Anomalous magnetoresistance near the superconductor-insulator transition in ultrathin films of α-MoxSi1-x

We have made systematic studies for both the zero-field and field-driven superconductor-insulator transitions in a series of 4-nm-thick films of amorphous MoxSi1-x at temperatures T down to similar to 0.05 K and fields B up to 15 T. For superconducting films, we have observed an anomalous peak in the magnetoresistance R(B) and a subsequent decrease in R(B) with increasing B at low temperatures in fields higher than the critical field B-xxC. This result, together with finding that the magnetoresistance is always positive for insulating films, may suggest the presence of localized Cooper pairs even at B>B-xxC in the limit of T-->0.