Spectroscopic evidence for unconventional superconductivity in twisted bilayer graphene*

The pairing mechanism of superconductivity in magic-angle twisted bilayer graphene (MATBG)  is one of the most interesting questions in the study of moiré materials. Whether MATBG is a conventional BCS superconductor or an unconventional one, such as those found in other correlated materials remains unknown. We tune the carrier density of MATBG between different ground states, including the superconducting state well below its transition temperature (T_c), and probe the Bogoliubov quasiparticle excitation spectrum using scanning tunneling microscopy (STM), thereby providing key information on the nature of MATBG’s pairing state and its symmetry. We observed several spectroscopic features that support the unconventional pairing in MATBG. Our spectroscopic measurements show that the ratio of the tunneling gap to T_c far exceeds (2Δ/k_BT_c ~ 25) that of the mean-field BCS ratio. We find a V-shaped gap consistent with that of a nodal superconductor. Suppressing the zero resistance states with a magnetic field or raising the temperature above Tc, the tunneling spectra show the presence of a pseudogap phase, similar to those found in other unconventional superconductors. These spectroscopic observations are irreconcilable with a conventional BCS mechanism of superconductivity [1].