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

We have determined the upper critical field Hc2as 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 Hc2decreased 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 ξ0closely tracks the gap measured by photoemission. We discuss implications for understanding the doping dependence of the critical temperature$T_{c0}$.