The magnetic states of lightly hole-doped cuprates in the clean limit

We have performed extensive zero field muSR experiments on pure YBa2Cu3O6+y and diluted Y-rare-earth substituted Y0.92Eu0.08Ba2Cu3O6+y and Y0.925Nd0.075Ba2Cu3O6+y at light hole-doping. A common magnetic behavior is detected for all the three families, demonstrating negligible effects of the isovalent Y-substituent disorder. Two distinct regimes are identified, separated by a crossover, whose origin is attributed to the concurrent thermal activation of spin and charge degrees of freedom: a thermally activated and a re-entrant quenched-hole antiferromagnetic regime. The peculiar temperature and hole density dependence of the magnetic moment m(h,T) fit a model with a (spin) activation energy for the crossover between the two regimes throughout the entire investigated range. The magnetic moment is suppressed by a simple dilution mechanism both in the reentrant regime (0<h<0.056) and in the so-called Cluster Spin Glass state coexisting with superconductivity (0.056<h<0.08). We argue a common magnetic ground state for these two doping regions and dub it quenched-hole antiferromagnet. Conversely frustration prevails in the thermally activated regime, that vanishes at the same concentration where superconductivity emerges, suggesting the presence of a quantum critical point at h_c=0.056(2).