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  • 1
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    Collapse of the low temperature insulating state in Cr-doped V2O3 thin films (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-09-19
    Description: We have grown epitaxial Cr-doped V 2 O 3 thin films with Cr concentrations between 0% and 20% on (0001)-Al 2 O 3 by oxygen-assisted molecular beam epitaxy. For the highly doped samples (〉3%), a regular and monotonous increase of the resistance with decreasing temperature is measured. Strikingly, in the low doping samples (between 1% and 3%), a collapse of the insulating state is observed with a reduction of the low temperature resistivity by up to 5 orders of magnitude. A vacuum annealing at high temperature of the films recovers the low temperature insulating state for doping levels below 3% and increases the room temperature resistivity towards the values of Cr-doped V 2 O 3 single crystals. It is well-know that oxygen excess stabilizes a metallic state in V 2 O 3 single crystals. Hence, we propose that Cr doping promotes oxygen excess in our films during deposition, leading to the collapse of the low temperature insulating state at low Cr concentrations. These results suggest that slightly Cr-doped V 2 O 3 films can be interesting candidates for field effect devices.
    Print ISSN: 0003-6951
    Electronic ISSN: 1077-3118
    Topics: Physics
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  • 2
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    Optical conductivity of topological insulator thin films (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-05-06
    Description: We present a detailed theoretical study on the optoelectronic properties of topological insulator thin film (TITFs). The k · p approach is employed to calculate the energy spectra and wave functions for both the bulk and surface states in the TITF. With these obtained results, the optical conductivities induced by different electronic transitions among the bulk and surface states are evaluated using the energy-balance equation derived from the Boltzmann equation. We find that for Bi 2 Se 3 -based TITFs, three characteristic regimes for the optical absorption can be observed. (i) In the low radiation frequency regime (photon energy ℏ ω 〈 200  meV), the free-carrier absorption takes place due to intraband electronic transitions. An optical absorption window can be observed. (ii) In the intermediate radiation frequency regime ( 200 〈 ℏ ω 〈 300  meV), the optical absorption is induced mainly by interband electronic transitions from surface states in the valance band to surface states in the conduction band and an universal value σ 0 = e 2 / ( 8 ℏ ) for the optical conductivity can be obtained. (iii) In the high radiation frequency regime ( ℏ ω 〉 300  meV), the optical absorption can be achieved via interband electronic transitions from bulk and surface states in the valance band to bulk and surface states in the conduction band. A strong absorption peak can be observed. These interesting findings indicate that optical measurements can be applied to identify the energy regimes of bulk and surface states in the TITF.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
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  • 3
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    Stable half-metallic monolayers of FeCl2 (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-05-14
    Description: The structural, electronic, and magnetic properties of single layers of Iron Dichloride (FeCl 2 ) were calculated using first principles calculations. We found that the 1T phase of the single layer FeCl 2 is 0.17 eV/unit cell more favorable than its 1H phase. The structural stability is confirmed by phonon calculations. We found that 1T-FeCl 2 possess three Raman-active (130, 179, and 237 cm −1 ) and one infrared-active (279 cm −1 ) phonon branches. The electronic band dispersion of the 1T-FeCl 2 is calculated using both gradient approximation of Perdew-Burke-Ernzerhof and DFT-HSE06 functionals. Both functionals reveal that the 1T-FeCl 2 has a half-metallic ground state with a Curie temperature of 17 K.
    Print ISSN: 0003-6951
    Electronic ISSN: 1077-3118
    Topics: Physics
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  • 4
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    Distinct magnetic signatures of fractional vortex configurations in multiband superconductors (2014)
    American Institute of Physics (AIP)
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    Publication Date: 2014-12-10
    Description: Vortices carrying fractions of a flux quantum are predicted to exist in multiband superconductors, where vortex core can split between multiple band-specific components of the superconducting condensate. Using the two-component Ginzburg-Landau model, we examine such vortex configurations in a two-band superconducting slab in parallel magnetic field. The fractional vortices appear due to the band-selective vortex penetration caused by different thresholds for vortex entry within each band-condensate, and stabilize near the edges of the sample. We show that the resulting fractional vortex configurations leave distinct fingerprints in the static measurements of the magnetization, as well as in ac dynamic measurements of the magnetic susceptibility, both of which can be readily used for the detection of these fascinating vortex states in several existing multiband superconductors.
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    Electronic ISSN: 1077-3118
    Topics: Physics
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  • 5
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    Veselago lensing in graphene with a p-n junction: Classical versus quantum effects (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-10-21
    Description: The feasibility of Veselago lensing in graphene with a p-n junction is investigated numerically for realistic injection leads. Two different set-ups with two narrow leads are considered with absorbing or reflecting side edges. This allows us to separately determine the influence of scattering on electron focusing for the edges and the p-n interface. Both semiclassical and tight-binding simulations show a distinctive peak in the transmission probability that is attributed to the Veselago lensing effect. We investigate the robustness of this peak on the width of the injector, the position of the p-n interface, and different gate potential profiles. Furthermore, the influence of scattering by both short- and long-range impurities is considered.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
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  • 6
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    Derivatization and diffusive motion of molecular fullerenes: Ab initio and atomistic simulations (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-07-11
    Description: Using first principles density functional theory in combination with the nonequilibrium Green's function formalism, we study the effect of derivatization on the electronic and transport properties of C 60 fullerene. As a typical example, we consider [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM), which forms one of the most efficient organic photovoltaic materials in combination with electron donating polymers. Extra peaks are observed in the density of states (DOS) due to the formation of new electronic states localized at/near the attached molecule. Despite such peculiar behavior in the DOS of an isolated molecule, derivatization does not have a pronounced effect on the electronic transport properties of the fullerene molecular junctions. Both C 60 and PCBM show the same response to finite voltage biasing with new features in the transmission spectrum due to voltage induced delocalization of some electronic states. We also study the diffusive motion of molecular fullerenes in ethanol solvent and inside poly(3-hexylthiophene) lamella using reactive molecular dynamics simulations. We found that the mobility of the fullerene reduces considerably due to derivatization; the diffusion coefficient of C 60 is an order of magnitude larger than the one for PCBM.
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    Electronic ISSN: 1089-7550
    Topics: Physics
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  • 7
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    Theoretical study of electronic transport properties of a graphene-silicene bilayer (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-06-09
    Description: Electronic transport properties of a graphene-silicene bilayer system are studied using density-functional theory in combination with the nonequilibrium Green's function formalism. Depending on the energy of the electrons, the transmission can be larger in this system as compared to the sum of the transmissions of separated graphene and silicene monolayers. This effect is related to the increased electron density of states in the bilayer sample. At some energies, the electronic states become localized in one of the layers, resulting in the suppression of the electron transmission. The effect of an applied voltage on the transmission becomes more pronounced in the layered sample as compared to graphene due to the larger variation of the electrostatic potential profile. Our findings will be useful when creating hybrid nanoscale devices where enhanced transport properties will be desirable.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
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  • 8
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    Terahertz plasmon-polariton modes in graphene driven by electric field inside a Fabry-Pérot cavity (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-06-11
    Description: We present a theoretical study on plasmon-polariton modes in graphene placed inside an optical cavity and driven by a source-to-drain electric field. The electron velocity and electron temperature are determined by solving self-consistently the momentum- and energy-balance equations in which electron interactions with impurities, acoustic-, and optic-phonons are included. Based on many-body self-consistent field theory, we develop a tractable approach to study plasmon-polariton in an electron gas system. We find that when graphene is placed inside a Fabry-Pérot cavity, two branches of the plasmon-polariton modes can be observed and these modes are very much optic- or plasmon-like. The frequencies of these modes depend markedly on driving electric field especially at higher resonant frequency regime. Moreover, the plasmon-polariton frequency in graphene is in terahertz (THz) bandwidth and can be tuned by changing the cavity length, gate voltage, and driving electric field. This work is pertinent to the application of graphene-based structures as tunable THz plasmonic devices.
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    Topics: Physics
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  • 9
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    Structural transitions and long-time self-diffusion of interacting colloids confined by a parabolic potential (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-01-10
    Description: We report on the ordering and dynamics of interacting colloidal particles confined by a parabolic potential. By means of Brownian dynamics simulations, we find that by varying the magnitude of the trap stiffness, it is possible to control the dimension of the system and, thus, explore both the structural transitions and the long-time self-diffusion coefficient as a function of the degree of confinement. We particularly study the structural ordering in the directions perpendicular and parallel to the confinement. Further analysis of the local distribution of the first-neighbors layer allows us to identify the different structural phases induced by the parabolic potential. These results are summarized in a structural state diagram that describes the way in which the colloidal suspension undergoes a structural re-ordering while increasing the confinement. To fully understand the particle dynamics, we take into account hydrodynamic interactions between colloids; the parabolic potential constricts the available space for the colloids, but it does not act on the solvent. Our findings show a non-linear behavior of the long-time self-diffusion coefficient that is associated to the structural transitions induced by the external field.
    Print ISSN: 0021-9606
    Electronic ISSN: 1089-7690
    Topics: Chemistry and Pharmacology , Physics
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  • 10
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    Anisotropic electronic, mechanical, and optical properties of monolayer WTe2 (2016)
    American Institute of Physics (AIP)
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    Publication Date: 2016-02-20
    Description: Using first-principles calculations, we investigate the electronic, mechanical, and optical properties of monolayer WTe 2 . Atomic structure and ground state properties of monolayer WTe 2 (T d phase) are anisotropic which are in contrast to similar monolayer crystals of transition metal dichalcogenides, such as MoS 2 , WS 2 , MoSe 2 , WSe 2 , and MoTe 2 , which crystallize in the H-phase. We find that the Poisson ratio and the in-plane stiffness is direction dependent due to the symmetry breaking induced by the dimerization of the W atoms along one of the lattice directions of the compound. Since the semimetallic behavior of the T d phase originates from this W-W interaction (along the a crystallographic direction), tensile strain along the dimer direction leads to a semimetal to semiconductor transition after 1% strain. By solving the Bethe-Salpeter equation on top of single shot G 0 W 0 calculations, we predict that the absorption spectrum of T d -WTe 2 monolayer is strongly direction dependent and tunable by tensile strain.
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    Topics: Physics
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