Doping dependence of the upper critical field in untwinned YBa2Cu3O7−δ thin films

We report on measurements of the doping dependence of the upper critical field Hc,2 in 50 nm thick YBa2Cu3O7−δ films. The films are untwinned and are characterized by a small in-plane compressive strain. We find that the Hc,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 Hc,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-Tc materials under strain, and opens up the possibility for investigating the interplay between CDW order and superconductivity tuned by strain.