Electronic structure, superconductivity, and magnetism in the C15 compounds ZrV2, ZrFe2, and ZrCo2

Abstract

We have calculated the self-consistent paramagnetic electronic structure of cubic Laves-phase Zr${\mathrm{V}}_2$, Zr${\mathrm{Fe}}_2$, and Zr${\mathrm{Co}}_2$, using the augmented-plane-wave method and the local-density-theory form of exchange-correlation potential. Using the mean-field Stoner theory, we have determined the spin susceptibilities and magnetic moments of these compounds. We find that Zr${\mathrm{V}}_2$ remains paramagnetic but with large Stoner enhancement, while the system $\mathrm{Zr}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2$ is ferromagnetic for $0\le x\lesssim 0.5$, in agreement with experiment. However, the Stoner theory yeilds an average magnetic moment which is generally much too small. The electron-phonon interaction has been calculated and, using estimates of the phononic properties from specific-heat measurements, we compare the theoretical estimates of the superconducting transition temperature with experiment. For Zr${\mathrm{V}}_2$, we have found that it is crucial to account for the drop in density of states at the Fermi energy due to the structural phase transition in this material (cubic to rhombohedral at $T_m\sim 100$ K). Estimates of this drop (∼30%) have been obtained by analyzing the temperature-dependent spin susceptibility above and below $T_m$.