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Published November 1, 2023 | Published
Journal Article Open

Detection of collective modes in unconventional superconductors using tunneling spectroscopy


We propose using tunneling spectroscopy with a superconducting electrode to probe the collective modes of unconventional superconductors. The modes are predicted to appear as peaks in dI/dV at voltages given by eV = ωᵢ/2 where ωᵢ denotes the mode frequencies. This may prove to be a powerful tool to investigate the pairing symmetry of unconventional superconductors. The peaks associated with the collective modes appear at fourth order in the single-particle tunneling-matrix element. At the same fourth order, multiple Andreev reflection (MAR) leads to peaks at voltage equal to the energy gaps, which, in BCS superconductors, coincides with the expected position of the amplitude (Higgs) mode. The peaks stemming from the collective modes of unconventional superconductors do not suffer from this coincidence. For scanning tunneling microscopes, we estimate that the magnitude of the collective mode contribution is smaller than the MAR contribution by the ratio of the energy gap to the Fermi energy. Moreover, there is no access to the mode dispersion. Conversely, for planar tunnel junctions the collective mode peak is expected to dominate over the MAR peak, and the mode dispersion can be measured. We discuss systems where the search for such collective modes is promising.

Copyright and License

© 2023 American Physical Society.


We thank Shuqiu Wang and Seamus Davis for sharing their data on UTe₂ which stimulated this investigation. We thank Nicholas Poniatowski, Leonid Glazman, and Iliya Esin for helpful discussions. P.A.L. acknowledges support by DOE (USA) office of Basic Sciences Grant No. DE-FG02-03ER46076. J.F.S. acknowledges support by the Air Force Office of Scientific Research under Award No. FA9550-22-1-0339.


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Additional details

November 9, 2023
November 9, 2023