Discovery of an Excited Pair State in Superfluid 3He
J.P. Davis, J. Pollanen, H. Choi, J.A. Sauls and W.P. Halperin
- Address: Department of Physics & Astronomy, Northwestern University, Evanston, IL 60208
- Date: January 9, 2008
Nature Physics 4, 571-575 (2008)
Order parameter collective modes are the fingerprint of a condensed phase. The spectroscopy of these modes in superfluid
3He 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 3He-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 2Δ. Based on the selection rules for coupling of transverse sound
to a collective mode in 3He-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 3He.
- Comment: 5 pages, 3 figures plus Supplementary Information (3 pages, 1 figure).