Gradual emergence of superconductivity in underdoped LSCO

TL;DR

This study uses neutron scattering to investigate how superconductivity gradually develops in underdoped LSCO, revealing a competition between magnetic fluctuations and superconductivity and showing a surprising magnetic field response.

Contribution

It provides the first direct evidence of the gradual emergence of superconductivity and its competition with magnetic fluctuations in underdoped LSCO.

Findings

Superconductivity appears gradually with doping in LSCO.

Magnetic fluctuations are suppressed by magnetic fields across dopings.

Two-dimensional superconductivity is weak at x=0.05.

Abstract

We present triple-axis neutron scattering studies of low-energy magnetic fluctuations in strongly underdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ with $x=0.05$, $0.06$ and $0.07$, providing quantitative evidence for a direct competition between these fluctuations and superconductivity. At dopings $x=0.06$ and $x=0.07$, three-dimensional superconductivity is found, while only a very weak signature of two-dimensional superconductivity residing in the CuO$_2$ planes is detectable for $x=0.05$. We find a surprising suppression of the low-energy fluctuations by an external magnetic field at all three dopings. This implies that the response of two-dimensional superconductivity to a magnetic field is similar to that of a bulk superconductor. Our results provide direct evidence of a very gradual onset of superconductivity in cuprates.