Quasi-particle tunneling in zinc doped yttrium barium copper oxide

Abstract

Point contact junctions of Al---Al₂O₃---YBa₂Cu_3-xZn_xO_7-y (where x = 0.025, 0.05 and 0.1) were made in order to study quasi-particle tunneling at 4.2 K. Such studies are expected to yield information on the superconducting energy gap parameter, Δ, as a function of zinc concentration, x. Zinc doping brings down the temperature of zero resistance, T_C. In these specimens, symmetric conductance curves with a fairly large conductance overshoot were obtained. The tunneling curves obtained at different spots on the same sample showed peaks at nearly the same values of the applied voltage. Multiple peaks were observed in the conductance curves. It was seen that the separation of the symmetrically situated inner peaks decreased systematically as the concentration of zinc was increased. This shows that Δ decreases with decreasing T_C. 2Δ/k_BT_capproaches the BCS weak coupling limit as T_C decreases with increasing zinc concentration.