Low field63Cu NMR study of indirect nuclear spin-spin coupling in YBa2Cu3O6.9. A new approach to probe strong correlation effects in high-Tc superconductors

The authors carried out a low field (6.3 kOe) NMR measurement of the Gaussian component of63Cu nuclear spin-spin relaxation rate 1/T2G at the planar Cu site in a high-Tc superconductor YBa2Cu3O6.9 (Tc=92 K). They demonstrate that the results provide quantitative information concerning the static spin susceptibility χ′(q) at non-zero wave vector q. The detailed analysis of the data assuming a model susceptibility peaked at the corner of first Brillouin zone q=Q (Q=(π/a, θ/a),a: lattice constant) shows that χ′(q) satisfies a Curie-Weiss law around q=Q. Stoner enhancement over the calculated Lindhard function at q=Q is estimated to be of the order of ∼10–20. They also demonstrate that combined analysis of 1/T1 and 1/T2G allows one to judge whether anomalous shift of the low frequency spectral weight of χ″(Q,ω) to higher frequencies (i.e.pseudo gap) exists or not. Application of the method to YBa2Cu3O6.9 revealed no appreciable pseudo gap in the material.