Origin of Superconductivity in Rhombohedral Trilayer Graphene: Quasiparticle Pairing within the Inter-Valley Coherent Phase

Superconductivity (SC) is observed in rhombohedral trilayer graphene (RTG) with an extremely low charge carrier densities, between the normal metal state and a probable inter-valley coherent (IVC) state. The measured coherence length in RTG is roughly two orders of magnitude shorter than the value predicted by a conventional BCS theory. To resolve these discrepancies, we propose that the RTG SC phase arises from the pairing of quasiparticles within the IVC phase. We illustrate the SC behavior using gapped Dirac cones with the chemical potential close to the conduction band's bottom and establish the Ginzburg-Landau theory. Our findings indicate that the mean-field transition temperature, $T_\mathrm{MF}$, is primarily determined by the density of states of quasiparticles within the IVC phase, and is more suppressed in comparison with the BCS relation. Interestingly, we discover that the coherence length behaves according to $\xi\sim 1/\sqrt{T_\mathrm{MF}}$. When the chemical potential lies within the band gap, the conventional coherence length becomes small, while the quantum metric contribution can diverge. Applying our approach to a microscopic model for RTG, our predictions align well with experimental data and effectively capture key measurable features of quantities such as the mean-field transition temperature $T_\mathrm{MF}$ and the coherence length $\xi$. Furthermore, we reveal that the quantum metric contribution, stemming from the multi-band properties of the IVC quasiparticles, plays a significant role in determining the coherence length around the transition regime from superconductivity to IVC. Lastly, experimental predictions are discussed on bebaviors of the upper critical field and coherence length at low temperatures.
Comment: 10+15 pages, 3+1 diagrams, 1+1 tables, supplementary included