Knight shift and spin-echo decay time of YBa2Cu4O8 and YBa2Cu3O7 in the superconducting state

Abstract

We report on calculations of the Knight shift and spin-echo decay time, T2G, in the superconducting state which are based on a nearly antiferromagnetic Fermi-liquid description of the spin-fluctuation spectrum, in which a single spin component is responsible for the dynamic spin behavior and the magnetic interaction between the planar quasiparticles brings the system close to antiferromagnetic behavior. The dynamic spin susceptibility is described by a random-phase-approximation-like expression, with a restoring force which is unchanged from the normal state, and an irreducible particle-hole susceptibility which reflects the dx2−y2 symmetry of the gap parameter and a quasiparticle Fermi surface consistent with photoemission experiments. We obtain a quantitative fit to the Knight shift results for YBa2Cu4O8 with a dx2−y2 strong coupling gap of maximum magnitude 2.8kBTc, and show that quantitative agreement with the recent measurements of T2G by Corey et al. [Phys. Rev. B 53, 5907 (1996)], may be obtained with this dx2−y2 pairing state and a quite substantial antiferromagnetic enhancement (∼140) of the static "band structure" spin susceptibility at wave vectors in the vicinity of Q=(π, π). We demonstrate that the experimental results of Corey et al. rule out an anisotropic s-wave state. Analogous calculations for the Knight shift of YBa2Cu3O7 suggest that the dx2−y2 strong coupling gap possesses a maximum magnitude, 2.6kBTc, and on this basis, we predict a reduction in T2G−1 of some 5% for the smaller antiferromagnetic enhancement (∼38) expected for this system.

Source-reported materials — not catalogue approval

Similar papers