FIR-spectroscopy of the FI-SDW excitation spectrum in a quasi-1D organic conductor

The anisotropic quasi one-dimensional conductor (TMTSF)2X (X = ClO4, PF6) exhibits a cascade of field-induced spin density wave (FI-SDW) phase transitions. In this contribution, we present a magneto-optical study of the far infrared excitation spectrum of (TMTSF)2ClO4 as a function of magnetic field and temperature covering the low field part of the phase diagram of this system. served transition temperatures of the various FI-SDW subphases follow closely the established phase diagram. Our results show that the magnitude of the SDW gap depends neither on magnetic field nor on temperature. In addition we find evidence for the existence of a collective mode excitation in the SDW state, indicating a relatively strong pinning to the lattice. Further indication of a large electron lattice coupling is given by the strong phonon renormalisation in the SDW state.