Anisotropic gap and quasiparticle-damping effects on NMR measurements of high-temperature superconductors

Gap anisotropy strongly affects the temperature dependence of nuclear-spin–lattice relaxation in the superconducting state. A model is presented which contains a simple form for the gap anisotropy expected of a weakly coupled system of superconducting layers. Phonon-induced quasiparticle damping is also included and is shown to contribute significantly to the relaxation rate. This is in contrast to the case of conventional superconductivity where phonons play only a minor role. The presence of both gap anisotropy and phonon quasiparticle damping eliminate the usual BCS ${\mathit{T}}_1^{\mathrm{-}1}$ enhancement below ${\mathit{T}}_{\mathit{c}}$, in agreement with experiments on ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$.