63,65Cu NMR investigation ofCuGeO3ssingle crystals:mThe uniform and the dimerized spin-Peierls phase

Abstract

We report a Cu NMR study performed under high magnetic field on single crystals of the inorganic spin-Peierls ${\mathrm{CuGeO}}_{3}$ compound, in its uniform and dimerized phases. The temperature (T) dependence of the magnetic hyperfine shift, i.e., the local static spin 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 ${\mathrm{Cu}}^{++}$ ion in an approximately axially symmetric environment. The nuclear spin-lattice relaxation rate (${\mathrm{T}}_{1}^{\mathrm{\ensuremath{-}}1g}$) in the dimerized phase is found to be activated for temperature below \ensuremath{\sim}${\mathrm{T}}_{\mathrm{SP}}$ /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 ${\mathrm{T}}_{1}^{\mathrm{\ensuremath{-}}1g}$ (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 ${\mathrm{T}}_{1}$ (T\ensuremath{\gtrsim}1.5 ${\mathrm{T}}_{\mathrm{SP}}$ ) in the range 8.9--14.9 T.