Hydrogenation effects on the structural stability and superconducting properties of calcium-intercalated bilayer graphene C2CaC2

Abstract

Two-dimensional (2D) materials have attracted significant interest due to their exceptional properties and potential applications in condensed matter physics and nanotechnology. Elemental substitution is a common approach to tuning the physical properties of materials. Among these strategies, intercalation has been shown to enhance superconductivity in 2D materials. Likewise, hydrogenation of pristine 2D materials has been extensively studied for its potential to improve superconducting properties. Recently, it has been proposed that ca-intercalated bilayer graphene, C₂CaC₂, is stable and exhibits a superconducting critical temperature of T_c = 18.9 K. In this study, we investigate the effects of hydrogenation on the structural stability and superconducting properties of C₂CaC₂. Using first-principles calculations, we examine various hydrogenation configurations and identify the most stable phase, the HC₂CaC₂, which is found to be dynamically and thermally stable at room temperature, as confirmed by phonon dispersion and ab initio molecular dynamics (AIMD) simulations. The system exhibits metallic behavior, with electronic states at the Fermi level primarily contributed by carbon p_z orbitals. The electron-phonon coupling constant is calculated to be λ = 0.56, with low-frequency vibrations of Ca and C atoms dominating the coupling. The superconducting critical temperature, estimated using the well-known Allen-Dynes formula, yields a typical value of T_c = 6.1 K for a standard Coulomb pseudopotential (μ* = 0.1). Despite the lower T_c compared to pristine C₂CaC₂, hydrogenation preserves structural stability and metallicity, offering insights into tunable superconductivity in intercalated 2D materials.