Heavy transition metal induced superconductivity in high hardness rock-salt structure tantalum carbide

It is widely acknowledged that strong electron-phonon coupling in conventional superconductors predominantly arises from the high vibrational frequencies of light elements. In this work, we report an anomalous superconducting system where the superconducting behavior in the covalent metallic compound TaC originates predominantly from heavy transition metal atoms. Polycrystalline samples of the ‘covalent metal’ TaC (space group Fm-3 m) were synthesized using a high-pressure high-temperature (HPHT) technique. Comprehensive characterization through direct magnetic susceptibility and electrical resistivity measurements confirms that TaC exhibits characteristics of a weakly-coupled type-II superconductor, demonstrating both a remarkably high upper critical magnetic field and a superconducting transition temperature (T_c) of 9.0 K. Intriguingly, the material displays exceptional mechanical properties as evidenced by its asymptotic Vickers hardness values, which significantly surpass these of conventional superconducting materials. The composition with Ta:C = 1:0.9 demonstrates optimal performance balance, achieving a maximum Vickers hardness of 18.7 GPa while maintaining robust superconducting transition characteristics. First-principles calculations reveal that the elevated T_c primarily stems from substantial electronic contributions from tantalum atoms. Our findings demonstrate that controlled introduction of carbon vacancies serves as an effective strategy for tailoring these dual functional characteristics, providing novel design principles for developing specialized superconducting devices capable of operating under extreme mechanical and electromagnetic conditions.