Contributions of spin-lattice relaxation to the echo decay of planar Cu in high-temperature superconductors

Measurement of T-2G, the Gaussian component of the spin-echo envelope of planar Cu nuclei in high-temperature superconductors, gives important information about the real part of the Cu electron spin susceptibility. In the traditional picture of the planar Cu echo decay, the internuclear coupling is assumed to remain static with respect to spin-lattice relaxation and mutual exchange fluctuations. In some circumstances, however, this assumption breaks down. We calculate the internuclear corrections arising from spin-lattice relaxation to the conventional theory of T-2G and show that T-2G can be easily corrected for these effects. We argue that mutual exchanges due to the perpendicular indirect couplings are suppressed in these materials. For YBa2Cu4O8, we iind a correction on the order of 10% in T(2)G and using the corrected values we find that the isotope ratio T-63(2G)/T-65(2G) agrees with theory. (C) 1998 Academic Press.