Low-frequency spins and the ground state in high-Tc cuprates

TL;DR

This study reveals that in high-Tc cuprates, low-frequency spin fluctuations coexist with superconductivity and are linked to a quantum transition at a critical doping level, affecting the superfluid density and ground state properties.

Contribution

It provides evidence for a quantum transition involving magnetic order that separates two distinct superconducting ground states in high-Tc cuprates.

Findings

Spin fluctuations coexist with superconductivity over a large doping range.

A critical doping point p_c=0.19 marks the disappearance of spin fluctuations and the pseudogap.

Superfluid density shows an abrupt change at p_c, indicating a crossover in superconducting properties.

Abstract

We study the spin fluctuations of pure and Zn-substituted high temperature superconductors (HTS) using the muon spin relaxation (muSR) technique. Superconductivity is found to coexist with low frequency spin fluctuations over a large region of the superconducting phase diagram. The characteristic temperature of spin fluctuations detected by muSR decreases with increasing doping and vanishes above a critical doping pc=0.19 where the normal state pseudogap vanishes. Furthermore, it is at pc that our penetration depth studies show an abrupt change in the doping dependence of the superfluid density. For p>pc the absolute value of the superfluid density is large and nearly independent of carrier concentration whereas for p<pc it drops rapidly, signalling a crossover to weak superconductivity. These results are discussed in terms of a quantum transition involving fluctuating short-range magnetic order that separates the superconducting phase diagram of HTS into two distinct ground states.