Large Dynamical Fluctuations in the Microwave Conductivity of YBa₂Cu₃O_7-δ above T_c

We report a systematic investigation of dynamical fluctuation effects in the frequency-dependent microwave conductivity (45 MHz–45 GHz) of YBa₂Cu₃O_7-δ thin films for T≳T_c. Our measurements directly yield a dynamical critical exponent z in the range 2.3–3.0, and the fluctuation lifetime τ^fl, which diverges more quickly than Gaussian theory predicts as the temperature approaches T_c from above, independent of sample quality. In addition, both the temperature and the frequency dependence of the fluctuation conductivity σ^fl exhibit scaling behavior for temperatures 1–2 K above T_c, and can be collapsed onto the same universal curve.