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

TL;DR

This study demonstrates that Nb-substituted MoB$_2$ adopts a high-temperature stable MgB$_2$-like structure and exhibits superconductivity at ambient pressure with a critical temperature up to 8 K and high critical fields.

Contribution

The paper shows that Nb substitution stabilizes the MgB$_2$ structure in MoB$_2$, enabling high critical field superconductivity at ambient pressure, expanding the potential of diborides for superconducting applications.

Findings

Nb substitution stabilizes MgB$_2$ structure in MoB$_2$.

Superconductivity observed at up to 8 K in Nb-doped MoB$_2$.

Critical magnetic fields approach 6 T in the synthesized material.

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.