Critical Behavior in the Low-Energy Spin Fluctuations in High- Tc Superconductors.

We study the spin fluctuations of pure and Zn-substituted La_{2 − x}Sr _xCuO₄ 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 μSR 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. [ABSTRACT FROM AUTHOR]