Evidence for unconventional superconductivity in twisted trilayer graphene

Abstract

Magic-angle twisted trilayer graphene (MATTG) has emerged as a moiré material that exhibits strong electronic correlations and unconventional superconductivity 1,2 . However, local spectroscopic studies of this system are still lacking. Here we perform high-resolution scanning tunnelling microscopy and spectroscopy of MATTG that reveal extensive regions of atomic reconstruction favouring mirror-symmetric stacking. In these regions, we observe symmetry-breaking electronic transitions and doping-dependent band-structure deformations similar to those in magic-angle bilayers, as expected theoretically given the commonality of flat bands 3,4 . Most notably in a density window spanning two to three holes per moiré unit cell, the spectroscopic signatures of superconductivity are manifest as pronounced dips in the tunnelling conductance at the Fermi level accompanied by coherence peaks that become gradually suppressed at elevated temperatures and magnetic fields. The observed evolution of the conductance with doping is consistent with a gate-tunable transition from a gapped superconductor to a nodal superconductor, which is theoretically compatible with a sharp transition from a Bardeen-Cooper-Schrieffer superconductor to a Bose-Einstein-condensation superconductor with a nodal order parameter. Within this doping window, we also detect peak-dip-hump structures that suggest that superconductivity is driven by strong coupling to bosonic modes of MATTG. Our results will enable further understanding of superconductivity and correlated states in graphene-based moiré structures beyond twisted bilayers 5 .

Promotion of superconductivity in magic-angle graphene multilayers

“…S3), indicating that the corresponding Ginzburg–Landau coherence lengths ξGL (~10 to 30 nm) are substantially smaller than those observed in TBG and deviate from the weak-coupling prediction, ξGL≈ℏvF/πΔ with Δ≈1.76kBTc; this suggests a strong-coupling origin of superconductivity ( 3 , 4 ) [see section 2 in ( 7 )]. When combined with other recent experiments ( 5 , 14 , 15 ), these observations affirm the unconventional nature of superconductivity within the entire class of graphene moiré systems. Further, the measurements on three to five layers indicate that the addition of layers promotes superconductivity over a broader filling window despite the coexisting dispersive bands as well as the ostensibly increased vulnerability to disorder—both from the additional twist angles and from the sensitivity to the relative displacement between layers (fig.…”

Helical Trilayer Nanographenes with Tunable Interlayer Overlaps

“…Multilayer (e.g., bilayer and trilayer) graphenes are composed of stacked graphenic layers . These systems exhibit outstanding electronic and optical features not available with single-layer graphene, including bandgap opening, , bound excitons, , superconductivity, − and electrocatalytic activity . Despite various advances in the synthesis of multilayer graphene achieved in the last decade, most of these structures were bound to the substrate and only a few flakes could be obtained using layer-by-layer assembly .…”

Seeing moiré: convolutional network learning applied to twistronics

“…Since the discovery of strongly correlated electronic phases in twisted bilayer graphene (TBG) [1,2], moiré materials made of multiple layers [3][4][5][6][7][8] or 2D crystals other than graphene [9][10][11][12] have been under intense study. A primary goal in these efforts is to understand how the rich phase diagram of TBG can be realized in other materials, and if phases distinct from those which appear in graphene superlattices are possible.…”