PAIR FUNNELING IN THE LOW-TEMPERATURE CONDUCTIVITY OF THE CD-SB ALLOYHIGH-RESISTANCE STATE NEAR THE SUPERCONDUCTOR-INSULATOR TRANSITION

We report a study of the temperature dependence, down to 30 mK, of the magnetoresistance of Cd-Sb alloy in the insulating phase obtained by annealing the quenched metallic superconducting (T-c approximate to 4. 5 K) phase of the alloy. Even though the sample in this state is no lo nger superconducting, the observed negative magnetoresistance points t o single-particle tunneling in the presence of a superconducting gap i n the spectrum. At magnetic fields B<2 T the ratio (alpha(T,B)=R(T,B)/ R(T,B=4 T) is found to be maximum at a temperature of about 0.1 K. Thi s behavior indicates a change of the conductivity mechanism from singl e-particle tunneling to incoherent two-particle tunneling as the tempe rature decreases. (C) 1996 American Institute of Physics.