Crossover from 2D to 3D behavior in the superfluid density of thin YBa2Cu3O7−δ films

Ultrathin underdoped films of YBa2Cu3O7−δ (YBCO) exhibit a prominent and abrupt downturn in ab-plane superfluid density nS(T) at a temperature consistent with a 2D vortex-pair unbinding transition. In this paper, we show that this characteristic feature of 2D superconductors diminishes with increasing film thickness consistent with theory, provided one uses the full film thickness rather than a copper-oxide bilayer thickness to calculate TC. In thick films, there is no evidence for 3D-XY fluctuations preceding the 2D downturn, as would be expected in a quasi-2D superconductor.