Determining the upper magnetic critical field in YBCO films

In the quest to increase the critical temperature of superconductors, it is essential to identify the factors that limit the strength of superconductivity. The upper critical field H_c,2 is a fundamental measure of that strength. Only in recent years, its magnitude and doping dependence has been measured in cuprate superconductors, but only when they are in single crystal form. It is therefore very interesting to measure its behavior in thin films, where the ground state can be modified through strain and intertwining and competition between different orders can be enhanced.

For the abovementioned reasons, in the work, written by our postdoc Eric Wahlberg and published on Superconductor Science and Technology (link), we have reported 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 what 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 order competes with superconductivity.

This work demonstrates the potential of using thin films for studying the phase diagram of high-Tc materials under strain, and opens up the possibility to investigate the interplay between charge density wave order and superconductivity tuned by strain.