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

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 p(c) approximate to 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, p(s.) For p > p(c) the absolute value of p(s) is large and nearly independent of carrier concentration whereas for p < p(c) 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. (C) 2003 Elsevier Science Ltd. All rights reserved.