Thickness dependence of superfluid density scaling in DyBa₂Cu₃O_7-x thin films

Understanding the nature of the pseudogap phase and the relationship between charge order and superconductivity (SC) in the cuprates remains an outstanding challenge [1]. Recently, 2D SC has gained much interest due to its discovery in a variety of unconventional materials, and advancements in MBE growth of thin films have shown that 2D SC can also exist in the cuprates [2]. One interesting approach, therefore, is to investigate the evolution of cuprate physics in the ultra-thin film limit where only a few superconducting CuO₂ planes are present. Here, we report a study of the temperature dependence of the superfluid density as a function of thickness as well as measurements of the terahertz and infrared optical conductivity in a series of DyBa₂Cu₃O_7-x SC thin films. We find that in the ultra-thin film limit the temperature scaling rapidly approaches that expected for dirty s-wave SC, while with decreasing oxygen content the scaling shows the opposite trend. These results suggest an increasing isotropic order parameter component below a threshold thickness due to distortions of the Fermi surface in the 2D limit.
[1] B. Keimer et. al., Nature 518, pg. 179–186.
[2] G. Logvenov et. al., Science 326, no. 5953, pg. 699–702.