Pressure-Induced Transition from Spin to Superconducting States in Novel MnN2.

The connection between magnetism and superconductivity has long been discussed since the discovery of Fe-based superconductors. Here, we report the discovery of a pressure-induced transition from a spin to a superconducting state in novel MnN2 based on ab initio calculations. The superconducting state can be obtained in two ways: the first is the pressure-induced transition from an AFM-P21/m to an NM-I4/mmm phase at 30 GPa, while the other is the pressure-induced transition from an FM-I4/mmm phase to magnetic vanishing at 14 GPa, which leads to a structural transition with the distortion of octahedrons to tetragonal pyramids. NM-I4/mmm-MnN2 is superconductive with T c ≈ 17.6 K at 0 GPa. In the second way, electronic structure calculations indicate that the system transforms from a high-spin state to a low-spin state due to increasing crystal-field splitting, causing disappearance of magnetism; more electron occupancy around the Fermi level drives the emergence of superconductivity. Remarkably, I4/mmm-MnN2 can achieve mutual spin-to-superconducting state transformation by pressure. Moreover, the AFM-P21/m-MnN2 phase is extremely incompressible with the hardness above 20 GPa. Our results provide a reasonable and systematic interpretation for the connection between magnetism and superconductivity and give clues for achieving spin-to-superconducting switching materials with certain crystal features.