Biennial Report 2005-2007 - Saha Institute of Nuclear Physics

Condensed Matter Physics 127have been evaluated as a function of electron-phonon (e-ph) coupling strength and temperature (T).The effects of site diagonal disorder on the above properties have been investigated. The disorderis found to suppress the kinetic energy and the Drude weight, reduces the spatial extension ofthe polaron, and makes the large-to-small polaron crossover smoother. Increasing temperature alsoplays similar role. For strong coupling the kinetic energy arises mainly from the incoherent hoppingprocesses owing to the motion of electrons within the polaron and it is almost independent of thedisorder strength. A second order strong coupling perturbation method has also been followedconsidering an infinite lattice system to show that the above is true for the Hostein model of anysize and any dimension. For strong coupling there is almost no size dependence on the resultspresented. At higher temperatures the kinetic energy shows a 1/T dependence. From the coherentand incoherent contributions to the kinetic energy, the temperature above which the incoherentpart dominates is determined as a function of e-ph coupling strength.AN Das, S Sil†TCMP4.2.1.6 Tunneling Conductance of graphene NIS junctionsDetails of the work: We show that in contrast to conventional normal metal-insulatorsuperconductor(NIS) junctions, the tunneling conductance of a NIS junction in graphene is anoscillatory function of the effective barrier strength of the insulating region, in the limit of a thinbarrier. The amplitude of these oscillations are maximum for aligned Fermi surfaces of the normaland superconducting regions and vanishes for large Fermi surface mismatch. The zero-bias tunnelingconductance, in sharp contrast to its counterpart in conventional NIS junctions, becomesmaximum for a finite barrier strength. We also suggest experiments to test these predictions.Subhro Bhattacharjee†, K SenguptaTCMP4.2.1.7 Josephson effect in graphene SBS junctionsDetails of the work: We study Josephson effect in graphene superconductor- barrier- superconductorjunctions with short and wide barriers, which can be created using a gate voltage. We show thatthe amplitude of the Josephson current of such graphene junctions, in complete contrast to theirconventional counterparts, is an oscillatory function of the effective barrier strength. We demonstratethat this oscillatory behavior occurs due to transmission resonance of Dirac-Bogoliubov-deGennes (DBdG) quasiparticles in superconducting graphene and propose simple experiments totest our predictions.Moitri Maiti, K SenguptaTCMP4.2.1.8 Theory of Tunneling Conductance of graphene NIS junctionsDetails of the work: We calculate the tunneling conductance of a graphene normal metal-insulatorsuperconductor(NIS) junction with a barrier of thickness d and with an arbitrary voltage V 0 appliedacross the barrier region. We demonstrate that the tunneling conductance of such a NIS junction