Flat band surface state superconductivity in thick rhombohedral graphene

TL;DR

This study reveals that thick rhombohedral graphene exhibits flat-band surface states hosting various magnetic phases and localized superconductivity, with evidence of unconventional pairing and persistence at zero displacement field.

Contribution

It demonstrates the existence of surface flat-band states and superconductivity in thick rhombohedral graphene, including at zero displacement field, expanding understanding of correlation effects in layered materials.

Findings

Detection of flat-band surface states in ~13-layer rhombohedral graphene.

Observation of surface-localized superconductivity with unconventional pairing.

Superconductivity persists at zero displacement field, indicating robust surface states.

Abstract

Rhombohedral multilayer graphene has recently emerged as a rich platform for studying correlation driven magnetic, topological and superconducting states. While most experimental efforts have focused on devices with N$\leq 9$ layers, the electronic structure of thick rhombohedral graphene features flat-band surface states even in the infinite layer limit. Here, we use layer resolved capacitance measurements to directly detect these surface states for $N\approx 13$ layer rhombohedral graphene devices. Using electronic transport and local magnetometry, we find that the surface states host a variety of ferromagnetic phases, including both valley imbalanced quarter metals and broad regimes of density in which the system spontaneously spin polarizes. We observe several superconducting states localized to a single surface state. These superconductors appear on the unpolarized side of the density-tuned spin transitions, and show strong violations of the Pauli limit consistent with a dominant attractive interaction in the spin-triplet, valley-singlet pairing channel. In contrast to previous studies of rhombohedral multilayers, however, we find that superconductivity can persist to zero displacement field where the system is inversion symmetric. Energetic considerations suggest that superconductivity in this regime is described by the existence of two independent surface superconductors coupled via tunneling through the insulating single crystal graphite bulk.