High critical field superconductivity at ambient pressure in MoB${}_2$ stabilized in the P6/mmm structure via Nb substitution

Abstract

Recently it was discovered that, under elevated pressures, MoB${}_2$ exhibits superconductivity at a critical temperature, $T_c$, as high as 32 K. The superconductivity appears to develop following a pressure-induced structural transition from the ambient pressure R$\bar{3}$m structure to an MgB${}_2$-like P6/mmm structure. This suggests that remarkably high $T_c$ values among diborides are not restricted to MgB${}_2$ as previously appeared to be the case, and that similarly high $T_c$ values may occur in other diborides if they can be coerced into the MgB${}_2$ structure. In this paper, we show that density functional theory calculations indicate that phonon free energy stabilizes the P6/mmm structure over the R$\bar{3}$m at high temperatures across the Nb${}_{1-x}$Mo${}_x$B${}_2$ series. X-ray diffraction confirms that the synthesized Nb-substituted MoB${}_2$ adopts the MgB${}_2$ crystal structure. High magnetic field electrical resistivity measurements and specific heat measurements demonstrate that Nb${}_{1-x}$Mo${}_x$B${}_2$ exhibits superconductivity with $T_c$ as high as 8 K and critical fields approaching 6 T.

6 pages, 6 figures, 1 table

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• Diboride compounds doped with transition metals$\unicode{x2013}$a route to superconductivity through structure stabilization as well as defects
• Nb-substitution suppresses the superconducting critical temperature of pressurized MoB${}_2$