The transition from the d- to s-state due to thermal fluctuation for high-T/sub c/ superconductors as an evidence from the microwave penetration-depth measurement

A temperature dependence of the penetration depth /spl lambda/(T) measurement for the high-T/sub c/ superconductors YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// and Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 7/ thin films elucidates a T/sup 2/ dependence at low temperatures and an exponential dependence at high temperatures. The transition temperature for the shift from T/sup 2/ to exponential dependence decreases as the duration for the samples exposed to air increases. An impurity scattered mechanism to fluctuate a pure d-wave to the s-wave by thermal fluctuation is proposed for the pairing states of these high-T/sub c/ superconducting films.