Homogeneous He-He solid solutions in the pre-separation region

Temperature dependences of the pressure P(T) in homogeneous solid ³He-⁴He mixtures have been studied experimentally in the wide range of concentrations (35.0%, 62.0%, 68.3%, 74.1%, 75.0%, and 89.3% ³He) above and below the equilibrium phase separation temperature T_s. An anomalous behaviour of the pressure in the vicinity of T_s is found for all investigated samples. With decreasing temperature, as T_s is approached, the pressure increases instead of expected reduction due to decrease in the phonon contribution (P_ph∼T⁴). Such an increase in pressure continues in the metastable region below T_s until the mixture separates. Theoretical interpretation of the observed effects based on a rigorous thermodynamic approach is proposed. The found experimentally pressure behaviour can be described only with the consistent account for fluctuations in the impurity subsystem which near T_s dominates over phonon contribution into the pressure. The obtained theoretical results are in good quantitative agreement with the experimental data. Density fluctuations in the concentrated mixtures give rise to a spontaneous formation of impuriton nano-clusters containing several hundreds of atoms. The fluctuation can be rigorously interpreted as a nucleus of the second phase in the pre-separated homogeneous solid mixture. The estimated size of the fluctuation nano-clusters agrees with the corresponding value for second phase nuclei obtained from the Lifshits-Slesov phenomenological theory of homogeneous nucleation.