Science Daily October 20, 2021
An international team of researchers (USA – Princeton University, Japan) combined tunnelling and Andreev reflection spectroscopy with the scanning tunnelling microscope to observe several key experimental signatures for unconventional superconductivity in magic angle twisted bilayer graphene (MATBG). They showed that the tunnelling spectra below the transition temperature Tc are inconsistent with those of a conventional s-wave superconductor, but rather resemble those of a nodal superconductor with an anisotropic pairing mechanism. They observed a large discrepancy between the tunnelling gap which far exceeds the mean-field BCS ratio, and the gap extracted from Andreev reflection spectroscopy. The tunnelling gap persisted even when superconductivity was suppressed, indicating its emergence from a pseudo gap phase. The pseudo gap and superconductivity were both absent when MATBG is aligned with hexagonal boron nitride (hBN). According to the researchers these findings and other observations reported here provided a preponderance of evidence for a non-BCS mechanism for superconductivity in MATBG…read more. TECHNICAL ARTICLE