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Discovery of an excited pair state in superfluid 3He

Nature Physics volume 4, pages 571–575 (2008)Cite this article

Abstract

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. Here, we use interferometry within an acoustic cavity—which is very sensitive to changes in the velocity of transverse sound—to reveal a new collective mode in the B phase of superfluid 3He (3He-B). We identify the mode as an excited bound state of Cooper pairs, which is weakly bound with an excitation energy within 1% of the pair-breaking edge. On the basis of 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 (orbital momentum L=3) with total angular momentum, J=4. The existence of a pair exciton with J=4 suggests an attractive, subdominant, f-wave pairing interaction in liquid 3He.

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Figure 1: Energy levels of the collective modes, J±, of superfluid 3He-B that have been observed or predicted to couple to either longitudinal or transverse sound and their Zeeman splitting in a magnetic field in the limit of weak quasiparticle interactions.
Figure 2: Acoustic cavity response to pressure as a function of energy normalized to the gap energy, at 88 MHz and ≈550 μK in zero magnetic field.
Figure 3: Pressure and magnetic-field dependence of the 2Δ mode.

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Acknowledgements

We acknowledge support from the National Science Foundation, DMR-0703656 and thank W. J. Gannon, M. J. Graf, Y. Lee, M. W. Meisel and B. Reddy for useful discussions.

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Authors and Affiliations

  1. Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, USA

    J. P. Davis, J. Pollanen, H. Choi, J. A. Sauls & W. P. Halperin

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Correspondence to W. P. Halperin.

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Davis, J., Pollanen, J., Choi, H. et al. Discovery of an excited pair state in superfluid 3He. Nature Phys 4, 571–575 (2008). https://doi.org/10.1038/nphys969

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