Abstract: Using density-functional theory in combination with the nonequilibrium Green's function formalism, we study the effect of iodide/chloride and iodide/bromide mixing on the electronic transport in lead based organometallic perovskite CH3NH3PbI3, which is known to be an effective tool to tune the electronic and optical properties of such materials. We found that depending on the level and position of the halide mixing, the electronic transport can be increased by more than a factor of 4 for a given voltage biasing. The largest current is observed for small concentration of bromide substitutions located at the equatorial sites. However, full halide substitution has a negative effect on the transport properties of this material: the current drops by an order of magnitude for both CH3NH3PbCl3 and CH3NH3PbBr3 samples. (C) 2015 Elsevier B.V. All rights reserved.

Abstract: A Josephson phase shift can be induced in a Josephson junction by a strategically nearby pinned Abrikosov vortex (AV). For an asymmetric distribution of an imprinted phase along the junction (controlled by the position of the AV) such a simple system is capable of rectification of ac current in a broad and tunable frequency range. The resulting rectified voltage is a consequence of the directed motion of a Josephson antivortex which forms a pair with the AV when at local equilibrium. The proposed realization of the ratchet potential by an imprinted phase is more efficient than the asymmetric geometry of the junction itself, is easily realizable experimentally, and provides rectification even in the absence of an applied magnetic field.

Abstract: Vortex loops are generated by the inhomogeneous stray field of a magnetic dipole on top of a current-carrying mesoscopic superconductor. Cutting and recombination processes unfold under the applied drive, resulting in periodic voltage oscillations across the sample. We show that a direct and detectable consequence of the cutting and recombination of these vortex loops in the present setup is the onset of vortices at surfaces where they were absent prior to the application of the external current. The nonlinear dynamics of vortex loops is studied within the time-dependent Ginzburg-Landau theory to describe the profound three-dimensional features of their time evolution.

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: We demonstrate experimentally and theoretically that the dissipative state at high current densities of superconducting samples with a periodic array of holes consist of flux rivers resulting from a short range attractive interaction between vortices. This dynamically induced vortexvortex attraction results from the migration of quasiparticles out of the vortex core. We have directly visualized the formation of vortex chains by scanning Hall microscopy after freezing the dynamic state by a field cooling procedure at constant bias current. Similar experiments carried out in a sample without holes show no hint of flux river formation.

Abstract: Freestanding silicene is predicted to display comparable electronic properties as graphene. However, the yet synthesized silicenelike structures have been only realized on different substrates which turned out to exhibit versatile crystallographic structures that are very different from the theoretically predicted buckled phase of freestanding silicene. This calls for a different approach where silicene is stabilized using very weakly interacting surfaces. We propose here a route by using graphene bilayer as a scaffold. The confinement between the flat graphene layers results in a planar clustering of Si atoms with small buckling, which is energetically unfavorable in vacuum. Buckled hexagonal arrangement of Si atoms similar to freestanding silicene is observed for large clusters, which, in contrast to Si atoms on metallic surfaces, is only very weakly van der Waals coupled to the graphene layers. These clusters are found to be stable well above room temperature. Our findings, which are supported by density-functional tight-binding calculations, show that intercalating bilayer graphene with Si is a favorable route to realize silicene.

Abstract: Using numerical simulations, we study the dynamic properties of a superconducting stripe with a perpendicular magnetized ferromagnet on top in the presence of an applied dc current. In the resistive state conventional phase-slip lines are transformed into kinematic vortex-antivortex pairs with special dynamic behavior. In addition, the location of phase slippage in the sample is predetermined by the position of the magnetic dot. Both these effects directly influence the dynamics of the superconducting condensate and lead to periodic oscillations of the voltage across the sample with a frequency tunable both by the applied current and by the magnetization of the magnet. We found that the frequency of the voltage oscillations increases with increasing number of magnetic dots.

Abstract: Intermediate-state flux structures in mesoscopic type-I superconductors are studied within the Ginzburg-Landau theory. In addition to well-established tubular and laminar structures, the strong confinement leads to the formation of (i) a phase of singly quantized vortices, which is typical for type-II superconductors and (ii) a ring of a normal domain at equilibrium. The stability region and the formation process of these intermediate-state structures are strongly influenced by the geometry of the sample.

Abstract: The photo-response of a thin current-carrying superconducting stripe with a 90 degrees turn is studied within the time-dependent Ginzburg-Landau theory. We show that the photon acting near the inner corner (where the current density is maximal due to the current crowding [J. R. Clem and K. K. Berggren, Phys. Rev. B 84, 174510 (2011)]) triggers the nucleation of superconducting vortices at currents much smaller than the expected critical one, but does not bring the system to a higher resistive state and thus remains undetected. The transition to the resistive state occurs only when the photon hits the stripe away from the corner due to there uniform current distribution across the sample, and dissipation is due to the nucleation of a kinematic vortex-antivortex pair near the photon incidence. We propose strategies to account for this problem in the measurements. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4731627]

Abstract: <script type='text/javascript'>document.write(unpmarked('The motion of Abrikosov vortices in type-II superconductors results in a finite resistance in the presence of an applied electric current. Elimination or reduction of the resistance via immobilization of vortices is the \u0022holy grail\u0022 of superconductivity research. Common wisdom dictates that an increase in the magnetic field escalates the loss of energy since the number of vortices increases. Here we show that this is no longer true if the magnetic field and the current are applied parallel to each other. Our experimental studies on the resistive behavior of a superconducting Mo0.79Ge0.21 nanostrip reveal the emergence of a dissipative state with increasing magnetic field, followed by a pronounced resistance drop, signifying a reentrance to the superconducting state. Large-scale simulations of the 3D time-dependent Ginzburg-Landau model indicate that the intermediate resistive state is due to an unwinding of twisted vortices. When the magnetic field increases, this instability is suppressed due to a better accommodation of the vortex lattice to the pinning configuration. Our findings show that magnetic field and geometrical confinement can suppress the dissipation induced by vortex motion and thus radically improve the performance of superconducting materials.'));

Abstract: Our calculations, within known limitations of Ginzburg-Landau theory, are fully correct and valid for transport phenomena in asymmetric mesoscopic superconductors, deep in the superconducting state. We deemed the experiments of Burlakov et al. [JETP Lett. 86, 517 (2007)] relevant and important to mention in the general context of our paper since the observed shifts in the oscillations of different quantities are qualitatively similar, even though those measurements are performed close to the superconducting-normal state transition in the so-called Little-Parks regime.

Abstract: Density functional theory calculations are used to study gas adsorption properties of a recently synthesized CaO monolayer, which is found to be thermodynamically stable in its buckled form. Due to its topology and strong interaction with the CO2 molecules, this material possesses a remarkably high CO2 uptake capacity (similar to 0.4 g CO2 per g adsorbent). The CaO + CO2 system shows excellent thermal stability (up to 1000 K). Moreover, the material is highly selective towards CO2 against other major greenhouse gases such as CH4 and N2O. These advantages make this material a very promising candidate for CO2 capture and storage applications.

Abstract: We report an anomalous matching effect in MoGe thin films containing pairs of circular holes arranged in such a way that four of those pairs meet at each vertex point of a square lattice. A remarkably pronounced fractional matching was observed in the magnetic field dependences of both the resistance and the critical current. At the half matching field the critical current can be even higher than that at zero field. This has never been observed before for vortices in superconductors with pinning arrays. Numerical simulations within the nonlinear Ginzburg-Landau theory reveal a square vortex ice configuration in the ground state at the half matching field and demonstrate similar characteristic features in the field dependence of the critical current, confirming the experimental realization of an artificial ice system for vortices for the first time.