Identification of ³He-A by ultrasound experiments

On the basis of available experimental data Gould [Physica B 178 (1992) 266] has suggested that ³He-A, conventionally identified as the axial phase, may actually be an axi-planar phase. Some experiments have been done to clarify the situation [see, for example, T.R. Mullins et al., Phys. Rev. Lett. 72 (1994) 4177] but the problem is still open. While we take no position on the interpretation of the data, we note that the collective mode structure of the two phases differ and that appropriate measurements could resolve the issue. We investigate this problem within a simple, time-dependent Ginzburg-Landau (GL) model as well as by studying the second variation of the free energy functional. Both methods show that the spectrum in the axial phase is degenerate, while it is split in the axi-planar phase. This fact may serve as a sensitive test of the existence of the latter.