Approaching two-dimensional superconductivity in ultrathin DyBa$_2$Cu$_3$O$_{7-δ}$

Abstract

The temperature dependence of the superfluid density ρs(T ) has been measured for a series of ultrathin MBE-grown DyBa2Cu3O 7-δ superconducting (SC) films by sub-mm wave interferometry combined with time-domain THz spectroscopy and IR ellipsometry. We find that all films 10 u.c. and thicker show the same universal temperature dependence of ρs(T ), which follows the critical behavior characteristic of single crystal YBa2Cu3O 7-δ as T approaches Tc. In 7 u.c. thick films, ρs(T ) declines steeply upon approaching Tc, as expected for the Berezinskii-Kosterlitz-Thouless vortex unbinding transition. Our analysis provides evidence for a sharply defined 4 u.c. non-SC interfacial layer, leaving a quasi-2D SC layer on top. We propose that the SC state in this interfacial layer is suppressed by competing (possibly charge) order.