Abstract: Quantum transport calculations are conducted using nonequilibrium Green's functional formalism to study the effect of halogenation on the electronic transport properties of aromatic S-(C6H5)(2)X and alkanethiolate S-(CH2)(11)X molecules (with X = H, F, Cl, Br, or I) sandwiched between gold electrodes. In terms of conductance, both molecules show the same dependence on the halogen terminal groups despite their different electronic nature. For example, fluorination results in a reduction of the current by almost an order of magnitude, whereas iodine substitution leads to larger current as compared to the reference system (i.e. hydrogen termination). Regarding the asymmetry in the current-voltage characteristics, halogenation reduces the rectification level for the aromatic molecule with the smallest asymmetry for iodine termination. However, in the case of alkanethiolate molecule, halogen substitution increases the current rectification except for fluorination. A physical explanation of these results is obtained from the analysis of the behavior of the density of states, transmission spectra and transmission eigenstates. These findings are of practical importance in exploring the potential of halogenation for creating functional molecular self-assemblies on metallic substrates.

Abstract: The application of atmospheric pressure plasmas in medicine is increasingly gaining attention in recent years, although very little is currently known about the plasma-induced processes occurring on the surface of living organisms. It is known that most bio-organisms, including bacteria, are coated by a liquid film surrounding them, and there might be many interactions between plasma species and the liquid layer before the plasma species reach the surface of the bio-organisms. Therefore, it is essential to study the behaviour of the reactive species in a liquid film, in order to determine whether these species can travel through this layer and reach the biomolecules, or whether new species are formed along the way. In this work, we investigate the interaction of reactive oxygen species (i.e. O, OH, HO2 and H2O2) with water, which is assumed as a simple model system for the liquid layer surrounding biomolecules. Our computational investigations show that OH, HO2 and H2O2 can travel deep into the liquid layer and are hence in principle able to reach the bio-organism. Furthermore, O, OH and HO2 radicals react with water molecules through hydrogen-abstraction reactions, whereas no H-abstraction reaction takes place in the case of H2O2. This study is important to gain insight into the fundamental operating mechanisms in plasma medicine, in general, and the interaction mechanisms of plasma species with a liquid film, in particular.

Abstract: The critical current of a thin superconducting strip of width W much larger than the Ginzburg-Landau coherence length xi but much smaller than the Pearl length Lambda = 2 lambda(2)/d is maximized when the strip is straight with defect-free edges. When a perpendicular magnetic field is applied to a long straight strip, the critical current initially decreases linearly with H but then decreases more slowly with H when vortices or antivortices are forced into the strip. However, in a superconducting strip containing sharp 90 degrees or 180 degrees turns, the zero-field critical current at H = 0 is reduced because vortices or antivortices are preferentially nucleated at the inner corners of the turns, where current crowding occurs. Using both analytic London-model calculations and time-dependent Ginzburg-Landau simulations, we predict that in such asymmetric strips the resulting critical current can be increased by applying a perpendicular magnetic field that induces a current-density contribution opposing the applied current density at the inner corners. This effect should apply to all turns that bend in the same direction.

Abstract: The effect of vacancy defects on the structural properties and the thermal stability of free standing silicene – a buckled structure of hexagonally arranged silicon atoms – is studied using reactive molecular dynamics simulations. Pristine silicene is found to be stable up to 1500 K, above which the system transits to a three-dimensional amorphous configuration. Vacancy defects result in local structural changes in the system and considerably reduce the thermal stability of silicene: depending on the size of the vacancy defect, the critical temperature decreases by more than 30%. However, the system is still found to be stable well above room temperature within our simulation time of 500 ps. We found that the, stability of silicene can be increased by saturating the dangling bonds at the defect edges by foreign atoms (e.g., hydrogen).

Abstract: The energy barrier which has to be overcome for a single vortex to enter or exit the sample is studied for thin superconducting disks, rings, and squares using the nonlinear Ginzburg-Landau theory. The shape and the height of the nucleation barrier is investigated for different sample radii and thicknesses and for different values of the Ginzburg-Landau parameter kappa. It is shown that the London theory considerably overestimates (underestimates) the energy barrier for vortex expulsion (penetration). (c) 2005 American Institute of Physics.

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: Thin superconducting films can exhibit negative magnetoresistance when an in-plane external magnetic field is aligned parallel with the transport current. We explain this effect as due to appearance of parallel vortices in the plain of the film at the first critical magnetic field H-c1 which leads to an enhancement of the superconducting properties and impedes the motion of the current induced perpendicular vortices. Our theoretical results are based on a numerical solution of the time-dependent and stationary 3D Ginzburg-Landau equations.

Abstract: Using density functional theory in combination with the Green’s functional formalism, we study the effect of surface functionalization on the electronic transport properties of 1D carbon allotrope—carbyne. We found that both hydrogenation and fluorination result in structural changes and semiconducting to metallic transition. Consequently, the current in the functionalization systems increases significantly due to strong delocalization of electronic states along the carbon chain. We also study the electronic transport in partially hydrogenated carbyne and interface structures consisting of pristine and functionalized carbyne. In the latter case, current rectification is obtained in the system with rectification ratio up to 50%. These findings can be useful for developing carbyne-based structures with tunable electronic transport properties.

Abstract: Using density-functional theory in combination with the nonequilibrium Green's function formalism, we study the effect of the crystal lattice structure of organometallic perovskite CH3NH3PbI3 on its electronic transport properties. Both dispersive interactions and spin-orbit coupling are taken into account in describing structural and electronic properties of the system. We consider two different phases of the material, namely the orthorhombic and cubic lattice structures, which are energetically stable at low (< 160 K) and high (> 330 K) temperatures, respectively. The sizable geometrical differences between the two structures in term of lattice parameters, PbI6 octahedral tilts, rotation and deformations, have considerable impact on the transport properties of the material. For example, at zero bias and for all considered electron energies, the cubic phase has a larger transmission than the orthorhombic one, although both show similar electronic densities of states. Depending on the applied voltage, the current in the cubic system can be several orders of magnitude larger as compared to the one obtained for the orthorhombic sample. We attribute this enhancement in the transmission to the presence of extended states in the cubic phase due to the symmetrically shaped and ordered PbI6 octaherdra. (C) 2015 Elsevier B.V. All rights reserved.

Abstract: Using first-principles calculations and reactive force field molecular dynamics simulations, we study the structural properties and dynamics of a fluorine (F) atom, either adsorbed on the surface of single layer graphene (F/GE) or between the layers of AB stacked bilayer graphene (F@ bilayer graphene). It is found that the diffusion of the F atom is very different in those cases, and that the mobility of the F atom increases by about an order of magnitude when inserted between two graphene layers. The obtained diffusion constant for F/GE is twice larger than that experimentally found for gold adatom and theoretically found for C-60 molecule on graphene. Our study provides important physical insights into the dynamics of fluorine atoms between and on graphene layers and explains the mechanism behind the separation of graphite layers due to intercalation of F atoms.

Abstract: Using the anisotropic time-dependent Ginzburg-Landau theory we study the effect of ordered and disordered pinning on the time response of superconducting strips to an external current that switched on abruptly. The pinning centers result in a considerable delay of the response time of the system to such abrupt switching on of the current, whereas the output voltage is always larger when pinning is present. The resistive state in both cases are characterized either by dynamically stable phase-slip centers/lines or expanding in-time hot-spots, which are the main mechanisms for dissipation in current-carrying superconductors. We find that hot-spots are always initiated by the phase-slip state. However, the range of the applied current for the phase-slip state increases significantly when pinning is introduced. Qualitative changes are observed in the dynamics of the superconducting condensate in the presence of pinning.

Abstract: Using the mass-spectrometric method we studied the formation of multi-charged plasma ions during the interaction of laser radiation with solid targets irradiated by neutron beams. We found that structural defects, caused by the neutron irradiation, influence not only the efficiency of the process of material evaporation and emission of plasma, but also the ionization and recombination processes taking place at the initial stage of plasma formation and expansion. We also show the effect of the dose of neutron irradiation on the threshold of plasma formation from the surface of the target.

Abstract: Using the time-dependent GinzburgLandau formalism, we study the dynamic properties of a submicron superconducting loop in applied current and in presence of a perpendicular magnetic field. The resistive state of the sample is caused by the motion of kinematic vortexantivortex pairs. Vortices and antivortices move in opposite directions to each other, perpendicularly to the applied drive, and the periodic creation and annihilation of such pairs results in periodic oscillations of the voltage across the sample. The dynamics of these kinematic pairs is strongly influenced by the applied magnetic field, which for high fields leads to the flow of just vortices. Kinematic vortices can be temporarily pinned inside the loop with observable trace in the voltage vs. time characteristics.

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: We conduct ballistic transport calculations to study the electronic transport properties of diazatriptycene molecule which can be self-assembled on metallic surfaces with uniform coverage and upright orientation of the functional head group. Due to its structural asymmetry, the molecule shows a clear current rectification, where the level of the rectification depends on the nature of the head group. For example, current rectification can be increased by more than a factor of 2 by anchoring the molecules to the electrode by CN functional group or introducing insulating CH2 group between the thiol end group and the adjacent phenyl ring. Our findings show the possibility of creating self-assembled monolayer of DAT molecules with controlled electronic transport properties through functionalization of the head group.