Fluctuations at the superconductor/normal phase transition of YBCO thin films

The zero-field phase transition of high $T_c $ superconductors has been studied by a number of techniques, such as penetration depth, magnetic susceptibility, specific heat and thermal expansion, which reveal information about the static, properties of fluctuations. Transport properties (such as the conductivity) which probe the dynamics near $T_c $ are less explored, and a wide range of critical exponents were reported experimentally. We investigated fluctuation effects of $YBa_2 Cu_3 O_{7-d} $~(YBCO) around~$T_c $ by doing frequency-dependent microwave conductivity measurements and dc current-voltage characteristics on the same film. For each experiment the scaling behavior of the data was investigated. The critical exponents $\nu $ and z from the two different experiments will be extracted and compared. We also investigated YBCO frequency-dependent conductivity fluctuation effects for different powers or currents and for different film thickness. The finite size effects in both the microwave and DC voltage current experiments will also be discussed. (This work was supported by NSF grant number DMR-0302596)