Critical fluctuations and pseudogap observed in the microwave conductivity of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta },$ ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{10+\delta },$ and ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ thin films

Critical fluctuations have been studied in the microwave conductivity of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta },$ ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{10+\delta },$ and ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ 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 wave vector 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 $\nu =1\mathrm{}$ very close to $T_c,$ and a crossover to the region with $\nu =2/3\mathrm{}$ 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.