  The application of pressure to elemental bismuth reduces its
conduction-valence band overlap, and results in a semimetal-semiconductor
(SMSC) transition around 25 kbar. This transition is nominally of the
topological "Lifshitz" Fermi surface variety, but there are open questions
about the role of interactions at low charge densities. Using a novel pressure
cell with optical access, we have performed an extensive study of bismuth's
infrared conductivity under pressure. In contrast to the expected pure band
behavior we find signatures of enhanced interaction effects, including strongly
coupled charge-plasmon (plasmaron) features and a plasma frequency that remains
finite up to the transition. These effect are inconsistent with a pure Lifshitz
bandlike transition. We postulate that interactions play a central role in
driving the transition.
