Fluctuations in the microwave conductivity of YBa2Cu3O7- delta single crystals in zero dc magnetic field.

We present a quantitative analysis of finite frequency fluctuation conductivity above and below Tc in cuprate superconductors in zero dc magnetic field. In a YBa2Cu3O72d crystal showing a linear in temperature increase of the penetration depth at low temperatures, we find that two-dimensional ~2D! finite frequency Gaussian conductivity fluctuations above Tc cross over into a slower divergence of the conductivity as Tc is approached from above. We find that the critical regime above Tc is less than about 0.6 K wide. At and below Tc , 3D fluctuations dominate the conductivity, with evidence of 3D XY critical scaling of the imaginary part of the conductivity down to 5 K below Tc .