Spin gap, phase separation and d-wave pairing as revealed by electron spin-lattice relaxation in 123 and 124 YBaCuO systems doped with Gd3+

With an original modulation technique, the Gd3- electron spin-lattice relax ation has been investigated in normal and superconducting states of YBa2Cu3 O (123) and YBa2Cu4O8 (124) compounds doped with 1% Gd. In the 123 sample w ith x = 0.9 (T-c = 90 K), the T-1 behavior within 50 < T < 200 K reveals th e [1 - tanh(2)(Delta /2kT)]/T dependence typical of a spin gap opening with Delta approximate to 240 K. Below 50 K, the exponential slowing down of T, is limited by the Korringa-like behavior (T1T= const): the same Korringa-l ike law is found in the 123 sample with x = 0.59 (T-c = 56 K) within the to tal 4.2-200 K temperature range. This is interpreted in terms of microscopi c separation of the normal and superconducting phases allowing for the elec tron spin cross-relaxation between them. In the 124 sample (T-c = 82 K). th e Gd3- relaxation rate below 60 K is found to obey a power law T-n with an exponent n approximate to 3. Such a behavior (previously reported for nucle ar spin relaxation) is indicative of the d-wave superconducring pairing. Ad ditional paramagnetic centers characterized by relatively slow spin-lattice relaxation are found in both 123 and 124 systems. A well-pronounced change in the T, temperature dependence at TT* = 180-200 K is observed for these slowly relaxing centers as well as for the conventional, fast-relaxing Gd3 ions, suggesting microscopic phase separation and a change in the relaxati on mechanism due to electronic crossover related with the opening of the sp in gap. This hypothesis is supported by some "180 K anomalies" previously r eported by other authors.