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 2 Sr 2 CaCu 2 O 8+ δ , Bi 2 Sr 2 Ca 2 Cu 3 O 10+ δ , and Y Ba 2 Cu 3 O 7 − δ 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 ν = 1 very close to T c , and a crossover to the region with ν = 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 ν = 2 / 3, the normal conductivity is gradually recovered, i. e. the closing of the pseudogap is monitored. in dc conductivity studies. In this paper we present an analysis of the microwave fluctuation conductivity in Bi 2 Sr 2 CaCu 2 O 8+ δ (BSCCO-2212), Bi 2 Sr 2 Ca 2 Cu 3 O 10+ δ (BSCCO-2223), and Y Ba 2 Cu 3 O 7 − δ (YBCO) thin films. The central result is the temperature dependence of the coherence length which shows multiple critical regimes in each of the superconductors. Also shown in the present analysis is the reduction of the contribution of the normal conductivity in the measured real part σ 1 ( T ) of the complex conductivity. It is a direct evidence of the reduction of the one-electron density of states at the Fermi level, known as the pseudogap effect 24 .