Shedding light on collective modes in two-dimensional superconductors

The order parameter of a conventional single-band superconductor, being a complex number, lives in a two-dimensional configuration space. Accordingly, there are always at least two collective excitation modes associated with superconducting order: the condensate may fluctuate in amplitude or overall phase. In unconventional superconductors, finer order-parameter structure and/or subdominant pairing channels can provide additional directions for fluctuations to explore, yielding a richer spectrum of collective modes. I will discuss our theoretical study of collective mode spectroscopy in two-dimensional unconventional superconductors [1]. Using a simple toy model, I will show how AC capacitance measurements can detect Leggett and clapping modes in 2D superconductors. I will then discuss superconducting rhombohedral trilayer graphene (RTG) as an illustrative case study. I will show how certain proposed pairing mechanisms for RTG would yield an in-gap clapping mode, and how that mode could be directly probed in experiment. Our results show how in 2D superconductors, linear response to vertical electric fields can yield insight into the superconducting gap structure, and even into the nature of the underlying microscopic interactions responsible for pairing.

[1] B. A. Levitan, Y. Oreg, E. Berg, M. Rudner, and I. Iorsh. Linear spectroscopy of collective modes and the gap structure in two-dimensional superconductors. arXiv:2406.08706 (2024), to appear in Physical Review Research.