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.
@article{arxiv.cond-mat/0210363,
title = {Low-frequency spins and the ground state in high-Tc cuprates},
author = {C. Panagopoulos and J. L. Tallon and B. D. Rainford and J. R. Cooper and C. A Scott and T. Xiang},
journal= {arXiv preprint arXiv:cond-mat/0210363},
year = {2009}
}