Acoustic phonon dispersion in single-crystal ?

The longitudinal elastic constants of 0953-8984/11/4/008/img7 single crystals were measured in the 0953-8984/11/4/008/img8 and 0953-8984/11/4/008/img9 directions near 0953-8984/11/4/008/img10 by an ultrasonic CW resonance technique in the MHz region. The comparison with previous low-frequency (f = 1 Hz) elastic measurements yields a huge acoustic phonon dispersion near 0953-8984/11/4/008/img11. We show that the dispersion is due to the crossover from isothermal to adiabatic behaviour.