Vacancies, disorder-induced smearing of the electronic structure, and its implications for the superconductivity of anti-perovskite MgC$_{0.93}$Ni$_{2.85}$

TL;DR

This study combines experimental and theoretical methods to analyze how vacancies and disorder in MgC$_{0.93}$Ni$_{2.85}$ affect its electronic structure and superconductivity, revealing that disorder smears the Fermi surface but does not drastically alter its shape.

Contribution

It provides the first combined experimental and theoretical analysis of vacancy-induced disorder effects on the electronic structure of MgC$_{0.93}$Ni$_{2.85}$ and its implications for superconductivity.

Findings

Fermi surface matches between experiment and calculations.

Disorder causes smearing but preserves Fermi surface shape.

Superconducting $T_c$ reduction is less than expected due to suppressed spin fluctuations.

Abstract

The anti-perovskite superconductor MgC$_{0.93}$Ni$_{2.85}$ was studied using high-resolution x-ray Compton scattering combined with electronic structure calculations. Compton scattering measurements were used to determine experimentally a Fermi surface that showed good agreement with that of our supercell calculations, establishing the presence of the predicted hole and electron Fermi surface sheets. Our calculations indicate that the Fermi surface is smeared by the disorder due to the presence of vacancies on the C and Ni sites, but does not drastically change shape. The 20\% reduction in the Fermi level density-of-states would lead to a significant ($\sim 70\%$) suppression of the superconducting $T_c$ for pair-forming electron-phonon coupling. However, we ascribe the observed much smaller $T_c$ reduction at our composition (compared to the stoichiometric compound) to the suppression of pair-breaking spin fluctuations.