The decay rate of critical fluctuations in3He-4He mixtures near the gas-liquid critical point

Abstract

We have measured the critical light scattering intensity and the Rayleigh line shape for3He and for3He-4He mixtures with compositionX(3He)=0.95, 0.79, and 0.63 along their respective critical isochores near the plait point. The experimental linewidth of3He is compared with the calculated one from heat conductivity and equation of state measurements, and satisfactory agreement is obtained. For mixtures, gravity effects in our cell of finite height prevent us from reaching the critical point along a path at strictly constant composition and density. HenceTc cannot be determined directly. Using the prediction that the scattered light intensity in the mixtures has the same diverging behavior as for the pure fluid, we determine the reduced temperaturet≡[T − Tc(X)]/Tc from the intensity. The measured Rayleigh line shape can be expressed by a single decay rate Γ as a function oft for a given scattering angle of the light beam. Our experiments show that Γ in the mixtures is only weakly dependent on composition. Our analysis leads to the determination of the mass diffusion coefficientD, which is found to be nearly independent of composition and nearly equal to the thermal diffusivityDT measured for3He. The results are discussed in the light of the predictions from mode coupling theory.