Superconductivity in rhombohedral trilayer graphene

Area of Science:

• Condensed Matter Physics
• Materials Science
• Nanoelectronics

Background:

• Accessing superconductivity via electric field effect in 2D devices is a key nanoelectronics goal.
• Superconductivity in graphene moiré heterostructures has been achieved, but often suffers from mechanical instability and disorder.
• Rhombohedral trilayer graphene offers a structurally stable, clean 2D material for exploring superconductivity.

Purpose of the Study:

• To investigate superconductivity in crystalline rhombohedral trilayer graphene using electric field effect.
• To characterize the superconducting states and their relationship to the normal state Fermi surface.
• To provide a model system for testing superconductivity theories in a clean, 2D material.

Main Methods:

• Fabrication of a clean, two-dimensional device using crystalline rhombohedral trilayer graphene.
• Electrical transport measurements, including resistivity and quantum oscillations, at sub-kelvin temperatures.
• Gate-tuning to induce and probe superconducting states (SC1 and SC2) and normal state properties.

Main Results:

• Observation of superconductivity (low/vanishing resistivity) in two distinct gate-tuned regions (SC1 and SC2) at sub-kelvin temperatures.
• Superconductivity is confirmed to be in the clean limit, with a high ratio of mean free path to superconducting coherence length.
• Quantum oscillations reveal both superconducting states emerge from an annular Fermi sea, near an isospin-symmetry-breaking transition.
• SC2 exhibits behavior violating the Pauli limit by an order of magnitude, emerging from a spin-polarized, valley-unpolarized half-metal.

Conclusions:

• Superconductivity in clean rhombohedral trilayer graphene provides a simplified system for theoretical investigations.
• The observed phenomena offer insights into exotic superconducting mechanisms, including phonon-mediated, isospin-fluctuation-driven, and annular Fermi liquid instabilities.
• This work enables the development of novel field-effect controlled electronic devices leveraging correlated electron phenomena and ballistic transport.