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

## TL;DR

This study investigates how Nb-substitution affects the high-pressure superconducting properties of MoB$_2$, revealing a significant suppression of the critical temperature compared to pure MoB$_2$, with insights from experiments and calculations.

## Contribution

It provides new experimental data on Nb-substituted MoB$_2$'s structure and superconductivity under high pressure, highlighting the suppression of $T_c$ due to Nb-substitution.

## Key findings

- Nb-substitution stabilizes the P6/mmm structure down to ambient pressure.
- Superconducting $T_c$ is suppressed from 8 K to 4-5.5 K under high pressure.
- Reduced electron-phonon coupling partly explains the suppression, but other factors may contribute.

## 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.