EVIDENCE FOR 2-PARTICLE NORMAL-STATE TUNNELING IN BA1-XKXBIO3 NATIVE-BARRIER TUNNEL-JUNCTIONS

The asymmetric background conductance linear in bias voltage usually o bserved in Ba1-xKxBiO3 superconductor-insulator-normal (SIN) tunnel ju nctions is associated with a zero-bias conductance linear in temperatu re above the superconducting transition. The ratio of the temperature coefficient and the voltage coefficient provides insight into the mech anism causing the linear background conductance. We present a simple m odel involving tunneling through two-particle states (negative-U, bipo laron) in the barrier that explains the linear background, the asymmet ry, and the linear temperature dependence of the conductance. Measurem ents of native-barrier Ba1-xKxBiO3 tunnel junctions are in quantitativ e agreement with the model.