Tunable Phase Boundaries and Ultra-Strong Coupling Superconductivity in Mirror Symmetric Magic-Angle Trilayer Graphene

TL;DR

This paper introduces mirror symmetric magic-angle twisted trilayer graphene (MATTG) as a highly tunable moiré superconductor, revealing rich phase boundaries, suppression near van Hove singularities, and ultra-strong coupling characteristics, advancing understanding of strong coupling superconductivity.

Contribution

The study demonstrates a new moiré superconductor with extensive tunability, showing suppression of superconductivity near van Hove singularities and achieving ultra-strong coupling regimes, which was not previously observed in similar systems.

Findings

Superconductivity is connected to broken symmetry phases.

Superconducting phase is suppressed near van Hove singularities.

System reaches ultra-strong coupling regime with large $T_{BKT}/T_{F}$ ratios.

Abstract

Moir\'e superlattices have recently emerged as a novel platform where correlated physics and superconductivity can be studied with unprecedented tunability. Although correlated effects have been observed in several other moir\'e systems, magic-angle twisted bilayer graphene (MATBG) remains the only one where robust superconductivity has been reproducibly measured. Here we realize a new moir\'e superconductor, mirror symmetric magic-angle twisted trilayer graphene (MATTG) with dramatically richer tunability in electronic structure and superconducting properties. Hall effect and quantum oscillations measurements as a function of density and electric field allow us to determine the system's tunable phase boundaries in the normal state. Zero magnetic field resistivity measurements then reveal that the existence of superconductivity is intimately connected to the broken symmetry phase emerging from two carriers per moir\'e unit cell. Strikingly, we find that the superconducting phase gets suppressed and bounded at the van Hove singularities (vHs) partially surrounding the broken-symmetry phase, which is difficult to reconcile with weak-coupling BCS theory. Moreover, the extensive in situ tunability of our system allows us to achieve the ultra-strong coupling regime, characterized by a Ginzburg-Landau coherence length reaching the average inter-particle distance and very large $T_\mathrm{BKT}/T_{F}$ ratios in excess of 0.1, where $T_\mathrm{BKT}$ and $T_F$ are the Berezinskii-Kosterlitz-Thouless transition and Fermi temperatures, respectively. These observations suggest that MATTG can be electrically tuned close to the two-dimensional BCS-BEC crossover. Our results establish a new generation of tunable moir\'e superconductors with the potential to revolutionize our fundamental understanding and the applications of strong coupling superconductivity.