ABSTRACTWe have studied the elastic properties of the Bechgaard salts (TMTSF)2PF6 and (TMTSF)2C104 through their Spin Density Wave (SDW) and anion ordering transitions. Using a vibrating reed technique to measure the sound velocity and attenuation, we observed a large softening of sound velocity immediately above the SDW transition temperature, 12K, of (TMTSF)2PF6 followed by a large hardening below the transition. The softening results from a strong dependence of Tc on stress in the most conducting direction, while the hardening results from the loss of screening from the conduction electrons and yields a value for the long wavelength electron-phonon coupling constant. The anion ordering transition at 24K in (TMTSF)2ClO4 is marked by a hardening of the lattice that is considerably larger than expected from the lattice constant changes. Our most interesting finding, however, is the observation of a series of steps in the sound velocity as the Field Induced Spin Density Wave (FISDW) transitions are crossed by increasing the magnetic field at low temperatures. The fact that the sound velocity steps are small at the beginning of the cascade of transitions and increase with field, even as extrapolated to low temperature, is an unexpected result from present theory and may indicate a more direct coupling of the lattice stiffness to the SDW order parameter.
