关键词: superconductivity, electronic structure, density functional theory, molecular dynamics

Abstract: The recent discovery of type-VII boron-carbon clathrates with calculated superconducting transition temperatures approaching $\sim 100 $ K has sparked interest in exploring new conventional superconductors that may be stabilized at ambient pressure. The electronic structure of the clathrate is highly tunable based on the ability to substitute different metal atoms within the cages, which may also be large enough to host small molecules. Here we introduce molecular hydrogen (H$_{2}$) within the clathrate cages and investigate its impact on electron-phonon coupling interactions and the superconducting transition temperature ($T_{\rm c}$). Our approach involves combining molecular hydrogen with the new diamond-like covalent framework, resulting in a hydrogen-encapsulated clathrate, (H$_{2}$)B$_{3}$C$_{3}$. A notable characteristic of (H$_{2}$)B$_{3}$C$_{3}$ is the dynamic behavior of the H$_{2}$ molecules, which exhibit nearly free rotations within the B-C cages, resulting in a dynamic structure that remains cubic on average. The static structure of (H$_{2}$)B$_{3}$C$_{3}$ (a snapshot in its dynamic trajectory) is calculated to be dynamically stable at ambient and low pressures. Topological analysis of the electron density reveals weak van der Waals interactions between molecular hydrogen and the B-C cages, marginally influencing the electronic structure of the material. The electron count and electronic structure calculations indicate that (H$_{2}$)B$_{3}$C$_{3}$ is a hole conductor, in which H$_{2}$ molecules donate a portion of their valence electron density to the metallic cage framework. Electron-phonon coupling calculation using the Migdal-Eliashberg theory predicts that (H$_{2}$)B$_{3}$C$_{3}$ possesses a $T_{\rm c}$ of 46 K under ambient pressure. These results indicate potential for additional light-element substitutions within the type-VII clathrate framework and suggest the possibility of molecular hydrogen as a new approach to optimizing the electronic structures of this new class of superconducting materials.

Key words: superconductivity, electronic structure, density functional theory, molecular dynamics