89Y and 63Cu NMR-NQR and susceptibility study of underdoped superconducting Y1-xCaxBa2Cu3O6.1

Y-89 NMR relaxation has been used to study the spin freezing process in Y1-xCaxBa2Cu3O6.1. Substituting Ca2+ for Y3+ in the parent antiferromagnetic YBa2Cu3O6.1 leads the system in the underdoped superconducting phase. In the normal state of the sample at x=0.15 the spin-lattice relaxation rate, which is not affected by the antiferromagnetic correlation of the Cu2+ magnetic moments, still evidences a decrease of (1/T1T) on cooling, indicative of a pseudo-gap opening in the density of states around the Fermi energy. The mast relevant result is found in the superconducting phase, where the recovery of the Y-89 NMR signal displays two distinct laws, pointing out different relaxation mechanisms without common spin temperature. One exponential recovery law yields a decay rate which decreases on cooling, as expected in the superconducting phase. The second relaxation process turns out to be described by a stretched exponential, with fast relaxation rates, strongly increasing on cooling, with a characteristic spin fluctuations frequency reaching the 20MHz range at about 8 K. This observation is interpreted as the direct experimental evidence of the coexistence, at mesoscopic level, of superconductive and spin-glass like phases, the spin freezing process being described by the fast rate of the nuclear relaxation.