Brillouin Scattering from Superfluid 3He-4He Mixtures
We are using Brillouin scattering to study thermally excited first sound. second sound, and concentration fluctuations in superfluid 3He-4He mixtures. The spectrum of the scattered light consists of five lines. One pair of lines arises from first sound and is shifted from the incident laser frequency by an amount v, Ku t /2n,where u t is the velocity of first sound and K is the scattering wave vector. The half-width at half-height of the lines 6v t is a measure of the amplitude attenuation coefficient a of first sound at that frequency: Sy,cqu t /21t. Another pair of lines results from scattering by second sound. The frequency shift of these lines is v2 Ku 2 /27r and their half-width is given by Sv2 a 2 u 2 /2n,where u2 and a2 are the velocity and attenuation coefficient of second sound, respectively. The fifth line is due to the presence in the mixture of concentration fluctuations at constant pressure. This line is not shifted in frequency relative to the incident light, indicating that concentration fluctuations do not propagate but merely decay away. The decay time constant is inversely proportional to the width of the unshifted line.
KeywordsAttenuation Coefficient Concentration Fluctuation Decay Time Constant Scatter Wave Vector Total Light Intensity
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