Critical Fluctuations in the Microwave Complex Conductivity of BSCCO and YBCO Thin Films

Critical fluctuations above $T_c$ are studied in the microwave complex conductivity of BSCCO-2212, BSCCO-2223, and YBCO thin films. The analysis of the experimental data yields the temperature dependences of the reduced coherence length $\xi(T)/\xi_0$. Two critical regimes are observed, having the static critical exponent $\nu=1$ close to $T_c$, and a crosover to $\nu=2/3$ at higher temperatures. In more anisotropic superconductors the reduced coherence lengths are larger, and the critical states extend to higher temperatures.