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_o closely tracks the gap measured by photoemission. We discuss implications for understanding the doping dependence of the critical temperature T_co.