Abstract: The effect of substitutional doping of fluorographene with boron and nitrogen atoms on its electronic and magnetic properties is investigated using first-principles calculations. It is found that boron dopants can be readily incorporated in the fluorographene crystal where they act as shallow acceptors and cause hole doping, but no changes in the magnetic properties are observed. Nitrogen dopants act as deep donors and give rise to a magnetic moment, but the resulting system becomes chemically unstable. These results are opposite to what was found for substitutional doping of graphane, i.e., hydrogenated graphene, in which case B substituents induce magnetism and N dopants do not.

Abstract: The conductance, the transmission, and the reflection probabilities through rectangular potential barriers and p-n junctions are obtained for bilayer graphene taking into account the four bands of the energy spectrum. We have evaluated the importance of the skew hopping parameters gamma(3) and gamma(4) to these properties and show that for energies E > gamma(1)/100 their effect is negligible. For high energies two modes of propagation exist and we investigate scattering between these modes. For perpendicular incidence both propagation modes are decoupled, and scattering between them is forbidden. This extends the concept of pseudospin as defined within the two-band approximation to a four-band model and corresponds to the (anti) symmetry of the wave functions under in-plane mirroring. New transmission resonances are found that appear as sharp peaks in the conductance which are absent in the two-band approximation. The application of an interlayer bias to the system (1) breaks the pseudospin structure, (2) opens a band gap that results in a distinct feature of suppressed transmission in the conductance, and (3) breaks the angular symmetry with respect to normal incidence in the transmission and reflection.

Abstract: We consider a junction between two topological insulators and calculate the properties of the interface states with an effective low-energy Hamiltonian for topological insulators with a single cone on the surface. This system bears a close resemblance to bilayer graphene, as both result from the hybridization of Dirac cones. We find gapless interface states not only when the helicity directions of the topological surface states are oppositely oriented, but they can also exist if they are equally oriented. Furthermore, we find that the existence of the interface states can be understood from the closing of the bulk gap when the helicity changes orientation. Recently superluminal tachyonic excitations were also claimed to exist at the interface between topological insulators. However, here we show that these interface states do not exist. DOI: 10.1103/PhysRevB.87.115113

Abstract: We derive an exact solution to the problem of spin snake states induced in a nonhomogeneous magnetic field by a combined action of the Rashba spin-orbit and Zeeman fields. The electron spin behavior as a function of the cyclotron orbit center position and an external homogeneous magnetic field was obtained. It is shown that in an antisymmetric magnetic field the electron spin in the snake states has only an in-plane projection, perpendicular to the magnetic interface, which vanishes at large positive momenta. Applying an external homogeneous magnetic field adds a finite out-of-plane spin component and simultaneously gaps out the spectral branches, which results in regular beating patterns of the spin current components.

Abstract: We investigate the link between superconductivity and atomic vibrations in Nb3Sn films with a thickness ranging from 10 to 50 nm. The challenge of measuring the phonon density of states (PDOS) of these films has been tackled by employing the technique of nuclear inelastic scattering by Sn-119 isotopes to reveal the Sn-partial phonon density of states. With the support of ab initio calculations, we evaluate the effect of reduced film thickness on the PDOS. This approach allows us to estimate the changes in superconducting critical temperature T-c induced by phonon confinement, which turned out to be limited to a few tenths of K. The presented method is successful for the Nb3Sn system and paves the way for more systematic studies of the role of phonon confinement in Sn-containing superconductors.

Abstract: Density-functional tight-binding and classical molecular dynamics simulations are used to investigate the structural deformations and melting of planar carbon nanoclusters C-N with N = 2-55. The minimum-energy configurations for different clusters are used as starting configurations for the study of the temperature effects on the bond breaking and rotation in carbon lines (N < 6), carbon rings (5 < N < 19), and graphene nanoflakes. The larger the rings (graphene nanoflakes) the higher the transition temperature (melting point) with ring-to-line (perfect-to-defective) transition structures. The melting point was obtained by using the bond energy, the Lindemann criteria, and the specific heat. We found that hydrogen-passivated graphene nanoflakes (CNHM) have a larger melting temperature with a much smaller dependence on size. The edges in the graphene nanoflakes exhibit several different metastable configurations (isomers) during heating before melting occurs. DOI: 10.1103/PhysRevB.87.134103

Abstract: Using a numerical tight-binding approach based on the Chebyshev–Bogoliubov–de Gennes method we describe Josephson junctions made of multilayer graphene contacted by top superconducting gates. Both Bernal (ABA) and rhombohedral (ABC) stacking are considered and we find that the type of stacking has a strong effect on the proximity effect and the supercurrent flow. For both cases the pair amplitude shows a polarization between dimer and nondimer atoms, being more pronounced for rhombohedral stacking. Even though the proximity effect in nondimer sites is enhanced when compared to single-layer graphene, we find that the supercurrent is suppressed. The spatial distribution of the supercurrent shows that for Bernal stacking the current flows only in the topmost layers while for rhombohedral stacking the current flows throughout the whole structure.

Abstract: We study the mobility of Dirac fermions in monolayer graphene on a GaAs substrate, limited by the combined action of the extrinsic potential of piezoelectric surface acoustical phonons of GaAs (PA) and of the intrinsic deformation potential of acoustical phonons in graphene (DA). In the high-temperature (T) regime, the momentum relaxation rate exhibits the same linear dependence on T but different dependencies on the carrier density n, corresponding to the mobility mu proportional to 1 root n and 1/n, respectively for the PA and DA scattering mechanisms. In the low-T Bloch-Gruneisen regime, the mobility shows the same square-root density dependence mu proportional to root n, but different temperature dependencies mu proportional to T-3 and T-4, respectively for PA and DA phonon scattering. DOI: 10.1103/PhysRevB.87.075443

Abstract: We report a method to reliably count the number of atoms from high-angle annular dark field scanning transmission electron microscopy images. A model-based analysis of the experimental images is used to measure scattering cross sections at the atomic level. The high sensitivity of these measurements in combination with a thorough statistical analysis enables us to count atoms with single-atom sensitivity. The validity of the results is confirmed by means of detailed image simulations. We will show that the method can be applied to nanocrystals of arbitrary shape, size, and atom type without the need for a priori knowledge about the atomic structure.

Abstract: Magnetic flux patterns are known to strongly differ in the intermediate state of type-I and type-II superconductors. Using a type-I/type-II bilayer we demonstrate hybridization of these flux phases into a plethora of unique new ones. Owing to a complicated multibody interaction between individual fluxoids, many different intriguing patterns are possible under applied magnetic field, such as few-vortex clusters, vortex chains, mazes, or labyrinthal structures resembling the phenomena readily encountered in soft-matter physics. However, in our system the patterns are tunable by sample parameters, magnetic field, current, and temperature, which reveals transitions from short-range clustering to long-range ordered phases such as parallel chains, gels, glasses, and crystalline vortex lattices, or phases where lamellar type-I flux domains in one layer serve as a bedding potential for type-II vortices in the other, configurations clearly beyond the soft-matter analogy.

Abstract: The instability of a free-standing one-sided hydrogenated/fluorinated graphene nanoribbon, i.e., graphone/fluorographene, is studied using ab initio, semiempirical, and large-scale molecular dynamics simulations. Free-standing semi-infinite armchairlike hydrogenated/fluorinated graphene (AC-GH/AC-GF) and boatlike hydrogenated/fluorinated graphene (B-GH/B-GF) (nanoribbons which are periodic along the zigzag direction) are unstable and spontaneously transform into spiral structures. We find that rolled, spiral B-GH and B-GF are energetically more favorable than spiral AC-GH and AC-GF which is opposite to the double-sided flat hydrogenated/fluorinated graphene, i.e., graphane/fluorographene. We found that the packed, spiral structures exhibit an unexpected localized highest occupied molecular orbital and lowest occupied molecular orbital at the edges with increasing energy gap during rolling. These rolled hydrocarbon structures are stable beyond room temperature up to at least T = 1000 K within our simulation time of 1 ns. DOI: 10.1103/PhysRevB.87.075448

Abstract: Neutral excitons in strained axially symmetric In(Ga)As/GaAs quantum dots with a ringlike shape are investigated. Similar to experimental self-assembled quantum rings, the analyzed quantum dots have volcano-like shapes. The continuum mechanical model is employed to determine the strain distribution, and the single-band envelope function approach is adopted to compute the electron states. The hole states are determined by the axially symmetric multiband Luttinger-Kohn Hamiltonian, and the exciton states are obtained from an exact diagonalization. We found that the presence of the inner layer covering the ring opening enhances the excitonic Aharonov-Bohm (AB) oscillations. The reason is that the hole becomes mainly localized in the inner part of the quantum dot due to strain, whereas the electron resides mainly inside the ring-shaped rim. Interestingly, larger AB oscillations are found in the analyzed quantum dot than in a fully opened quantum ring of the same width. Comparison with the unstrained ringlike quantum dot shows that the amplitude of the excitonic Aharonov-Bohm oscillations are almost doubled in the presence of strain. The computed oscillations of the exciton energy levels are comparable in magnitude to the oscillations measured in recent experiments. DOI: 10.1103/PhysRevB.87.085314

Abstract: Nonlinear dynamics of Josephson vortices (fluxons) in artificial stacks of superconducting-normal-superconducting Josephson junctions under simultaneously applied time-periodic ac and constant biasing dc currents is studied using the time dependent Ginzburg-Landau formalism with a Lawrence-Doniach extension. At zero external magnetic field and dc biasing current the resistive state of the system is characterized by periodic nucleation and annihilation of fluxon-antifluxon pairs, relative positions of which are determined by the state of neighboring junctions. Due to the mutual repulsive interaction, fluxons in different junctions move out of phase. Their collective motion can be synchronized by adding a small ac component to the biasing dc current. Coherent motion of fluxons is observed for a broad frequency range of the applied drive. In the coherent state the maximal output voltage, which is proportional to the number of junctions in the stack, is observed near the characteristic frequency of the system determined by the crossing of the fluxons across the sample. However, in this frequency range the dynamically synchronized state has an alternative-a less ordered state with smaller amplitude of the output voltage. Collective behavior of the junctions is strongly affected by the sloped sidewalls of the stack. Synchronization is observed only for weakly trapezoidal cross sections, whereas irregular motion of fluxons is observed for larger slopes of the sample edge.

Abstract: The thermodynamic properties of the electron gas in multilayer graphene depend strongly on the number of layers and the type of stacking. Here we analyze how those properties change when we vary the number of layers for rhombohedral stacked multilayer graphene and compare our results with those from a conventional two-dimensional electron gas. We show that the highly degenerate zero-energy Landau level which is partly filled with electrons and partly with holes has a strong influence on the values of the different thermodynamic quantities.