The thermodynamics of the A–B transition in superfluid 3He as a probe of the A-phase gap node geometry

We have used a magnetic field gradient to stabilise the phase boundary between the A- and B-phases of superfluid He-3 inside a quasiparticle black body radiator down to 146 mu K; The radiator technique enables us to measure very accurately the density of quasiparticle excitations and therefore the temperature of the B-phase inside the radiator. As we ramp the held through the transition we can observe the cooling (warming) effect when the volume of A-phase is increased (decreased). From these measurements we deduce the temperature dependence of the latent heat, which provides the lowest temperature verification yet of the existence of nodes in the A-phase order parameter.