超流~ 3 He中激发对态的发现
Discovery of an Excited Pair State in Superfluid 3He
摘要：
序参量集合模是凝聚相的指纹。超流$^3$ He和非常规超导体中这些模式的光谱可以提供关于凝聚体对称性以及负责基态和激发能的微观配对机制的关键信息。我们报道了在超流$^3$ He-B中发现的一种新的集体模，我们将其确定为库珀对的激发束缚态。我们在一个对横向声速变化非常敏感的声腔中使用干涉测量法。我们的声速和模式频率的测量，连同声双折射的观察表明，这种新的模式是弱约束与激发能的1%内的对破断边缘的$2Δ$。根据$^3$ He-B中横向声与集体模耦合的选择定则，结合集体模频率附近的声双折射观测，我们推断新的集体模很可能是自旋三重态（S=1），$f$波对激子（L=3），总角动量J=4. J=4的激子对的存在表明在液态$^3$ He中存在一种吸引的次主导的$f$波对相互作用。
英文摘要：
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 $2Δ$. 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.
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