Abstract | I will be presenting our observation of ferromagnetism and superconductivity in rhombohedral trilayer graphene. Rhombohedral trilayer graphene is a two-dimensional electron system where three layers of graphene are stacked to form a rhombohedral lattice. The band structure of rhombohedral trilayer graphene features van Hove singularities in the low energy regime, where the density of states diverges. The van Hove singularities can be enhanced by applying an electrical displacement field perpendicular to the sample, inducing Fermi surface instability. By combining cryogenic electrical transport measurement and quantum capacitance measurement, we found the Fermi surface instability drives spontaneous ferromagnetic polarization of the electron system into one or more spin- and valley flavors. The interplay of magnetic phase transitions and the change of the Fermi surface topology lead to a complex phase diagram in the density – displacement field space. More interestingly, superconductivity is observed near some phase boundaries, featuring zero resistance below 100mK that can be eliminated by sufficiently large electrical current or magnetic field. While the major features of the magnetic phase diagram can be captured by a simple Stoner model, the origin of superconductivity remains unclear. Our observation of the ferromagnetism and superconductivity in an itinerant electronic system may enable a new class of field-effect controlled mesoscopic electronic devices combining correlated electron phenomena. |
