Ultrasonic Studies of the SDW transition in (TMTSF)2PF6

Acoustic measurements of (TMTSF)2PF6 have been performed in the temperature range 2 K < T < 40 K. The behavior of the sound velocity and attenuation observed at the transition to the SDW ground state 12 K depends on the particular mode propagating in the crystal: Mode I shows a jump of the sound velocity at tsdw; for mode II the velocity increases with decreasing temperature as the square of the order parameter; a third type exhibits only a kink in the temperature dependence of the sound velocity at tsdw. We apply various models and discuss the coupling of the SDW to the lattice.