Far-Infrared Studies of Spin-Peierls Materials in a Magnetic Field

We report the low-temperature far-infrared response of two prototypical spin-Peierls (SP) materials as a function of magnetic field in order to characterize the microscopic nature of the SP and high-field incommensurate phases. For the linear chain inorganic compound, GeCuO3, we observe that the B3u shearing mode is sensitive to the high-field phase boundary in the H−T phase diagram, and we find Zeeman splitting of the zone−center spin-Peierls gap within the dimerized phase. In contrast, for the organic molecular conductor MEM(TCNQ)2, neither the electron−phonon coupling modes nor the low-energy lattice modes were found to be sensitive to the high-field phase boundary. We attribute this difference to the extended vs molecular structure of the two solids as well as to the extent of spin localization.