Discovery of High-Temperature Superconductivity (T_c = 55 K) in B-Doped Q-Carbon

We have achieved a superconducting transition temperature (T_c) of 55 K in 27 at% B-doped Q-carbon. This value represents a significant improvement over previously reported T_c of 36 K in B-doped Q-carbon and is the highest T_c for conventional BCS (Bardeen-Cooper-Schrieffer) superconductivity in bulk carbon-based materials. The B-doped Q-carbon exhibits type-II superconducting characteristics with H_c2(0) ∼ 10.4 T, consistent with the BCS formalism. The B-doped Q-carbon is formed by nanosecond laser melting of B/C multilayered films in a super undercooled state and subsequent quenching. It is determined that ∼67% of the total boron exists with carbon in a sp³ hybridized state, which is responsible for the substantially enhanced T_c. Through the study of the vibrational modes, we deduce that higher density of states near the Fermi level and moderate to strong electron-phonon coupling lead to a high T_c of 55 K. With these results, we establish that heavy B doping in Q-carbon is the pathway for achieving high-temperature superconductivity.