Superconductivity in Carbon Cage-Like Substance C6X (X = B, Na) under Pressure

Abstract Since the discovery of superconductivity, increasing the critical temperature (TC) has always been a core challenge. This study is based on the “element substitution” strategy, proposing to construct new C6X systems by replacing atoms in the CaH6 cage-like structure. The study systematically explores the multidimensional coupling mechanism of “structure - electron - phonon” in these systems. The research focuses on the stability, electron–phonon coupling, and superconducting properties of the C6X (X = B, Na) systems, and reaches the following conclusions. The C6B system can maintain dynamic stability at normal pressure, but its dominant low-frequency phonon mode limits the superconducting transition temperature (29.13 K). Second, C6Na exhibits structural stability at 20 GPa and demonstrates enhanced electron–phonon coupling (EPC) strength (λ = 3.29), attributable to its pronounced charge transfer capability, high-frequency phonon modes, and highly symmetric, degeneracy-protected density of states (DOS), collectively elevating the superconducting critical temperature (TC) to 85.94 K. Finally, applying pressure leads to a decrease in the TC of the C6X system. Additionally, the study further proposes design principles for enhancing the TC of the C6X system. By enhancing the Fermi surface DOS through high charge transfer, inducing degenerate states near the Fermi energy (EF) with high symmetry, and leveraging multichannel electron–phonon coupling to break through the traditional coupling strength limitations. This work provides a theoretical basis for understanding the element regulation mechanism of carbon-based cage superconductors and optimizing superconducting properties, and points out the direction for the design of carbon-based superconducting materials at ambient pressure.