Convective instability in rotating liquid 3He-4He mixtures

Abstract

Thermal convection is investigated experimentally in a dilute liquid mixture of 3He in 4He at four temperatures between 20 and 100 mK above the superfluid transition temperature, chosen for their proximity to the codimension-two and hydrodynamic tricritical points. Two experimental cells of aspect ratio 2.76 and 1.00 were used. For the cell with the higher aspect ratio, two convective transitions at each of the four temperatures were observed above a critical angular velocity, and only one observed below. At temperatures lower than that of the hydrodynamic tricritical point the transition with the lower critical Rayleigh number is hysteretic for all angular velocities; above this temperature hysteresis is absent. The critical Rayleigh numbers are compared with theoretical predictions that take into account the existence of convection modes with azimuthal angular dependence. In the case of the cell with the smaller aspect ratio thermal relaxation oscillations were observed when heating from below. Convective thresholds were again observed but their critical Rayleigh numbers are almost independent of angular velocity. Some suggestions are advanced for this unexpected behaviour.