Cu NMR study of lightly doped La$_{2-x}$Sr$_{x}$CuO$_4$

TL;DR

This study uses Cu NMR to investigate how lightly doped La$_{2-x}$Sr$_{x}$CuO$_4$ transitions from an insulator to a superconductor, revealing nanoscale phase separation and a connectivity transition around 5% doping.

Contribution

It provides the first microscopic evidence of a transition from disconnected metallic islands to a granular metal in lightly doped LSCO, highlighting nanoscale phase separation.

Findings

Evidence for a low-temperature transition at x=5% doping.

Identification of nanoscale electronic phase separation.

Observation of a connectivity transition in the electronic system.

Abstract

We present a single crystal Cu NMR study of the cuprate superconductor La$_{2-x}$Sr$_{x}$CuO$_4$ with hole doping levels between $x=2$\% and $8$\%. Measurements with short spin echo times enable us to systematically study the local properties of the electronic spin system in the region of the phase diagram where the material evolves from the insulating antiferromagnetic parent phase to the superconducting state. We find evidence for qualitative changes as the Sr concentration increases through $x=5$\% in both NMR spectra and relaxation times, which we interpret as signatures of a low-temperature transition from a state with disconnected metallic islands to a granular metal with tunneling between the grains. These results provide microscopic insight into the physics of a doped charge-transfer insulator in the presence of quenched substitutional disorder, indicate that a connectivity transition occurs, and demonstrate that complex nanoscale electronic phase separation is ubiquitous in LSCO around the doping level where superconductivity first appears in the phase diagram.