FAR-INFRARED STUDIES OF SPIN-PEIERLS MATERIALS IN A MAGNETIC-FIELD

We report the low-temperature far-infrared response of two prototypica l spin-Peierls (SP) materials as a function of magnetic field in order to characterize the microscopic nature of the SP and high-field incom mensurate phases. For the linear chain inorganic compound, GeCuO3, we observe that the B-3u shearing mode is sensitive to the high-field pha se boundary in the H-T phase diagram, and we find Zeeman splitting of the zone-center spin-Peierls gap within the dimerized phase. In contra st, for the organic molecular conductor MEM(TCNQ)(2), neither the elec tron-phonon coupling modes nor the low-energy lattice modes were found to be sensitive to the high-field phase boundary. We attribute this d ifference to the extended vs molecular structure of the two solids as well as to the extent of spin localization.