Scanning Intrinsic Superconductivity and the Spin Hall Effect in Niobium Borides

Although niobium borides have been extensively studied on their mechanical properties, a systematic investigation on their potential metastable phases, superconducting excitations, and spin Hall effects (SHE) is still lacking. Herein, we carry out a global structure search in the Nb-B system based on the evolutionary algorithm and density functional theory. Thermodynamical, dynamical, mechanical, and quasiharmonic approximation investigations unveil four new phases, Nb₃B₅, Nb₂B₅, Nb₃B₇, and Nb₅B₄, which are promising candidates for experimental preparations. Moreover, the superconducting transitions of all Nb-B compounds are performed from electron–phonon calculations. In addition, all Nb-B compounds are predicted to be topologically nontrivial. More intriguingly, Nb₂B₅ is unveiled as a promising noncentrosymmetric superconductor to explore topological superconducting excitation. Furthermore, nonzero spin Hall conductivity (SHC) tensor elements of all Nb-B compounds are predicted, and NbB is predicted to own a maximal SHC value of 320 (ℏ/e) (S/cm) among all Nb-B compounds. Our theoretical results fill in the gap on superconducting and SHC parameters of Nb-B binaries, demonstrating that Nb-B binaries are a potential choice to explore superconducting excitations and topological states and to investigate the SHE.