Universal relationship between the penetration depth and the normal-state conductivity in YBaCuO

The absolute values of the conductivity in the normal state σn and of the low-temperature penetration depths λ(0) were measured for a number of different samples of the YBaCuO family. We found a striking correlation between σn and λ−2 regardless of doping, oxygen reduction or defects, thus providing a simple method to predict the superconducting penetration depth and to have an estimate of the sample quality by measuring the normal-state conductivity.