Nb-substitution suppresses the superconducting critical temperature of pressurized MoB${}_2$

Abstract

A recent work has demonstrated that MoB${}_2$, transforming to the same structure as MgB${}_2$ ($P6/mmm$), superconducts at temperatures above 30 K near 100 GPa [C. Pei $et$ $al$. Natl. Sci. Rev., nwad034 (2023)], and Nb-substitution in 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 MoB${}_2$ (Nb${}_{0.25}$Mo${}_{0.75}$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 $T_c$ 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 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 Nb${}_{0.25}$Mo${}_{0.75}$B${}_2$, but do not account fully for the observed suppression, which may also arise from inhomogeneity and enhanced spin fluctuations.

8 pages, 5 figures, 1 table

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