Rhombohedral Graphene Exhibits Flat Band Surface State Superconductivity and Diverse Ferromagnetic Phases

Rhombohedral multilayer graphene presents a fascinating arena for exploring novel electronic states, and recent investigations reveal a wealth of correlated magnetic and superconducting behaviour. Yi Guo, Owen Sheekey, and Trevor Arp, alongside colleagues from the University of California at Santa Barbara and Aalto University School of Science, now demonstrate the existence of superconductivity originating from flat band surface states within thick rhombohedral graphene devices. Their work directly detects these surface states using layer-resolved capacitance measurements, and reveals a surprising robustness of superconductivity even when the material possesses full symmetry. This discovery challenges existing understanding of superconductivity in these materials, and suggests a unique mechanism involving coupled surface superconductors operating through the insulating bulk, potentially paving the way for new electronic devices and a deeper understanding of correlated electron systems.

This work explores how applying strain can control the electronic properties of this material, specifically focusing on the emergence of insulating states and their response to external stimuli. Researchers used high-resolution scanning tunneling microscopy and spectroscopy, alongside theoretical calculations, to examine the local electronic structure and distribution of these insulating states. The results demonstrate that applying uniaxial strain can transform the material from a metallic to an insulating state, significantly increasing the energy gap, reaching up to 70 meV. Importantly, this insulating state is highly sensitive to in-plane magnetic fields, with the energy gap substantially reduced at fields as low as 1 Tesla. These findings establish a method for manipulating the electronic and magnetic properties of rhombohedral multilayer graphene through strain engineering, potentially leading to new electronic devices and a deeper understanding of correlated electron systems.

Multilayer Graphene Superconductivity and Electronic Phases

Detailed experiments on multilayer graphene, specifically 13 and 14 layers, have revealed a complex landscape of electronic properties, including multiple superconducting states. Researchers systematically investigated the material’s electronic phases, stacking order, and spin polarization using a variety of techniques including transport measurements, differential conductance measurements, and Raman spectroscopy. The team identified four distinct superconducting states, each appearing under specific conditions of carrier density and electric field. These states exhibit unique characteristics observable through changes in electrical resistance and local density of states. The research also mapped out detailed electronic phase diagrams, revealing regions of spin polarization where the material exhibits spontaneous spin alignment. The stacking order of the graphene layers, either rhombohedral or Bernal, was also determined and shown to significantly influence the material’s electronic properties.

Surface Superconductivity Violates Pauli Limit

Recent investigations of rhombohedral multilayer graphene have uncovered a rich variety of correlated electronic states, including magnetism, topology, and superconductivity. Researchers have now directly detected surface states within these materials using layer-resolved capacitance measurements, revealing a range of ferromagnetic phases, including valley-imbalanced quarter metals and regions of spontaneous spin polarization. Notably, the team observed several superconducting states localized to single surface states, appearing on the unpolarized side of density-tuned spin transitions. These superconductors demonstrate a strong violation of the Pauli limit, by a factor of at least 5, suggesting a dominant attractive interaction in a specific pairing channel. Remarkably, superconductivity persists even when the material exhibits inversion symmetry, indicating the presence of two independent surface superconductors coupled through the insulating bulk. The onset of superconductivity coincides precisely with the onset of spontaneous spin polarization on the graphene surfaces, and applying an in-plane magnetic field enhances the superconducting domain, extending it into the single-surface state regime.

Surface Superconductivity and Magnetic Phases in Graphene

Researchers have demonstrated the existence of superconductivity and a variety of magnetic phases within multilayer rhombohedral graphene, revealing a complex interplay between electronic structure and emergent quantum phenomena. Through layer-resolved capacitance measurements and electronic transport studies, the team directly detected flat-band surface states, which host both valley-imbalanced quarter metals and regions of spontaneous spin polarization. The team observed superconducting states localized to these surface states, appearing alongside the onset of spin polarization and persisting even when the material exhibits inversion symmetry. This suggests coupling between two independent surface superconductors. The findings indicate that superconductivity arises from a dominant attractive interaction in a specific pairing channel, and the observed magnetic behavior is strongly linked to the high density of states within the flat surface bands. While theoretical calculations show approximate agreement with the experimentally determined boundaries of the spin-polarized phase, discrepancies suggest the potential for additional, hidden symmetry-breaking phases.