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

TL;DR

This study investigates the superfluid density in ultrathin DyBa₂Cu₃O₇₋δ films, revealing a transition from 3D to 2D superconductivity and suggesting interfacial layer effects suppressing superconductivity.

Contribution

It provides experimental evidence of a transition to two-dimensional superconductivity in ultrathin films and identifies an interfacial layer affecting superconducting properties.

Findings

Films 10 u.c. and thicker show universal superfluid density behavior.

7 u.c. films exhibit BKT transition characteristics.

A 4 u.c. non-superconducting interfacial layer is identified.

Abstract

The temperature dependence of the superfluid density $\rho_s(T)$ has been measured for a series of ultrathin MBE-grown DyBa$_2$Cu$_3$O$_{7-\delta}$ 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 $\rho_s(T)$, which follows the critical behavior characteristic of single crystal YBa$_2$Cu$_3$O$_{7-\delta}$ as $T$ approaches $T_c$. In 7 u.c. thick films, $\rho_s(T)$ declines steeply upon approaching $T_c$, 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.