Cu-63,Cu-65 NMR investigation of CuGeO3 single crystals: The uniform and the dimerized spin-Peierls phase

We report a Cu NMR study performed under high magnetic field on single crystals of the inorganic spin-Peierls CuGeO3 compound, in its uniform and dimerized phases. The temperature(T) dependence of the magnetic hyperfine shift, i.e., the local static spill susceptibility, is found to scale the macroscopic susceptibility . This allows the determination of the hyperfine coupling tensor, which can be well accounted for by the on-site coupling associated with the localized electronic spin of a Cu++ ion in an approximately axially symmetric environment. The nuclear spin-lattice relaxation rate (T-1(-1)) in the dimerized phase is found to be activated for temperature below similar to T-SP/2. The magnetic-field dependence of the activation energy is in good agreement with that of the energy gap determined by neutron inelastic scattering. In the uniform phase, neither the magnitude of T-1(-1) (an order of magnitude smaller) nor its T dependence (approximately linear in T instead of being nearly constant) correspond to the theoretical predictions for a simple S=1/2, one-dimensional Heisenberg antiferromagnet, unless we admit for a filtering of the antiferromagnetic fluctuations, due to a small supertransferred hyperfine coupling. As expected, we found no magnetic-field dependence of T-1(T greater than or equal to 1.5 T-SP) in the range 8.9-14.9 T.