Spectroscopic Evidence for Correlated Chern Insulators and Unconventional Superconductivity in Magic-Angle Graphene*

The flat electronic bands of magic-angle twisted bilayer graphene (MATBG) are highly conducive to producing a wide variety of correlated ground states, including correlated insulating, superconducting, and magnetic phases [1]. In this talk, I will discuss a set of experiments that leverage the spectroscopic resolution of the scanning tunneling microscope (STM) to probe these correlated phases. First, I will introduce a local spectroscopic technique using the STM that we use to detect a sequence of topological insulators in MATBG that break time-reversal symmetry spontaneously [2]. Second, I will discuss a series of experiments that uncover key spectroscopic signatures of unconventional superconductivity in MATBG, which strongly parallel phenomena observed in many known correlated superconductors [3]. Although transport studies of superconductivity in MATBG have suggested an unconventional pairing mechanism, conclusive evidence for any mechanism beyond the Bardeen-Cooper-Schrieffer (BCS) paradigm requires spectroscopic resolution. Tunnelling spectroscopy shows that the spectra in MATBG below the critical transition temperature T_c are inconsistent with those of a conventional s-wave superconductor, but rather resemble those of a nodal superconductor with an anisotropic pairing mechanism. Combining these tunneling measurements with point-contact spectroscopy measurements not only allows us to identify a large discrepancy between the tunnelling gap and the gap extracted from Andreev reflection spectroscopy, but also allows us to disentangle superconductivity from a precursor pseudogap phase that persists well-above the critical temperature and magnetic field of MATBG. In total, these findings provide a preponderance of evidence for a non-BCS mechanism for superconductivity in MATBG.