10,865 research outputs found

    Model for the magnetoresistance and Hall coefficient of inhomogeneous graphene

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    We show that when bulk graphene breaks into n-type and p-type puddles, the in-plane resistivity becomes strongly field dependent in the presence of a perpendicular magnetic field, even if homoge- neous graphene has a field-independent resistivity. We calculate the longitudinal resistivity \rho_{xx} and Hall resistivity \rho_{xy} as a function of field for this system, using the effective-medium approximation. The conductivity tensors of the individual puddles are calculated using a Boltzmann approach suit- able for the band structure of graphene near the Dirac points. The resulting resistivity agrees well with experiment, provided that the relaxation time is weakly field-dependent. The calculated Hall resistivity has the sign of the majority carrier and vanishes when there are equal number of n and p type puddles.Comment: 5 pages, 4 figure

    Numerical Study of Energy Loss by a Nanomechanical Oscillator Coupled to a Cooper Pair Box

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    We calculate the dynamics of a nanomechanical oscillator (NMO) coupled capacitively to a Cooper pair box (CPB), by solving a stochastic Schrodinger equation with two Lindblad operators. Both the NMO and the CPB are assumed dissipative, and the coupling is treated within the rotating wave approximation. We show numerically that, if the CPB decay time is smaller than the NMO decay time, the coupled NMO will lose energy faster, and the coupled CPB more slowly, than do the uncoupled NMO and CPB. The results show that the efficiency of energy loss by an NMO can be substantially increased if the NMO is coupled to a CPB.Comment: 10 pages, 3 figure

    Tunable Hydrogen Storage in Magnesium - Transition Metal Compounds

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    Magnesium dihydride (\mgh) stores 7.7 weight % hydrogen, but it suffers from a high thermodynamic stability and slow (de)hydrogenation kinetics. Alloying Mg with lightweight transition metals (TM = Sc, Ti, V, Cr) aims at improving the thermodynamic and kinetic properties. We study the structure and stability of Mgx_xTM1−x_{1-x}H2_2 compounds, x=[0x=[0-1], by first-principles calculations at the level of density functional theory. We find that the experimentally observed sharp decrease in hydrogenation rates for x≳0.8x\gtrsim0.8 correlates with a phase transition of Mgx_xTM1−x_{1-x}H2_2 from a fluorite to a rutile phase. The stability of these compounds decreases along the series Sc, Ti, V, Cr. Varying the transition metal (TM) and the composition xx, the formation enthalpy of Mgx_xTM1−x_{1-x}H2_2 can be tuned over the substantial range 0-2 eV/f.u. Assuming however that the alloy Mgx_xTM1−x_{1-x} does not decompose upon dehydrogenation, the enthalpy associated with reversible hydrogenation of compounds with a high magnesium content (x=0.75x=0.75) is close to that of pure Mg.Comment: 8 pages, 5 figure

    Tunable Band Gap in Graphene with a Non-Centrosymmetric Superlattice Potential

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    We show that, if graphene is subjected to the potential from an external superlattice, a band gap develops at the Dirac point provided the superlattice potential has broken inversion symmetry. As a numerical example, we calculate the band structure of graphene in the presence of an external potential due to periodically patterned gates arranged in a triangular graphene superlattice (TGS) with broken inversion symmetry, and find that a band gap is created at both the original and "second generation" Dirac point. The gap can be controlled, in principle, by changing the external potential and the lattice constant of the TGS.Comment: 6 figures, Phys. Rev. B 79, 20543

    Transient radiative energy transfer in incompressible laminar flows

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    Analysis and numerical procedures are presented to investigate the transient radiative interactions of nongray absorbing-emitting species in laminar fully-developed flows between two parallel plates. The particular species considered are OH, CO, CO2, and H2O and different mixtures of these. Transient and steady-state results are obtained for the temperaure distribution and bulk temperature for different plate spacings, wall temperatures, and pressures. Results, in general, indicate that the rate of radiative heating can be quite high during earlier times. This information is useful in designing thermal protection systems for transient operations
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