Overview

We use transverse sound spectroscopy to probe excited states of the Cooper pairs in the B-phase of superfluid 3He. In particular we study those excited states with non-zero energy at zero momentum, the so called gapped, or exciton, modes. This includes precise measurements of the imaginary squashing mode energies as well as its Zeeman splitting in a magnetic field. The latter is measured through the acoustic Faraday effect, a phenomenon of acoustic birefringence induced by Zeeman splitting. Additionally, we have made measurements of the absolute velocities of transverse sound and as a result have discovered a previously unobserved collective mode near the pair-breaking threshold, 2Delta, which also exhibits acoustic birefringence. We measure its coupling strength to transverse sound and Verdet constant. Using the selection rules which govern the coupling to transverse sound, we argue that this is the so called J=4 exciton mode, which results from sub-dominant f-wave pairing interactions in this p-wave superfluid. Additionally, we have made measurements of the transverse acoustic response of 3He confined within 98% porous aerogel to explore the possibility of stabilizing new superfluid phases of 3He due to anisotropic quasiparticle scattering. We find global axial anisotropy introduced via preferential radial shrinkage produces signatures of phase transitions on warming from the B-like phase to the A-like phase, suggestive of a region of stable A-like phase, while global axial anisotropy introduced via axial compression affects only the metastable supercooling of the A-like phase, possibly by inhibiting the nucleation of the B-like phase in aerogel. In the appendices we discuss: the implications of our 2Delta-mode measurements, combined with longitudinal sound measurements, on the energy of pair-breaking; possible observation of the J=1- collective mode; measurements of the real squashing mode using longitudinal sound; a new and unusual magneto-acoustic effect; surface Andreev bound states affecting the transverse acoustic impedance; and finally, some ideas for future experiments with acoustic techniques.

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9781243505842

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