We propose a theory of the thermal Casimir interaction for multilayered test bodies coated with a graphene sheet. The reflection coefficients on such structures are expressed in terms of the components of the polarization tensor and the dielectric permittivities of material layers. The developed theory is applied to calculate the gradient of the Casimir force between an Au-coated sphere and a graphene sheet deposited on a SiO${}_2$ film covering a Si plate, which is the configuration of a recent experiment performed by means of a dynamic atomic force microscope. The theoretical results are found to be in very good agreement with the experimental data. We thus confirm that graphene influences the Casimir interaction and can be used for tailoring the force magnitude in nanostructures.

In this paper, we propose the idea of using chiral negatively refractive media to realize three-dimensional (3D) complete band gaps in the one-dimensional (1D) photonic crystals with a two-layer unit cell. We explain why at least a three-layer unit cell is necessary for realizing the 3D complete band gaps in 1D achiral photonic crystals, and show how we can reduce the number of layers of the unit cell to two if chirality is introduced. Although the media with strong chirality supply negative refraction only for one of the two eigenwaves, the complete band gaps could be still obtained with an even simpler lattice structure compared to the achiral counterpart.

We report on transport properties of monolayer graphene with a laterally modulated potential profile, employing striped top gate electrodes with spacings of 100 to 200 nm. Tuning of top and back gate voltages gives rise to local charge carrier density disparities, enabling the investigation of transport properties either in the unipolar ($n{n}^{\prime }$) or the bipolar ($n{p}^{\prime }$) regime. In the latter, pronounced single- and multibarrier Fabry-Pérot (FP) resonances occur. We present measurements of different devices with different numbers of top gate stripes and spacings. The data are highly consistent with a phase coherent ballistic tight-binding calculation and quantum capacitance model, whereas a superlattice effect and modification of band structure can be excluded.

We investigate the transport properties of a graphene layer in the presence of Rashba spin-orbit interaction. Quite generally, spin-orbit interactions induce spin splittings and modifications of the graphene band structure. We calculate within the scattering approach the linear electric and thermoelectric responses of a clean sample when the Rashba coupling is localized around a finite region. We find that the thermoelectric conductance, unlike its electric counterpart, is quite sensitive to external modulations of the Fermi energy. Therefore, our results suggest that thermocurrent measurements may serve as a useful tool to detect nonhomogeneous spin-orbit interactions present in a graphene-based device. Furthermore, we find that the junction thermopower is largely dominated by an intrinsic term independently of the spin-orbit potential scattering. We discuss the possibility of canceling the intrinsic thermopower by resolving the Seebeck coefficient in the subband space. This causes unbalanced populations of electronic modes which can be tuned with external gate voltages or applied temperature biases.

We investigate the chiral tunneling in a bilayer graphene $n\text{−}p\text{−}n$ junction in time aspect. The phase and dwell times are evaluated for various situations, including the effects of trigonal warping and band gap due to an external gate field. In the absence of band gap and for normal incidence, the chirality induces Klein effects; when the trigonal warping is excluded, the tunneling times are the same as the ordinary barrier tunneling, but including it leads to a perfect reflection and a new type of phase time is found. In the presence of band gap and/or for an oblique incidence, the Klein effects disappear and the tunneling times have peaks corresponding to resonant transmission maxima due to the Fabry-Pèrot-type interference of the oscillating waves allowed inside the barrier. The trigonal warping also leads to valley-polarized transmission for an oblique incidence in the absence or presence of band gap. As a result, the tunneling times and scattering angles of particles from one valley are different from the other valley. We observe that the Hartman effect exists only when the chirality selects pure evanescent waves as transmission channels.

We have done a study of graphene/hexagonal boron nitride ($h$-BN) thin films within the framework of density functional theory, and find that the interlayer interaction energy of a graphene/$h$-BN monolayer thin film is inversely proportional to the layer number. This analysis based on a method which simulates the interlayer interactions in lattice-mismatched thin films shows that thin films with four or more layers can have stable lattice-matched stacking geometries. We find that the maximum value of the band gap of the lattice-matched thin films having the same layer number, but different stacking sequences, decreases with respect to the layer number, even though one can consider several different stable stacking sequences of these feasible lattice-matched thin films. In addition, the band gap can be tuned by using an external electric field. We also propose six-layer graphene/$h$-BN bilayer thin films with 99-meV band gap or graphenelike linear dispersion depending on the stacking sequences.

The topological crystalline insulator SnTe has been grown epitaxially on a Bi${}_2$Te${}_3$ buffer layer by molecular beam epitaxy. In a 30-nm-thick SnTe film, $p$- and $n$-type carriers are found to coexist, and Shubnikov–de Haas oscillation data suggest that the $n$-type carriers are Dirac fermions residing on the SnTe (111) surface. This transport observation of the topological surface state in a $p$-type topological crystalline insulator became possible due to a downward band bending on the free SnTe surface, which appears to be of intrinsic origin.

One-dimensional valence bond solid (VBS) states represent the simplest symmetry-protected topological phases. We show that their ground state entanglement spectrum contains both topological and nontopological structures. For the $\text{SO}\left(3\right)$ symmetric VBS states with odd-integer spins, the twofold topological degeneracy is associated with an underlying $Z_2\times Z_2$ symmetry that protects the corresponding topological phase. In general, for the $\text{SO}(2S+1)$ symmetric VBS states with integer spins $S$, the corresponding protecting symmetry is identified as the ${(Z_2\times Z_2)}^S$ symmetry, yielding the $2^S$-fold topological degeneracy. The topological degeneracy and associated protecting symmetry can be identified by a nonlocal unitary transformation, which changes the topological order of the VBS states into conventional ferromagnetic order.

The nature of the elemental cerium phases, undergoing an isostructural volume collapse transition, cannot be understood using conventional solid-state concepts. Using the combination of density functional theory and dynamical mean-field theory, we study the magnetic properties of both the $\alpha$ and the $\gamma$ phases. We compute the magnetic form factor and show that it is very close to the free ion behavior in both the local moment $\gamma$ phase as well as the more itinerant $\alpha$ phase, in agreement with neutron scattering experiments. In sharp contrast, the dynamic local magnetic susceptibility of the two phases is strikingly different. In the $\gamma$ phase, the sharp low energy peak due to local moment formation and consequently low Kondo temperature dominates the spectra. In $\alpha$ phase two broad peaks can be identified, the first is due to Kondo screening and the second is due to Hund's coupling. This shows that hybridization plays a central role in the $\alpha$-$\gamma$ transition in cerium, and that from the point of view of magnetic properties, the $4f$ electrons are strongly correlated in both phases.

X-ray scattering was used to investigate the magnetically induced ionic displacements in the low-temperature commensurate ferroelectric/antiferromagnetic phase of the multiferroic HoMn${}_2$O${}_5$. The structural modulation signal appearing at twice the magnetic wave vector ${\mathbf{k}}_m=\left(\frac{1}{2}0\frac{1}{4}\right)$ has been used, combined with symmetry analysis, to determine a model for the ionic displacements up to a precision of ${10}^{-3}\text{Å}$. The symmetry-breaking operations that are associated to the active irreducible representation have been experimentally determined from the analysis of the modulation modes. They reveal a lowering of symmetry to the polar point group $m2m$. Calculations based on the determined model show that the magnetic structure along the c direction is stabilized via nearest-neighbor and next-nearest-neighbor interactions through the rare-earth layer. We also show that by knowing both the magnetic and the magnetically induced strain waves patterns, it is possible to remove any phase ambiguity between the crystal and magnetic structures.

The tunnel magnetoresistance ratio (TMR) of fully epitaxial magnetic tunnel junctions with an off-stoichiometric Co${}_2$MnSi Heusler alloy has been shown to exhibit a systematic dependence on Mn content, reaching 1135% at 4.2 K for Co${}_2$Mn${}_{1.29}$Si. In this paper, we explain the behavior of the observed TMR ratio using ab initio calculations and hard x-ray photoelectron spectroscopy (HAXPES). For the Mn-deficient samples, we show that the the drop of the TMR is caused by Co antisite atoms, which impose extra states into the minority-spin band gap. On the other hand, Mn-excess composition shows nearly half-metallic behavior. This result can be intuitively understood since both Co${}_2$MnSi and Mn${}_2$CoSi are theoretically predicted to be half-metallic ferromagnets.

We report the magnetoresistance, magnetothermopower, and quantum oscillation study of ${\mathrm{Sb}}_2{\mathrm{Te}}_2\mathrm{Se}$ single crystal. The in-plane transverse magnetoresistance exhibits a crossover at a critical field $B^{*}$ from semiclassical weak-field $B^2$ dependence to the high-field unsaturated linear magnetoresistance which persists up to the room temperature. The low-temperature Seebeck coefficient is negative in zero field contrary to the positive Hall resistivity, indicating the multiband effect. The magnetic field induced the sign reversion of the Seebeck coefficient between 2 and 150 K. The quantum oscillation of crystals reveals the quasi-two-dimensional (quasi-2D) Fermi surface. These effects are possibly attributed to the large Fermi surface which touches the Brillouin-zone boundary to become quasi-2D and the variation in the chemical potential induced by the magnetic field.

The photoluminescence polarizations of (In,Ga)As/GaAs quantum dots annealed at different temperatures are studied as a function of external magnetic field (Hanle curves). In these dependencies, remarkable resonant features appear due to all-optical nuclear magnetic resonances (NMR) for optical excitation with modulated circular polarization. Application of an additional radio-frequency field synchronously with the polarization modulation strongly modifies the NMR features. The resonances can be related to transitions between different nuclear spin states split by the strain-induced gradient of the crystal field and by the externally applied magnetic field. A theoretical model is developed to simulate quadrupole and Zeeman splittings of the nuclear spins in a strained quantum dot. Comparison with the experiment allows us to uniquely identify the observed resonances. The large broadening of the NMR resonances is attributed to variations of the quadrupole splitting within the quantum dot volume, which is well described by the model.

We report resistance measurements in HgTe wells with an inverted band structure near the charge neutrality point (CNP), where the system is expected to be a two-dimensional topological insulator with a dominant edge states contribution. The sample resistance is found to be about 100 times higher than the resistance quantum $h/2e^2$. Surprisingly, instead of a strong temperature dependence expected in such a seemingly insulating state the resistance at the CNP is found to be temperature independent at low temperatures. The experimental results are compared to the recent theoretical models.

Local vibrational spectra of meta-dichlorobenzene molecules adsorbed on different parts of the Au(111) reconstruction are investigated using a low-temperature scanning tunneling microscope. The spectra show substantial variations on subnanometer length scale. While for the molecule physisorbed on either the hcp or the fcc domain of the reconstruction only low-energy modes are beyond the detection limit, higher-energy modes are observed for the molecule chemisorbed at the elbow site. The different adsorption strengths of the molecules manifest themselves in an energy shift of the modes. These shifts are used to identify through which part the molecule is bonded to the surface.

We use nonequilibrium molecular-dynamics simulations to study the effect of structural defects on the thermal conductivity $\lambda$ of graphene. Focusing on 5-7 and 5-8 defects in the graphene honeycomb lattice, we find that $\lambda$ depends sensitively on whether the defects are isolated, form lines, or form extended arrangements in haeckelites. Our results indicate that the presence of defects makes $\lambda$ anisotropic and, depending on the temperature, quenches its value by one to two orders of magnitude with respect to graphene, mainly by reducing the phonon mean free path.

Typically, computational screens for new materials sharply constrain the compositional search space, structural search space, or both, for the sake of tractability. To lift these constraints, we construct a machine learning model from a database of thousands of density functional theory (DFT) calculations. The resulting model can predict the thermodynamic stability of arbitrary compositions without any other input and with six orders of magnitude less computer time than DFT. We use this model to scan roughly 1.6 million candidate compositions for novel ternary compounds ($A_x{B}_y{C}_z$), and predict 4500 new stable materials. Our method can be readily applied to other descriptors of interest to accelerate domain-specific materials discovery.

We report a high sensitivity to oxygen vacancy formation as well as the temperature dependence of anisotropic optical properties of a nanocrystalline ${\text{V}}_2$${\text{O}}_5$ thin film. Spectroscopic ellipsometry data show that the film has a uniaxial optical anisotropy due to a highly ordered nanocrystalline layer structure. The anisotropic optical properties of the film begin to change at ∼400 K in a high vacuum and can be linked to the changes in the electronic structure of the film upon reduction. An increase in the refractive indices, anisotropic changes of the optical conductivities, and a decrease in film thickness are observed after vacuum heating and are recovered to original values by heating in air up to 480 K. The experimental results are supported by the previous theoretical studies on the stability of reduced ${\text{V}}_2$${\text{O}}_5$ surface through the formation of oxygen vacancies. We discuss the formation of oxygen vacancies, vacancy-induced structural relaxations, and changes in the electronic structure of ${\text{V}}_2$${\text{O}}_5$ in conjunction with the experimental results.