Competing Orbital Magnetism and Superconductivity in electrostatically defined Josephson Junctions of Alternating Twisted Trilayer Graphene

TL;DR

This study reveals a competitive relationship between superconductivity and orbital magnetism in alternating twisted trilayer graphene, demonstrating how gate-tuned Josephson junctions can probe and manipulate these phases.

Contribution

It uncovers the coexistence and competition of superconductivity and orbital magnetism in TTG, and introduces gate-defined Josephson junctions as a tool to study magnetic phases.

Findings

Orbital magnetism is strongest near charge neutrality point.

Superconductivity emerges at larger moiré fillings, suppressing magnetism.

Asymmetry in Fraunhofer interference pattern indicates magnetic ordering.

Abstract

The coexistence of superconductivity and magnetism within a single material system represents a long-standing goal in condensed matter physics. Van der Waals-based moir\'e superlattices provide an exceptional platform for exploring competing and coexisting broken symmetry states. Alternating twisted trilayer graphene (TTG) exhibits robust superconductivity at the magic angle of 1.57{\deg} and 1.3{\deg}, with suppression at intermediate twist angles. In this study, we investigate the intermediate regime and uncover evidence of orbital magnetism. As previously reported, superconductivity is suppressed near the charge neutrality point (CNP) and emerges at larger moir\'e fillings. Conversely, we find orbital magnetism most substantial near the CNP, diminishing as superconductivity develops. This complementary behavior is similarly observed in the displacement field phase space, highlighting a competitive interplay between the two phases. Utilizing gate-defined Josephson junctions, we probe orbital magnetism by electrostatically tuning the weak links into the magnetic phase, revealing an asymmetric Fraunhofer interference pattern. The estimated orbital ferromagnetic ordering temperature is approximately half the superconducting critical temperature, coinciding with the onset of Fraunhofer asymmetry. Our findings suggest that the observed orbital magnetism is driven by valley polarization and is distinct from the anomalous Hall effect reported at integer fillings in twisted graphene systems. These results offer insights into the interplay between superconductivity and magnetism in moir\'e superlattices.