Relationship between the elastic properties and the order parameter of the spin-density wave phase of (TMTSF)2PF6

We present elastic constant data of the material (TMTSF)2PF6, with particular emphasis on the region about the phase transition, and at low temperatures. Specific heat, resistivity, and NMR measurements have indicated a very sharp transition at TSDW = 12.1K. Similarly sharp features have been observed in the temperature dependence of the Young's Modulus, followed by a continual stiffening below the transition. Comparison to recent NMR results indicate that the modulus varies as the square of the internal magnetic field below TSDW, thus constraining the form of the coupling of the order parameter to the strain field. There is no evidence for anomalies in the modulus below 4K, where it has been suggested that a transition to a commensurate state occurs