Robust ferrimagnetic ground state and suppressed superconductivity in two-dimensional C6H

Abstract

Two-dimensional hydrogenated graphene (C${}_6$H) represents a promising platform for exploring emergent electronic phases. Owing to its high electronic density of states at the Fermi level, C${}_6$H is expected to support phonon-mediated superconductivity, with a calculated critical temperature ($T_c$) of 37.4 K in the paramagnetic metallic phase. However, spin-polarized first-principles calculations reveal that C${}_6$H stabilizes in a ferrimagnetic ground state, which is energetically favored by 0.175 eV per unit cell over the paramagnetic metallic phase. This large energy difference significantly exceeds $k_{B}T$ at room temperature, indicating robust magnetic order. Although the superconducting condensation energy lowers the total energy by 7 meV, the superconducting phase remains metastable. These results highlight the dominant role of magnetism in C${}_6$H and illustrate how a high electronic density of states can drive competing instabilities in hydrogenated two-dimensional materials, offering design principles for carbon-based magnetic systems.