decreasing the temperature, resulting in a dramatic suppression of the resistivity and a sharpening of the transitions with decreasing bridge width $w$. The associated excess conductivity in all the bridges scales as $1/w$, which may suggest either the presence of a highly conducting region that is dominating the electric transport, or a change in the vortex dynamics in narrow enough bridges. We argue that this effect can only be observed in materials with sufficiently weak vortex pinning.

Based on ab initio band-structure calculations we formulate a general theoretical method for description of the temperature dependence of an electric-field gradient in solids. The method employs a procedure of averaging multipole electron-density component $\left(l\ne 0\right)$ inside a sphere vibrating with the nucleus at its center. As a result of averaging, each Fourier component $\left(K\ne 0\right)$ on the sphere is effectively reduced by the square root of the Debye-Waller factor $[exp(-W\left)\right]$. The electric-field gradient related to a sum of $K$ components most frequently decreases with temperature $\left(T\right)$, but under certain conditions because of the interplay between terms of opposite signs, it can also increase with $T$. The method is applied to calculations of the temperature evolution of the electric-field gradients of pristine zinc and cadmium crystallized in the hexagonal lattice. For calculations within our model, of crucial importance is the temperature dependence of mean-square displacements which can be taken from experiment or obtained from the phonon modes in the harmonic approximation. For the case of Zn, we have used data obtained from single-crystal x-ray diffraction. In addition, for Zn and Cd, we have calculated mean-square displacements with the density-functional perturbation treatment of the quantum espresso package. With the experimental data for displacements in Zn, our calculations reproduce the temperature dependence of the electric-field gradient very accurately. Within the harmonic approximation of the quantum espresso package, the decrease in electric-field gradients in Zn and Cd with temperature is overestimated. Our calculations indicate that the anharmonic effects are of considerable importance in the temperature dependence of electric-field gradients.

Static magnetization configurations of thin soft ferromagnetic films and nanodots, coupled to a hard antidot matrix with out-of-plane magnetization, are studied by micromagnetic simulations and analytical calculations. When the antidot matrix produces sufficient stray fields, having radial symmetry, these nanostructures support the formation of topologically nontrivial magnetic configurations—vortices and skyrmions in nanodots and films, respectively. It is demonstrated that the studied nanostructure reveals an additional degree of freedom—the helicity of the vortex or skyrmion—which can be tuned on demand by a variation of the material parameters and geometry. The variation of helicity $\gamma$ is not abrupt. In addition to Neel-like (radial) vortices and skyrmions $\left(\gamma =0,\pi \right)$, it is possible to achieve unconventional configurations with an intermediate helicity $\gamma \ne 0,\pm \pi /2,\pi$, which transform to common Bloch-like configurations $\left(\gamma =\pm \pi /2\right)$ in the limit of negligible stray fields from the matrix. We present an analytical model, which allows us to calculate the stability region of pure Neel-like states, outside which unconventional magnetization states with intermediate helicity are realized.

The edge of the electronic fractional quantum Hall (FQH) system obeys the law of the chiral Luttinger liquid theory due to its intrinsic topological properties and the relation of bulk-edge correspondence. However, in a realistic experimental system, such as the usual Hall bar setup, the softening of the background confinement potential can induce the reconstruction of the edge spectrum, which breaks the chirality and universality of the FQH edge. The entanglement spectrum (ES) of the FQH ground state has the same counting structure as that of the energy spectrum indicating the topological characters of the quantum state. In this work, we report that the ES can also have an edge reconstruction while sweeping the area of the subsystem in real space cut. Moreover, we found that the critical area of the subsystem matches accurately with the intrinsic building block of the FQH liquids, namely the correlation hole of the FQH liquids. The above results seem to be universal after our study of a series of typical FQH states, such as two Laughlin states at $\nu =1/3$ and $1/5$, and the Moore-Read state for $\nu =5/2$.

We demonstrate that the plasmon in one-dimensional Coulomb interacting electron fluids can develop a finite-momentum maxon-roton-like nonmonotonic energy-momentum dispersion. Such an unusual nonmonotonicity arises from the strongly interacting $1/r$ Coulomb potential going beyond the conventional band linearization approximation used in the standard bosonization theories of Luttinger liquids. We provide details for the nonmonotonic plasmon dispersion using both bosonization and random-phase approximation theories. We also calculate the specific heat including the nonmonotonicity and discuss possibilities for observing the nonmonotonic plasmon dispersion in various physical systems, including semiconductor quantum wires, carbon nanotubes, and the twisted bilayer graphene at subdegree twist angles, which naturally realize one-dimensional domain-wall states. We provide results for several different models of long-range interaction showing that the nonomonotonic charge collective mode dispersion is a generic phenomenon in one-dimensional strongly interacting electron systems.

The study of thin-film materials is a subject of growing interest. Some of these materials are insulating due to the presence of disorder, which also produces localization of charges. Kelvin probe force microscopy (KPFM) is a unique tool to characterize these materials, but a full quantitative interpretation of the results is still lacking. To address this problem, we propose a simple and fast procedure based on the image charge method that represents an advance in this direction since it is not limited to any film thickness or the nature of the underlying substrate. Even more, it can be combined with fast Fourier transform algorithms to generate theoretical images from known charge distributions or to obtain charge distributions from the Kelvin voltage images. Within this framework, we analyze the problem of the lateral resolution of the technique, providing a criterion to estimate it. Finally, we address the problem of systems with hopping conductivity where multiple localized charges coexist. We demonstrate that even in these complex systems, the KPFM gives valuable information, allowing us to distinguish between noninteracting and interacting electronic systems. Furthermore, it is possible to calculate the charge density in the noninteracting case.

We study the influence of epitaxial strain on the electronic properties of InAs/GaSb composite quantum wells (CQWs), host structures for quantum spin Hall insulators, by transport measurements and eight-band $\mathbf{k}·\mathbf{p}$ calculations. Using different substrates and buffer layer structures for crystal growth, we prepare two types of samples with vastly different strain conditions. CQWs with a nearly strain-free GaSb layer exhibit a resistance peak at the charge neutrality point that reflects the opening of a topological gap in the band-inverted regime. In contrast, for CQWs with 0.50% biaxial tensile strain in the GaSb layer, semimetallic behavior indicating a gap closure is found for the same degree of band inversion. Additionally, with the tensile strain, the boundary between the topological and nontopological regimes is located at a larger InAs thickness. Eight-band $\mathbf{k}·\mathbf{p}$ calculations reveal that tensile strain in GaSb not only shifts the phase boundary but also significantly modifies the band structure, which can result in the closure of an indirect gap and make the system semimetallic even in the topological regime. Our results thus provide a global picture of the topological-nontopological phase diagram as a function of layer thicknesses and strain.

The correlated electronic structure of the infinite-layer compounds ${\mathrm{NdNiO}}_2$ and ${\mathrm{SrCuO}}_2$ at stoichiometry and with finite hole doping is compared. Key differences are elucidated from an advanced first-principles many-body perspective. Contrary to the charge-transfer insulating cuprate, the self-doped nickelate remains noninsulating even for large interaction strength, though the Ni-$d_{x^2-y^2}$ spectral weight is also gapped in that limit. Hybridization between $\mathrm{Ni}\left(3d\right)$ and $\mathrm{Nd}\left(5d\right)$ is crucial for the appearance of the self-doping band. Upon realistic hole doping, ${\mathrm{Sr}}_{1-y}{\mathrm{CuO}}_2$ shows the expected mixed oxygen-Cu-$d_{x^2-y^2}$ (Zhang-Rice) states at low energy. In the case of ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2$, the self-doping band is shifted to higher energies and a doping-dependent $d_{z^2}$-versus-$d_{x^2-y^2}$ competition on Ni is revealed. The absence of prominent Zhang-Rice physics in infinite-layer nickelates might be relevant to understand the notable difference in the superconducting $T_{\mathrm{c}}$'s.

Trions are excitonic species with a positive or negative charge, and thus, unlike neutral excitons, the flow of trions can generate a net detectable charge current. Trions under favorable doping conditions can be created in a coherent manner using resonant excitation. In this work, we exploit these properties to demonstrate a gate controlled trion switch in a few-layer graphene/monolayer ${\mathrm{WS}}_2$/monolayer graphene vertical heterojunction. By using a high-resolution spectral scan through a temperature controlled variation of the band gap of the ${\mathrm{WS}}_2$ sandwich layer, we obtain a gate voltage dependent vertical photocurrent strongly relying on the spectral position of the excitation, and the photocurrent maximizes when the excitation energy is resonant with the trion peak position. Further, the resonant photocurrent thus generated can be effectively controlled by a back gate voltage applied through the incomplete screening of the bottom monolayer graphene, and the photocurrent strongly correlates with the gate dependent trion intensity, while the nonresonant photocurrent exhibits only a weak gate dependence—unambiguously proving a trion driven photocurrent generation under resonance. We estimate a sub-100 fs switching time of the device. The findings are useful towards demonstration of ultrafast excitonic devices in layered materials.

In hexagonal two-dimensional (2D) materials such as graphene, hexagonal boron nitride, or transition metal dichalcogenides, electrons possess intrinsic degrees of freedom (DOFs): sublattice pseudospin, spin, and valley. Independent polarization of the DOFs for carrying information can be harnessed in future semiconductor industry. Here we propose a general Hamiltonian based on the Haldane model with spin-orbit coupling (SOC) which realizes the independence of all the DOFs that can tune optical absorption spectra as a function of the SOC. The present result suggests a new 2D material or a new 2D electronic device which offers any combination of pseudospintronics, spintronics, and valleytronics applications.

We found that a surface state induced by hydrogenation on the surface of $\mathrm{SrTi}{\mathrm{O}}_3\left(001\right)$ (STO) did not obey the rigid-band model, which was confirmed by in situ electrical resistivity measurements in ultrahigh vacuum. With exposure of atomic hydrogen on the STO, a surface state (H-induced donor state, HDS) appears within the bulk band gap (an in-gap state), which donates electrons thermally activated to the conduction band, resulting in downward bending of the bulk bands beneath the surface. The doped electrons flow through the space-charge layer in two-dimensional manner parallel to the surface. The observed semiconductor behavior in the temperature dependence of electronic conductivity is explained by the thermal activation of carriers. The HDS and the conduction band are nonrigid in energy position; they come closer with increasing the hydrogen adsorption. Eventually the HDS saturates its position around 60 meV below the conduction-band minimum. The sheet conductivity, accordingly, also saturates at $\sim 1.0\mu \mathrm{S}\square$ with increasing hydrogen adsorption, corresponding to completion of the hydrogenation of the surface.

We describe a simple setup generating pure valley currents—valley transport without charge transport—in strained graphene nanoribbons with zigzag edges. The crucial ingredient is a uniaxial strain pattern which couples to the low-energy Dirac electrons as a uniform pseudomagnetic field. Remarkably, the resulting pseudo-Landau levels are not flat but disperse linearly from the Dirac points, with an opposite slope in the two valleys. We show how this is a natural consequence of an inhomogeneous Fermi velocity arising in the low-energy theory describing the system, which maps to an exactly solvable singular Sturm-Liouville problem. The velocity of the valley currents can be controlled by tuning the magnitude of strain and by applying bias voltages across the ribbon. Furthermore, applying an electric field along the ribbon leads to pumping of charge carriers between the two valleys, realizing a valley analog of the chiral anomaly in one spatial dimension. These effects produce unique signatures that can be observed experimentally by performing ordinary charge transport measurements and spectroscopy.

Electrons in graphene behave like relativistic Dirac particles which can reduce the velocity of light by two orders of magnitude in the form of plasmon-polaritons. Here we show how these properties lead to a peculiar nonlinear plasmon response in the quasiclassical regime of terahertz frequencies. On the one hand, we show how interband plasmon damping is suppressed by the relativistic Klein tunneling effect. On the other hand, we demonstrate huge enhancement of the nonlinear intraband response when the plasmon velocity approaches the resonance with the electron Fermi velocity. This extreme sensitivity to the plasmon intensity could be used for new terahertz technologies.

Recent interest in optical analogs to the quantum spin Hall and quantum valley Hall effects is driven by the promise to establish topologically protected photonic edge modes at telecommunication and optical wavelengths on a simple platform suitable for industrial applications. While first theoretical and experimental efforts have been made, these approaches so far both lack a rigorous understanding of the nature of topological protection and the limits of backscattering immunity. We here use a generic group theoretical methodology to fill this gap and obtain general design principles for purely dielectric two-dimensional topological photonic systems. The method comprehensively characterizes possible two-dimensional hexagonal designs and reveals their topological nature, potential, and limits.

The electronic and transport properties of the ordered B2-(${\mathrm{Fe}}_{60}{\mathrm{Al}}_{40}$) phase which undergoes a continuous transition into the disordered A2-(${\mathrm{Fe}}_{60}{\mathrm{Al}}_{40}$) phase are studied from first principles. The disordering is characterized as a gradual interchange of atoms between Fe and Al sublattices under the condition that the total amount of Fe and Al atoms is kept unchanged. This is the simplest model of gradual disordering of the ordered phase due to the ion irradiation. The physical properties are strongly influenced by varying local environment of Fe atoms on both sublattices. This leads to the transition between a high moment at large disorder and a very low moment in the ordered phase. Similar behavior is found also for anomalous Hall conductivity and anomalous Hall angle. Unusual behavior of the longitudinal conductivity as a function of degree of disorder is due to the shift of the Fermi level of majority states from the $sp$-like part to the $d$ states with increasing ordering. The disordered phase has a large anomalous Hall angle as contrasted with a negligible anomalous Hall angle for well ordered samples, which is in agreement with recent experiment.

CoFeB alloys are highly relevant materials for spintronic applications. In this work, the crystallization of CoFeB alloys triggered by thermal annealing was investigated by x-ray diffraction techniques and scanning electron microscopy, as well as spectroscopic ellipsometry and magneto-optical Kerr effect spectroscopy for annealing temperatures ranging from 300 to ${600}^{\circ }\mathrm{C}$. The transformation of ∼100-nm-thick ${\mathrm{Co}}_x{\mathrm{Fe}}_{(80-x)}{\mathrm{B}}_{20}$ films from amorphous to polycrystalline was revealed by the sharpening of spectral features observed in optical and magneto-optical dielectric functions spectra. The influence of B on the dielectric function was assessed both experimentally and by optical modeling. By analyzing the Drude component of the optical dielectric function, a consistent trend between the charge-carrier scattering time/resistivity and the annealing temperature was observed, in agreement with the electrical investigations by means of the four-point-probe method.

The effect of multiple topological transitions for electron-hole excitations is discovered in full shell proximitized semiconducting nanowires with trapped superconducting vortices, recently shown to be a promising platform for the realization of Majorana states at moderate longitudinal magnetic fields. The mechanism of such multiple transitions is uncovered and explained to be governed by the interplay of normal and Andreev reflections from the shell and by the textured spin-orbit coupling inside the semiconductor. The extensive analysis of emerging propagating and Majorana-type evanescent quasiparticle modes in such Andreev waveguides is performed. Experimentally, these modes reveal themselves in peculiarities of charge and spin-polarized heat transport.

We report on single-crystal growth, stoichiometry, structure and basic characterization of ${\mathrm{Rb}}_{1-x}{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_{2-z}{\mathrm{S}}_z$ crystals where Se is substituted by S. The temperature and magnetic field dependence of magnetic and thermodynamic properties of all samples was studied by differential-scanning calorimetry, magnetic susceptibility, electrical conductivity, and specific heat. The experimental results are discussed within a $T-z$ phase diagram, which includes vacancy-ordered and vacancy-disordered antiferromagnetic (AFM), superconducting (SC), and nonsuperconducting phases. The structural study reveals change in the local environment of the Fe tetrahedrons depending on substitution: a reduction of the Fe-Fe and Fe-Ch(chalcogen) bond lengths and a tendency for six out- of eight bond angles to approach values realizing a regular tetrahedron and hence, suggesting a reduction of structural distortions with substitution. With increasing substitution, a nonmonotonic decrease of the superconducting transition temperature $T_c$ was observed; the SC state disappears at a substitution level above $z=1.2$. The SC state coexists with the AFM state that persists in all samples independent of substitution. The transition temperature into the AFM state, $T_N$, decreases gradually with increasing substitution indicating a weakening of the AFM interactions. The AFM phase exhibits an iron-vacancy-ordered structure below the structural transition temperature $T_s$. $T_s$ shows a nonmonotonous variation: a decrease with increasing $z$ up to 1.3, followed by an increase on further increasing $z$. The electronic specific heat reveals a significant reduction of the anomaly at the SC transition temperature indicating a reduction of the density of states at the Fermi energy and a weakening of the electronic correlations that can explain the suppression of the superconductivity with substitution.