Bulk superconductivity in undoped T'-La$_{1.9}$Y$_{0.1}$CuO$_4$ probed by muon spin rotation

TL;DR

This study provides direct evidence of bulk superconductivity in undoped T'-La$_{1.9}$Y$_{0.1}$CuO$_4$ thin films through muon spin rotation, challenging the traditional view of the parent compound as a Mott insulator.

Contribution

It demonstrates bulk superconductivity in undoped T'-La$_{1.9}$Y$_{0.1}$CuO$_4$ using depth-resolved muon spin rotation, revealing the true ground state as a strongly correlated metal.

Findings

Confirmed Meissner effect in undoped films

Measured penetration depth of 466 nm

Parent compound is a strongly correlated metal

Abstract

The Meissner effect has been directly demonstrated by depth-resolved muon spin rotation measurements in high-quality thin films of the T'-structured cuprate, T'-La$_{1.9}$Y$_{0.1}$CuO$_4$, to confirm bulk superconductivity ($T_c\simeq21$ K) in its {\sl undoped} state. The gradual expelling of an external magnetic field is observed over a depth range of $\sim$100 nm in films with a thickness of 275(15) nm, from which the penetration depth is deduced to be 466(22) nm. Based on this result, we argue that the true ground state of the "parent" compound of the $n$-type cuprates is not a Mott insulator but a strongly correlated metal with colossal sensitivity to apical oxygen impurities.