Excess conductivity of overdoped Bi2Sr2CaCu2O8+x crystals well above Tc.

We have used a multiterminal technique in order to measure the (a,b) plane excess conductivity Δσ in several Bi2Sr2CaCu2O8+x single crystals. We find that the experimental Δσ does not follow a simple power law Δσ∼ε-α, with ε=ln(T/Tc), and that it drops faster than the two-dimensional Aslamazov-Larkin law, α=1, with increasing temperature. In addition, data for samples with different doping do not scale on a universal curve. We discuss our data in terms of microscopic and Ginzburg-Landau theories, where high-momentum fluctuations are either not excited, or phenomenologically cut off. The experimental Δσ drops even faster than the prediction of the extended microscopic theory. However, we can accurately describe all our data up to T≈1.3 Tc with the GL theory, assuming a sample-dependent cutoff value. We relate the cutoff parameter to the doping level of our samples.