Nucleation of stable superconductivity in YBCO-films

Abstract

By means of the linear dynamic conductivity, inductively measured on epitaxial films between 30mHz and 30 MHz, the transition line Tg(B) to generic superconductivity is studied in fields between B=0 and 19T. It follows closely the melting line Tm(B) described recently in terms of a blowout of thermal vortex loops in clean materials. The critical exponents of the correlation length and time near Tg(B), however, seem to be dominated by some intrinsic disorder. Columnar defects produced by heavy-ion irradiation up to field-equivalent-doses of B φ = 10T lead to a disappointing reduction of Tg(B → 0) while for B > B φ the generic line of the pristine film is recovered. These novel results are also discussed in terms of a loop-driven destruction of generic superconductivity.