Superconductivity at 28 K in Sodium Graphite Intercalation Compound under High Pressure

The discovery of 15.1 K superconductivity in first-stage CaC_{6} reignited interest in searching for high-temperature superconductors in graphite intercalation compounds (GICs). However, despite nearly two decades of intensive research, progress in exploring high-temperature superconductivity in GIC materials has remained stagnant. Currently, research on the superconductivity of GICs mainly focuses on first-stage GICs, while the superconductivity of high-stage GICs has been largely overlooked. Here, we report an experimental discovery of superconductivity with maximum critical temperature (T_{c}) of approximately 28 K in sodium carbide at about 14 GPa evidenced by a sharp drop of resistance and a characteristic decrease of T_{c} under magnetic fields up to 6 T. The upper critical field μ_{0}H_{c2}(0) was estimated to be 8.8 T based on fitting with the Ginzburg-Landau superconducting model while the coherent length is approximately 61.2  Å. X-ray diffraction measurements combined with crystal structure predictions identified the superconducting candidate material as the second-stage GIC NaC_{8}, in which sodium atoms occupy the interlayer spaces between AA-stacked bilayer graphene. The current findings not only achieve the long-sought high-T_{c} GICs, but more importantly, open up a new avenue for exploring high-temperature superconductors in light-element compounds.