Join us for the Physics Condensed Matter Seminar at 2:30 pm
Title: Spectroscopic evidence for unconventional superconductivity in magic-angle twisted bilayer graphene
A question regarding the superconducting state in magic-angle twisted bilayer graphene (MATBG) remains whether the pairing mechanism is that of a conventional superconductor or the unconventional cases such like high-Tc superconductors, although various features of the superconducting phase have been reported in transport measurements. Tunneling measurement, otherwise, provides key information for understanding superconducting nature by studying Bogoliubov quasiparticle excitation spectra. In this talk, we report the unconventional spectroscopic features of the superconducting gap of the MATBG by using density-tuned scanning tunneling spectroscopy combined with Andreev reflection spectroscopy. We show 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. We observe a large discrepancy between the tunnelling gap ΔT, which far exceeds the mean-field BCS ratio (with 2ΔT/kBTc ~ 25), and the gap ΔAR extracted from Andreev reflection spectroscopy (2ΔAR/kBTc ~ 6). The tunnelling gap persists even when superconductivity is suppressed, indicating its emergence from a pseudogap phase. Moreover, the pseudogap and superconductivity are both absent when MATBG is aligned with hexagonal boron nitride. These findings and other observations reported here provide a preponderance of evidence for a non-BCS mechanism for superconductivity in MATBG [1].
[1] M. Oh et al., “Evidence for unconventional superconductivity in twisted bilayer graphene,” Nature, 600, 240-245 (2021)
Join Zoom Meeting https://berkeley.zoom.us/j/99982021890?pwd=OTMyd3cwMGp5Tk9QTW1vNk9EMDJNZz09
Meeting ID: 999 8202 1890 Passcode: 954439
Location: 3 Physics North
Speaker: Myungchul Oh
Affiliation: Princeton University