Diboride compounds doped with transition metals$\unicode{x2013}$a route to superconductivity through structure stabilization as well as defects

TL;DR

This study shows that doping diboride compounds with transition metals stabilizes their structure at ambient pressure and induces superconductivity, with critical temperatures up to 8.5 K, highlighting a new route for ambient-pressure superconductors.

Contribution

Demonstrates that transition metal doping stabilizes the P6/mmm structure in diborides at ambient pressure and induces superconductivity, extending understanding of structure-superconductivity relationships.

Findings

Doping stabilizes P6/mmm structure at ambient pressure.

Superconductivity observed with T_c up to 8.5 K in doped samples.

Rapid cooling enhances T_c and retains superconductivity in MoB2.

Abstract

Recent investigations into MoB$_{2}$ have unveiled a direct connection between a pressure-induced structural transition to a P6/mmm space group structure and the emergence of superconductivity, producing critical temperatures up to 32 K at 100 GPa. This pressure-induced superconducting state underscores the potential of doped MoB$_{2}$ as a possible candidate for metastable superconductivity at ambient pressure. In this work, we demonstrate that doping by Zr, Hf, or Ta stabilizes the P6/mmm structure at ambient pressure and results in the realization of a superconducting state with critical temperatures ranging from 2.4 up to 8.5 K depending on the specific doping. We estimate the electron-phonon coupling $\lambda$ and the density of states based on resistivity and specific heat data, finding that $\lambda$ ranges from 0.4 - 0.6 for these compounds. Finally, to investigate the role of possible metastable defect structures on the critical temperature, we analyze MoB$_{2}$, MoB$_{2.5}$, and Nb/Zr-doped MoB$_{2}$ using rapid cooling techniques. Notably, splat-quenching produces samples with higher critical temperatures and even retains superconductivity in MoB$_{2}$ at ambient pressure, achieving a critical temperature of 4.5 K.