Dependence of Upper Critical Field and Pairing Strength on Doping in Cuprates

We have determined the upper critical field H c2 as a function of hole concentration in bismuth-based cuprates by measuring the voltage induced by vortex flow in a driving temperature gradient (the Nernst effect), in magnetic fields up to 45 tesla. We found that H c2 decreased steeply as doping increased, in both single and bilayer cuprates. This relationship implies that the Cooper pairing potential displays a trend opposite to that of the superfluid density versus doping. The coherence length of the pairs ξ 0 closely tracks the gap measured by photoemission. We discuss implications for understanding the doping dependence of the critical temperature T c0 .