Critical behavior in the low-energy spin fluctuations in high-T-c superconductors

We study the spin fluctuations of pure and Zn-substituted La2 – x Sr x CuO4 using the muon spin relaxation 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 SR decreases with increasing x and vanishes above a critical doping x c 0.19. This value of x c coincides with the doping at which the normal state pseudogap extrapolates to zero. These results are discussed in terms of a quantum transition that separates the superconducting phase diagram of high-temperature superconductors into two distinct ground states