Discovery of a New Excited Pair State in Superfluid $^{3}$He

In superfluid $^{3}$He, the order parameter collective modes correspond to excited states of the $^{3}$He Cooper pairs and are classified by their total angular momentum, \textit{J = L + S}. Many of these modes with $J \le $ 2 have been experimentally observed through longitudinal sound measurements or NMR. As a result of coupling to the collective mode with $J $= 2 and $m_{J }=\pm $1 there is an enhanced restoring force for transverse sound in superfluid $^{3}$He-B. Previously, we have used the interference of transverse sound waves to study this collective mode. Recently we have discovered a new coupling to transverse sound near the pair-breaking threshold with the classic signatures of a collective mode. Application of a magnetic field results in circular acoustic birefringence and a new acoustic Faraday effect, from which we extract the corresponding Verdet constant. Selection rules for the coupling to transverse sound and acoustic birefringence require this mode to have $J \ge $ 4, suggesting that this mode is most likely the $J $= 4 ($m_{J }=\pm $1) mode resulting from an attractive $f$-wave pairing interaction in this $p$-wave superfluid.