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

A temperature dependence of the penetration depth λ(T) measurement for the high-T_c superconductors YBa₂Cu ₃O_7-δ and Tl₂Ba₂CaCu ₂O₇ thin films elucidates a T² dependence at low temperatures and an exponential dependence at high temperatures. The transition temperature for the shift from T² 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_c superconducting films