High critical field superconductivity at ambient pressure in MoB2 stabilized in the P6/mmm structure via Nb substitution

Recently it was discovered that, under elevated pressures, MoB2 exhibits superconductivity at a critical temperature Tc as high as 32 K. The superconductivity appears to develop following a pressure-induced structural transition from the ambient pressure R3¯m structure to an MgB2-like P6/mmm structure. This suggests that remarkably high Tc values among diborides are not restricted to MgB2 as previously appeared to be the case, and that similarly high Tc values may occur in other diborides if they can be coerced into the MgB2 structure. In this paper, we show that density functional theory calculations indicate that phonon free energy stabilizes the P6/mmm structure over the R3¯m at high temperatures across the Nb1-xMoxB2 series. X-ray diffraction confirms that the synthesized Nb-substituted MoB2 adopts the MgB2 crystal structure. High magnetic field electrical resistivity measurements and specific heat measurements demonstrate that NbxMo1-xB2 exhibits superconductivity with Tc as high as 8 K and critical fields approaching 6 T.