Sound velocity and attenuation in nuclear-ordered U2D2 solid

Abstract We studied sound properties of nuclear-ordered U2D2 solid 3 He crystal with a single magnetic domain along the melting curve. We measured temperature dependences of sound velocities and attenuations for a longitudinal and two transverse sounds between the sound frequency f , 3 and 50 MHz below the nuclear-ordered transition temperature T N . The sound velocity v changes as Δ v / v = a ( T / T N ) 4 and the attenuation α approximately as Δ α = cf 3 ( T / T N ) 9 . The coefficients a and c depend on crystal orientations and sound modes. Assuming the tetragonal symmetry of U2D2 crystal, we determined six independent elastic constants of nuclear spin system from Δ v / v .