Investigation of solid He4 by stimulated brillouin scattering

Abstract

Solid helium is generally referred to as a quantum crystal because its zero point energy is cornpaFable to the binding energy. Consequently one expects its behaviour to be quite different from ordinary cIassical solids. The amplitude of the zero point motion, which is nearly 30 per cent of the interatomic distance, should cause anharmonicities in the latti& potential to play an important role. The strength of anhamonicity is expressed by the Grüneisen parameter, which can be determined experimentally from the pressure dependence of specific heat or velocity of sound. The values obtained for solid helium, however, are not too different from those of ordinary substances.It is well know that direct evidence about the anharmonic behaviour of crystals can be obtained by ultrasonic measurements. In particular, the acoustic attenuation in ideal crystals is determined by phonon-phonon interactions. Until now such measurements have been carrid out only in liquid helium. For solid helium there exist rather rough estimations at 10 MHz which only give an upper limit for the sound absorptions. This lack of experimental data is not surprising, since the application of the conventional uItrasonic methods to solid helium gives rise to serious technical diculties : In particular, ultrasonic measurements are hampered by the large acoustical mismatch btween the transducer and the sample. In addition they require a high degree of crystal perfection over a large volume. Finally, because of the large acbustic anisotropy of solid helium the energy of a sound wave generally does not travel in the &section of the wave vecEor, i.e. perpendicular to the transducer, which futthw complicata the measurements.All these dificulties can be avoided or become unimportant if one uses stimulated Brillouin scattering (SBS) for the determination of the sound absorption. The crystal, has to be of a high quality only in a small volume, where the laser light is focused. Crystal irregularities iduence the propagation of light only slightly as the index of refraction is close to unity and nearly isotropic. Because of the short distance through which the phwons travel during measurement (< 50 μm) > effects are negligible.