Doping dependence of the upper critical field in untwinned YBa2Cu3O 7 − δ thin films

We report on measurements of the doping dependence of the upper critical field H c , 2 in 50 nm thick YBa2Cu3O 7 − δ films. The films are untwinned and are characterized by a small in-plane compressive strain. We find that the H c , 2 shows a strong decrease in the underdoped region of the phase diagram, in agreement with that which has been measured in relaxed single crystals. The origin of the decrease of H c , 2 in the underdoped regime is discussed within a scenario where charge density wave (CDW) order competes with superconductivity. This demonstrates the potential of using thin films to study the phase diagram of high-T c materials under strain, and opens up the possibility for investigating the interplay between CDW order and superconductivity tuned by strain