Vacancies, disorder-induced smearing of the electronic structure, and its implications for the superconductivity of anti-perovskite MgC0.93Ni2.85

Abstract

The anti-perovskite superconductor MgC_0.93Ni_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 (~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.

Figure 1

Calculated electronic structure of stoichiometric and non-stoichiometric MgCNi₃. (a), Comparison of the total DOS for the stoichiometric (purple) and supercell configurational average (green) calculations. Calculated stoichiometric Fermi surface from bands 1 and 2 are shown in (b and c), respectively. Calculated MgC_0.875Ni_2.875 supercell configurationally averaged Fermi surface of bands 1 and 2 are shown in (d and e), respectively. The colours on the surfaces represent the occupation smearing of the configurational average, and are given by the standard deviation of the occupation density at the Fermi surface. The high-symmetry points of the simple cubic Brillouin zone are labelled in (b).

Figure 2

Comparison between the experimental and calculated Compton profiles. Difference, ΔJ(p _z), between Compton profiles with scattering vectors along the [110] and [100] directions for the experimental profiles (black dots), and those of the stoichiometric (red line) and supercell configurational average (blue line) calculations. The thickness of the blue line indicates the variation amongst the different supercell configurations and is two standard deviations wide. All of the calculations have been convoluted with a one-dimensional Gaussian with a full-width-at-half-maximum of 0.12 a.u. to approximate the experimental resolution. For clarity, the error bars are only plotted for every third data point and indicate statistical errors of one standard deviation.

Figure 3

Comparison between the reconstructed experimental and calculated projected occupation densities. (a), Experimental occupation density (left half), and theoretical k-space occupation densities, projected down [001], for the stoichiometric (top right), and supercell configurational average (bottom right) calculations. The plot is centred at the projected XΓ-point, and spans two Brillouin zones. The calculated projected occupation densities have been convoluted with a two-dimensional Gaussian approximating the experimental resolution function. The projected high-symmetry points are labelled. (b), Cuts through the [001]-projected experimental and theoretical occupation densities along certain (projected) high-symmetry directions. The thickness of the line for the supercell configurational average corresponds to two standard deviations and the experimental error bars indicate statistical errors of one standard deviation. All distributions have been normalised to unity at the projected RM-point.