Fluctuation effects in the microwave conductivity of cuprate superconductors

Observations on the surface impedance at 14, 25, and 36 GHz of high quality crystals of Bi2Sr2CaCu2O8+x (Bi2212), Tl2Ba2CuO6 (T12201), and YBa2(Cu0.97 5Zn0.025)(3)O7-delta (Y123/Zn) are reported, and the corresponding complex conductivity sigma=sigma'-i sigma " is analyzed in the critical region near T-c and at higher temperatures where fluctuations are important. The well known sharp peak in sigma' within the critical region is successfully analy zed using effective medium theory. At higher temperatures Bi2212 and Y123/Z n agree in form with the treatment of conductivity fluctuations of Aslamazo v and Larkin, extended to high frequency. T12201, however, shows an extra r esponse of long relaxation time which persists at high temperatures. Tentat ive explanations of this effect are considered, including the possibility t hat it is due to uncondensed bipolarons. The physics of conversion of super current to normal current within fluctuating regions is reviewed. [S0163-18 29(99)10401-6].