Spin susceptibility of neutron-irradiation-disordered YBa2Cu3O6.9 in the normal and superconducting states

Abstract

The spin-spin relaxation rate 63T 2 −1 of 63Cu nuclei in CuO2 layers is measured in the normal and superconducting states of the compound YBa2Cu3O6.9 (T c onset =94 K) subjected to radiation-induced disordering by a fast-neutron flux Φ to T c onset =68 K (Φ=7×1018 cm−2) and T c onset <4 K (Φ=12×1018 cm−2). It is found that as the structural disorder increases, the contribution of the indirect spin-spin interaction 63T 2G −1 , which is related to the value of the spin susceptibility at the boundary of the Brillouin zone of the copper planes χs(q={π/a; π/a}), decreases slightly at the transition to the superconducting state for the initial sample and remains unchanged for the weakly disordered sample. This behavior of the short-wavelength contribution to the spin susceptibility attests to the stability of the x2−y2 symmetry of the energy gap against structural disorder, in accordance with proposed theoretical models of Cooper pairing for high-Tc cuprates.