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

Abstract

With an original modulation technique, the Gd 3 + electron spin-lattice relaxation has been investigated in normal and superconducting states of YBa 2 Cu306+x (123) and YBa2Cu4G8 (124) compounds doped with 1 % Gd. In the 123 sample with x = 0.9 (T = 90 K), the T, behavior within 50 < T < 200 K reveals the [1 -tanh 2 (zl/2kT)]/T dependence typical of a spin gap opening with A 240 K. Below 50 K, the exponential slowing down of T, is limited by the Korringa-like behavior (TI T = const); the same Korringa-like law is found in the 123 sample with x = 0.59 (T = 56 K) within the total 4.2-200 K temperature range. This is interpreted in terms of microscopic separation of the normal and superconducting phases allowing for the electron spin cross-relaxation between them.In the 124 sample (T = 82 K), the Gd3 + relaxation rate below 60 K is found to obey a power law T" with an exponent n 3. Such a behavior (previously reported for nuclear spin relaxation) is indicative of the d-wave superconducting pairing. Additional paramagnetic centers characterized by relatively slow spin-lattice relaxation are found in both 123 and 124 systems. A well-pronounced change in the Ti temperature dependence at TT 180-200 K is observed for these slowly relaxing centers as well as for the conventional, fast-relaxing Gd 3 + ions, suggesting microscopic phase separation and a change in the relaxation mechanism due to electronic crossover related with the opening of the spin gap. This hypothesis is supported by some "180 K anomalies" previously reported by other authors.Among the puzzles of high-Te physics, three problems attracted special attention but remained unsolved for years. First, the so-called spin gap (or pseudo gap) which opens in underdoped oxide superconductors well above T c ; second, a microscopical (or mesoscopical) phase separation of spins and charges in high-Tc materials; and, finally, the competition between s-and d-wave superconducting pairing below T. In our contribution, some information on these topics as obtained with the help of electron paramagnetic resonance (EPR) and, especially, electron spin-lattice relaxation will be presented.

ESR and longitudinal response in metals containing localized paramagnetic centers with spinS>1/2

“…Unlike this, the relaxation rate of the total longitudinal magnetization of the sspins is equal to TIc-~ = T~ l + T~ t and so does not depend on M. Thus, ir the partial spin-lattice relaxation rate of the s-spins can be neglected, the following relation takes place: -1 -1 T2eer/Tl~ff = S(S + 1)-M(M + 1). Such a relation between the relaxation rate T~~-~ measured by the modulation longitudinal method and the homogeneous parts of the linewidths of the resolved FS components was observed experimentally in the normal phase of Y0.99Gd0.01Ba2Cu306+x [14] and superconducting phase of Y0.99Gd0.0q [15]. In both cases, the localized (s-) spins were represented by Gd 3+ ions (S = 7/2).…”

“…It should be noted that a similar effect was considered in studying the NMR spectra with quadrupolar splitting in antiferromagnets [13]; in this case, all nucleuslike modes merge into a single NMR line under the influence of strong electron-nucleus coupling. The condition of such merging has the forro [13]: Icoql y Ico21,1cog'l, (14) I! where co", co~ now are the s-e-coupling coefficients in the Bloch-Hasegawa equations for S = 1/2.…”