Low Field (63)Cu NMR Study of Indirect Nuclear Spin-Spin Coupling in YBa(2)Cu(3)O(6.9). A New Approach to Probe Strong Correlation Effects in High-T(c) Superconductors

The authors carried out a low field (6.3 kOe) NMR measurement of the Gaussian component of (63)Cu nuclear spin-spin relaxation rate 1/T(2G) at the planar Cu site in a high-T(c) superconductor YBa(2)Cu(3)O(6.9) (T(c) = 92 K). They demonstrate that the results provide quantitative information concerning the static spin susceptibility chi' (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 = (pi/a, pi/a), a: lattice constant) shows that chi'(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 similar to 0-20. They also demonstrate that combined analysis of 1/T(1) and 1/T(2G) allows one to judge whether anomalous shift of the low frequency spectral weight of chi ''(Q,omega) to higher frequencies (i.e. pseudo gap) exists or not. Application of the method to YBa(2)Cu(3)O(6.9) revealed no appreciable pseudo gap in the material.