Probing the Superconductivity Limit of Li-Doped Graphene

The introduction of superconductivity in graphene systems is highly desirable from both fundamental physics and application perspectives. In this article, a superlattice strategy to develop a series of Li-doped graphene is reported: deposition type-I (Li₂C₆, Li₂C₈, LiC₆, Li₃C₂₄, LiC₁₂, LiC₁₆, Li₂C₃₆, LiC₂₄), intercalation type-II (LiC₄, Li₂C₁₂, LiC₈, LiC₁₂, LiC₁₆), and coexisting deposition and intercalation type-III (Li₃C₁₂). With increasing concentration of Li atoms, both metallicity, and electron–phonon coupling (EPC) has dramatically increased, which is favorable for the emergence of superconductivity in the screened Li–C compounds. Notably, graphene superlattice structures with intercalated Li2 atoms have higher stability, while Li1-deposited graphene at the same concentration produces higher T_c. Among them, type-I-Li₂C₆, type-I-Li₂C₈, type-II-LiC₄, and type-III-Li₃C₁₂ are phonon-mediated superconductors with high transition temperatures (T_c) of 18, 12, 3.4, and 14 K, respectively. The EPC of type-I-Li₂C₆, type-I-Li₂C₈, and type-III-Li₃C₁₂ mainly arises from the coupling of the C-2p_z electron states with the low-frequency (0–800 cm⁻¹) deposition-Li_xy/Li_z, and out-of-plane-C_z vibrations. In contrast, the high-frequency (800–1600 cm⁻¹) vibration modes of in-plane-C_xy atoms are mainly responsible for the T_c of type-II-LiC₄. The findings provide comprehensive insights into the superconductivity limit of Li-doped graphene.