Linear spectroscopy of collective modes and the gap structure in two-dimensional superconductors

TL;DR

This paper demonstrates that linear spectroscopy of 2D superconductors can detect collective modes like Leggett and clapping modes, providing insights into the gap structure and pairing mechanisms, especially in materials like rhombohedral trilayer graphene.

Contribution

It introduces a simple model showing how AC gating response can reveal collective modes and details of the gap structure in multi-band 2D superconductors.

Findings

Detection of Leggett and clapping modes via optical response.

Trigonal warping facilitates clapping mode detection.

In-gap modes suggest unconventional pairing mechanisms.

Abstract

We consider optical response in multi-band, multi-layer two-dimensional superconductors. Within a simple model, we show that linear response to AC gating can detect collective modes of the condensate, such as Leggett and clapping modes. We show how trigonal warping of the superconducting order parameter can help facilitate detection of clapping modes. Taking rhombohedral trilayer graphene as an example, we consider several possible pairing mechanisms and show that all-electronic mechanisms may produce in-gap clapping modes. These modes, if present, should be detectable in the absorption of microwaves applied via the gate electrodes, which are necessary to enable superconductivity in this and many other settings; their detection would constitute strong evidence for unconventional pairing. Last, we show that absorption at frequencies above the superconducting gap $2 |\Delta|$ also contains a wealth of information about the gap structure. Our results suggest that linear spectroscopy can be a powerful tool for the characterization of unconventional two-dimensional superconductors.