High frequency ESR investigation on dynamical charge disproportionation and spin gap excitation in NaV2O5

A significant frequency dependence of the ESR line width is found in NaV2O5 between 34-100 K and the line width increases as the resonance frequency i s increased from 95 GHz to 760 GHz. The observed frequency dependence is qu alitatively explained in terms of the dynamical charge disproportionation. The present results show the essential role of the internal charge degree o f freedom in a V-O-V bond. We have also proposed the existence of the Dzyal oshinsky-Moriya interaction in the low temperature charge ordered phase con sidering the breaking of the selection rule of ESR realized as the direct o bservation of the spin gap excitation.