A recent study has demonstrated that ${\mathrm{MoB}}_2$, transforming to the same structure as ${\mathrm{MgB}}_2$ ($P6/mmm$), superconducts at temperatures above 30 K near 100 GPa [C. Pei et al., Natl. Sci. Rev. 10, nwad034 (2023)], and Nb substitution in ${\mathrm{MoB}}_2$ stabilizes the $P6/mmm$ structure down to ambient pressure [A. C. Hire et al., Phys. Rev. B 106, 174515 (2022)]. The current work explores the high-pressure superconducting behavior of Nb-substituted ${\mathrm{MoB}}_2$ (${\mathrm{Nb}}_{0.25}{\mathrm{Mo}}_{0.75}{\mathrm{B}}_2$). High-pressure x-ray diffraction measurements show that the sample remains in the ambient pressure $P6/mmm$ structure to at least 160 GPa. Electrical resistivity measurements demonstrate that from an ambient pressure $T_{\mathrm{c}}$ of 8 K (confirmed by specific heat to be a bulk effect), the critical temperature is suppressed to 4 K at 50 GPa, before gradually rising to 5.5 K at 170 GPa. The critical temperature at high pressure is thus significantly lower than that found in ${\mathrm{MoB}}_2$ under pressure (30 K), revealing that Nb substitution results in a strong suppression of the superconducting critical temperature. Our calculations indeed find a reduced electron-phonon coupling in ${\mathrm{Nb}}_{0.25}{\mathrm{Mo}}_{0.75}{\mathrm{B}}_2$, but do not account fully for the observed suppression, which may also arise from inhomogeneity and enhanced spin fluctuations.

• Received 27 February 2023
• Accepted 3 August 2023

DOI:https://doi.org/10.1103/PhysRevB.108.094501