Decrease of upper critical field with underdoping in cuprate superconductors

It is still unclear why the transition temperature T-c of cuprate superconductors falls with underdoping. The doping dependence of the critical magnetic field H-c2 is directly relevant to this question, but different estimates of H-c2 are in sharp contradiction. We resolve this contradiction by tracking the characteristic field scale of superconducting fluctuations as a function of doping, via measurements of the Nernst effect in La1.8-xEu0.2SrxCuO4. H-c2 is found to fall with underdoping, with a minimum where stripe order is strong. The same non-monotonic behaviour is observed in the archetypal cuprate superconductor YBa2Cu3Oy. We conclude that competing states such as stripe order weaken superconductivity and cause both H-c2 and T-c to fall as cuprates become underdoped.