Critical fluctuations and pseudogap observed in the microwave conductivity of BSCCO and YBCO thin films

Critical fluctuations have been studied in the microwave conductivity of Bi_2Sr_2CaCu_2O_(8+\delta), Bi_2Sr_2Ca_2Cu_3O_(10+delta), and YBa_2Cu_3O_(7-delta) thin films above T_c. It is found that a consistent analysis of the real and imaginary parts of the fluctuation conductivity can be achieved only if an appropriate wavevector or energy cutoff in the fluctuation spectrum is taken into account. In all of the three underdoped superconducting films one observes strong fluctuations extending far above T_c. The coherence length inferred from the imaginary part of the conductivity exhibits the static critical exponent nu = 1 very close to T_c, and a crossover to the region with nu = 2/3 at higher temperatures. In parallel, our analysis reveals the absence of the normal conductivity near T_c, i.e. fully opened pseudogap. Following the crossover to the region with nu = 2/3, the normal conductivity is gradually recovered, i.e. the closing of the pseudogap is monitored.