Discovery of an Excited Pair State in Superfluid 3He

TL;DR

This paper reports the discovery of a new excited pair state in superfluid helium-3, identified as a spin-triplet f-wave pair exciton with implications for understanding pairing interactions.

Contribution

The study identifies a previously unknown collective mode in superfluid helium-3, revealing an excited bound state of Cooper pairs with specific angular momentum characteristics.

Findings

Discovery of a new collective mode in superfluid $^3$He-B

Mode identified as an excited bound state of Cooper pairs

Indicates an attractive f-wave pairing interaction

Abstract

Order parameter collective modes are the fingerprint of a condensed phase. The spectroscopy of these modes in superfluid $^3$He and unconventional superconductors can provide key information on the symmetry of the condensate as well as the microscopic pairing mechanism responsible for the ground state and excitation energies. We report the discovery of a new collective mode in superfluid $^3$He-B which we identify as an excited bound state of Cooper pairs. We use interferometry within an acoustic cavity that is very sensitive to changes in the velocity of transverse sound. Our measurements of sound velocity and mode frequency, together with the observation of acoustic birefringence indicate that this new mode is weakly bound with an excitation energy within 1% of the pair-breaking edge of $2\Delta$. Based on the selection rules for coupling of transverse sound to a collective mode in $^3$He-B, combined with the observation of acoustic birefringence near the collective mode frequency, we infer that the new mode is most likely a spin-triplet (S=1), $f$-wave pair exciton (L=3) with total angular momentum, J=4. The existence of a pair exciton with J=4 suggests an attractive, sub-dominant, $f$-wave pairing interaction in liquid $^3$He.