SPIN SUSCEPTIBILITY OF NEUTRON-IRRADIATION-DISORDERED YBA2CU3O6.9 IN THE NORMAL AND SUPERCONDUCTING STATES

The spin-spin relaxation rate T-63(2)-1 Of Cu-63 nuclei in CuO2 layers is measured in the normal and superconducting states of the compound YBa2Cu3O6.9 (T-c(inset) = 94 K) subjected to radiation-induced disorde ring by a fast-neutron flux Phi to T-c(onset) = 68 K (Phi = 7X10(18) c m(-2)) and T-c(onset) < 4 K (Phi = 12X10(18) cm(-2)). It is found that as the structural disorder increases, the contribution of the indirec t spin-spin interaction T-63(2G)-1, which is related to the value of t he spin susceptibility at the boundary of the Brillouin zone of the co pper planes chi(s)(q = {pi/a; pi/a}), decreases slightly at the transi tion to the superconducting state for the initial sample and remains u nchanged for the weakly disordered sample. This behavior of the short- wavelength contribution to the spin susceptibility attests to the stab ility of the x(2)-y(2) symmetry of the energy gap against structural d isorder, in accordance with proposed theoretical models of Cooper pair ing for high-T-c cuprates. (C) 1998 American Institute of Physics.