Evidence for a generic novel quantum transition in high-Tc cuprates
  We study the low-energy spin fluctuations and superfluid density of a series of pure and Zn-substituted high-$T_c$ superconductors (HTS) using the muon spin relaxation and $ac$-susceptibility techniques. At a critical doping state, $p_c$, we find (i) simultaneous abrupt changes in the magnetic spectrum in the superconducting ground state and (ii) that the slowing down of spin fluctuations becomes singular at T=0. These results provide experimental evidence for a novel quantum transition that separates the superconducting phase diagram of HTS into two distinct ground states. 
C. Panagopoulos, J.L.Tallon, B.D. Rainford, T. Xiang, J.R. Cooper,   C.A. Scott