Thermal fluctuations in the microwave conductivity ofBi2Sr2CaCu2O8

Recently, a peak close to ${\mathit{T}}_{\mathit{c}}$ has been observed in the microwave conductivity of single crystals of the high-temperature superconductor ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$. This peak was interpreted as a coherence peak. In this paper we investigate an interpretation in terms of thermal-fluctuation effects. The fluctuation contribution to the conductivity calculated by Aslamasov and Larkin (generalized to finite frequencies by Schmidt) is of the magnitude of the observed effect, but leads to a narrow peak at ${\mathit{T}}_{\mathit{c}}$. In microwave experiments in the gigahertz range, however, which probe a surface layer with a distribution of ${\mathit{T}}_{\mathit{c}}$’s, thermal fluctuations lead to a broader peak slightly below the dc critical temperature, as observed. Strong pair breaking tends to shift this peak somewhat further below the dc critical temperature, and also suppresses the importance of other fluctuation contributions (Maki-Thompson), which in principle could lead to a fluctuation peak in the nuclear-spin-relaxation rate. Our results are consistent with the conclusion that there are no true coherence peaks in the conductivity or nuclear-spin relaxation as a result of strong pair breaking.