We analyze the impacts of the electric field on the Dzyaloshinskii-Moriya interaction, magnetocrystalline anisotropy, and intrinsic ferromagnetism of the recently discovered two-dimensional ferromagnetic chromium tri-iodide ($\mathrm{Cr}{\mathrm{I}}_3$) monolayer, by combining density functional theory and Monte Carlo simulations. By taking advantage of the counterbalancing effects of anisotropic symmetric exchange energy and antisymmetric exchange energy, it is shown that the intrinsic ferromagnetism can be manipulated by externally applied off-plane electric fields. The results quantitatively reveal the impacts of off-plane electric field on the lattice structure, magnetic anisotropy energy, symmetric and antisymmetric exchange energies, Curie temperature, magnetic hysteresis, and coercive field. The physical mechanism of all-electrical control of magnetism proposed here is useful for creating next-generation magnetic device technologies based on the recently discovered two-dimensional ferromagnetic crystals.

We have synthesized a spin-1/2 antiferromagnet, $\mathrm{C}{\mathrm{u}}_4{\left(\mathrm{OH}\right)}_6\mathrm{FCl}$, containing kagome lattices. The magnetic susceptibility gives a Curie-Weiss temperature ${\theta }_{\mathrm{CW}}=-135\mathrm{K}$, nearest-neighbor exchange $J=-166\mathrm{K}$, and two magnetic anomalies at $T_1=15\mathrm{K}$ and $T_2=4.8\mathrm{K}$ with weak ferromagnetism. However, the specific heat curve does not exhibit λ-like peaks expected for long-range magnetic order. The high-field ESR data are strongly correlated with the magnetism. There are a gapless mode ${\omega }_1$ observed at all temperatures and two gapped modes, ${\omega }_2$ and ${\omega }_3$, corresponding to different temperature regions: $T<T_1(=15\mathrm{K})$ and $T<T_2(=4.8\mathrm{K})$, respectively. The possible origins of the gapped modes are discussed.

We present a thorough study of doping dependent magnetic hysteresis and relaxation characteristics in single crystals of $\left(\mathrm{B}{\mathrm{a}}_{1-x}{\mathrm{K}}_x\right)\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ ($0.18\le x\le 1$). The critical current density $J_c$ reaches maximum in the underdoped sample $x=0.26$ and then decreases in the optimally doped and overdoped samples. Meanwhile, the magnetic relaxation rate S rapidly increases and the flux creep activation barrier $U_0$ sharply decreases in the overdoped sample $x=0.70$. These results suggest that vortex pinning is very strong in the underdoped regime, but it is greatly reduced in the optimally doped and overdoped regime. Transmission electron microscope (TEM) measurements reveal the existence of dislocations and inclusions in all three studied samples $x=0.38$, 0.46, and 0.65. An investigation of the paramagnetic Meissner effect (PME) suggests that spatial variations in $T_c$ become small in the samples $x=0.43$ and 0.46, slightly above the optimal doping levels. Our results support that two types of pinning sources dominate the $\left(\mathrm{B}{\mathrm{a}}_{1-x}{\mathrm{K}}_x\right)\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ crystals: (i) strong δl pinning, which results from the fluctuations in the mean free path $l$ and $\delta T_c$ pinning from the spatial variations in $T_c$ in the underdoped regime, and (ii) weak $\delta T_c$ pinning in the optimally doped and overdoped regime.

We investigated the superconductivity in a few-layer ${\mathrm{NbSe}}_2$-graphene heterojunction by differential conductance spectroscopy. Because of the gate-tunable Fermi level of the few-layer graphene, used here as a tunneling electrode in a nano-point-contact spectroscopy setup, the differential conductance of the heterojunction showed highly sensitive dependence on the gate voltage, which allowed us to probe the nature of the superconducting gap functions with unprecedented detail by continuously tuning the transparency of the junction between the spectroscopic tunneling and the Andreev reflection limits. Characteristic features associated with a two-gap superconductivity in ${\mathrm{NbSe}}_2$ were reproducibly observed in both limits and between, e.g., in the form of a central conductance dip with two sets of coherence peaks when the Fermi level was close to the charge neutrality point of graphene. From fits with the Blonder-Tinkham-Klapwijk model, two gaps with their temperature dependence were extracted. The two gaps associated with the two-band superconductivity in ${\mathrm{NbSe}}_2$ followed the expected temperature behavior in the limit of weak interband scattering, with a gap to $T_c$ ratio suggesting a weak to moderately strong coupling in few-layer systems.

We investigate the thickness effects on charge and magnetic orderings in Fe perovskite oxide ${\mathrm{La}}_{1/3}{\mathrm{Sr}}_{2/3}{\mathrm{FeO}}_3/{\mathrm{SrTiO}}_3$ thin films by hard x-ray and resonant soft x-ray scattering (RSXS) with changing thin-film thickness systematically. We found that the correlation lengths of the magnetic ordering along the in-plane and out-of-plane directions are comparable and proportional to the thickness, and show stronger thickness dependence than those of charge ordering. The magnetic ordered states disappear when the correlation length of magnetic ordering decreases to that of charge ordering through the intrinsic thickness effects. Surface-sensitive grazing-incident RSXS revealed that the orderings exist even in the surface region, which indicates that the observed orderings are not affected by surface effects like oxygen vacancies. Critical thickness is in 5–15 nm, which corresponds to 4–11 antiferromagnetic ordering period. This critical value seems to be common to other ferromagnetic oxide thin films.

We study in detail, via experimental measurements, atomistic simulations, and density functional theory transport calculations, the process of formation and the resulting electronic properties of atomic-sized contacts made of Au, Ag, and Cu. Our data analysis of both experimental results and simulations leads to a precise relationship between geometry and electronic transmission—we reestablish the significant influence of the number of first neighbors on the electronic properties of atomic-sized contacts. This result allows us also to interpret subtle differences between the metals during the process of contact formation as well as the characteristics of the resulting contacts.

We study the properties of a thin film of topological insulator material. We treat the coupling between helical states at opposite surfaces of the film in the properly-adapted tunneling approximation, and show that the tunneling matrix element oscillates as a function of both the film thickness and the momentum in the plane of the film for ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. As a result, while the magnitude of the matrix element at the center of the surface Brillouin zone gives the gap in the energy spectrum, the sign of the matrix element uniquely determines the topological properties of the film, as demonstrated by explicitly computing the pseudospin textures and the Chern number. We find a sequence of transitions between topological and nontopological phases, separated by semimetallic states, as the film thickness varies. In the topological phase, the edge states of the film always exist but only carry a spin current if the edge potentials break particle-hole symmetry. The edge states decay very slowly away from the boundary in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, making ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, where this scale is shorter, a more promising candidate for the observation of these states. Our results hold for free-standing films as well as heterostructures with large-gap insulators.

Lowering of the thickness of a thin-film three-dimensional topological insulator down to a few nanometers results in the gap opening in the spectrum of topologically protected two-dimensional surface states. This phenomenon, which is referred to as the anomalous finite-size effect, originates from hybridization between the states propagating along the opposite boundaries. In this work, we consider a bismuth-based topological insulator and show how the coupling to an intense high-frequency linearly polarized pumping can further be used to manipulate the value of a gap. We address this effect within recently proposed Brillouin-Wigner perturbation theory that allows us to map a time-dependent problem into a stationary one. Our analysis reveals that both the gap and the components of the group velocity of the surface states can be tuned in a controllable fashion by adjusting the intensity of the driving field within an experimentally accessible range and demonstrate the effect of light-induced band inversion in the spectrum of the surface states for high enough values of the pump.

We investigate the influence of hexagonal warping on the transport properties of topological insulators. We study the charge conductivity within Kubo formalism in the first Born approximation using low-energy expansion of the Hamiltonian near the Dirac point. The effects of disorder, magnetic field, and chemical-potential value are analyzed in detail. We find that the presence of hexagonal warping significantly affects the conductivity of the topological insulator. In particular, it gives rise to the growth of the longitudinal conductivity with the increase of the disorder and anisotropic anomalous in-plane magnetoresistance. Hexagonal warping also affects the quantum anomalous Hall effect and anomalous out-of-plane magnetoresistance. The obtained results are consistent with the experimental data.

Near-field radiative heat transfer (RHT) between two bodies can significantly exceed the far-field limit set by Planck's law due to the evanescent wave tunneling, which typically can only occur when the two bodies are separated at subwavelength distances. We show that the RHT between two SiC nanoparticles with separation distances much larger than the thermal wavelength can still exceed the far-field limit when the particles are located within a subwavelength distance away from a SiC substrate. In this configuration, the localized surface phonon polariton (SPhP) of the particles couples to the propagating SPhP of the substrate which then provides a new channel for the near-field energy transport and enhances the RHT by orders of magnitude at large distances. The enhancement is also demonstrated to appear in a chain of closely spaced SiC nanoparticles located in the near field of a SiC substrate. The findings provide a way for the long-distance transport of near-field energy.

Parafermionic zero modes are fractional topologically protected quasiparticles expected to arise in various platforms. We show that Coulomb charging effects define a parafermion box with unique access options via fractional edge states and/or quantum antidots. Basic protocols for the detection, manipulation, and control of parafermionic quantum states are formulated. With those tools, one may directly observe the dimension of the zero-mode Hilbert space, prove the degeneracy of this space, and perform on-demand digital operations satisfying a parafermionic algebra.

The consistency between the exchange-correlation functional used in pseudopotential construction and in the actual density functional theory calculation is essential for the accurate prediction of fundamental properties of materials. However, routine hybrid density functional calculations at present still rely on generalized gradient approximation pseudopotentials due to the lack of hybrid functional pseudopotentials. Here, we present a scheme for generating hybrid functional pseudopotentials, and we analyze the importance of pseudopotential density functional consistency for hybrid functionals. For the PBE0 hybrid functional, we benchmark our pseudopotentials for structural parameters and fundamental electronic gaps of the Gaussian-2 (G2) molecular dataset and some simple solids. Our results show that using our PBE0 pseudopotentials in PBE0 calculations improves agreement with respect to all-electron calculations.

We have calculated order parameters for the phases of the Kitaev chain with interaction and dimerization at a special symmetric point applying the Jordan-Wigner and other duality transformations. We use string order parameters (SOPs) defined via the correlation functions of the Majorana string operators. The SOPs are mapped onto the local order parameters of some dual Hamiltonians and easily calculated. We have shown that the phase diagram of the interacting dimerized chain comprises the phases with the conventional local order as well as the phases with nonlocal SOPs. From the results for the critical indices, we infer the two-dimensional Ising universality class of criticality at the particular symmetry point where the model is exactly solvable.

A bridging topological state with a conventional semiconductor platform offers an attractive route towards future spintronics and quantum device applications. Here, based on first-principles and tight-binding calculations, we demonstrate the existence of topological states hosted by a two-dimensional (2D) metal alloy in a Si surface, the BiAg/Si(111)-$4\times 4$ surface, which has already been synthesized experimentally. It exhibits a topological insulating state with an energy gap of 71 meV $(\sim 819\mathrm{K})$ above the Fermi level and a topological metallic state with quasiquantized conductance below the Fermi level. The underlying mechanism leading to the formation of such nontrivial states is revealed by analysis of the “charge-transfer” and “orbital-filtering” effect of the Si substrate. A minimal effective tight-binding model is employed to reveal the formation mechanism of the topological states. Our finding opens opportunities to detect topological states and measure its quantized conductance in a large family of 2D surface metal alloys, which have been or are to be grown on semiconductor substrates.

We analyze, both theoretically and numerically, the temperature-dependent thermal conductivity $\kappa$ of two-dimensional nanowires with surface roughness. Although each sample is characterized by three independent parameters, the diameter (width) of the wire, the correlation length, and the strength of the surface corrugation, our theory predicts that there exists a universal regime where $\kappa$ is a function of a single combination of all three model parameters. Numerical simulations of propagation of acoustic phonons across thin wires confirm this universality and predict a $d^{1/2}$ dependence of $\kappa$ on the diameter $d$.

For well-ordered ultrathin films of NiO(001) on Ag(001), a series of unoccupied states below the vacuum level has been found. The states show a nearly free electron dispersion and binding energies which are typical for image potential states. By time-resolved two-photon photoemission (2PPE), the lifetimes of the first three states and their dependence on oxide film thickness are determined. For NiO film thicknesses between 2 and 4 monolayers (ML), the lifetime of the first state is in the range of 28–42 fs and shows an oscillatory behavior with increasing thickness. The values for the second state decrease monotonically from 88 fs for 2 ML to 33 fs for 4 ML. These differences are discussed in terms of coupling of the unoccupied states to the layer-dependent electronic structure of the growing NiO film.