Predicted superconductivity of Ni2VAl and pressure dependence of superconductivity in Ni2NbX (X = Al, Ga and Sn) and Ni2VAl

A first-principles study of the electronic and superconducting properties of the Ni2VAl Heusler compound is presented. The electron–phonon coupling constant of λep=0.68 ?> is obtained, which leads to a superconducting transition temperature of Tc=∼4 ?> K (assuming a Coulomb pseudopotential μ∗=0.13 ?>), which is a relatively high transition temperature for Ni based Heusler alloys. The electronic density of states reveals a significant hybridization between Ni-eg and V-t2g states around the Fermi level. The Fermi surface, consisting of two electron pockets around the X-points of the Brillouin zone, exhibits nesting and leads to a Kohn anomaly of the phonon dispersion relation for the transverse acoustic mode TA2 along the (1, 1, 0) direction, which is furthermore found to soften with pressure. As a consequence, Tc ?> and λep ?> vary non-monotonically under pressure. The calculations are compared to similar calculations performed for the Ni2NbX (X = Al, Ga and Sn) Heusler alloys, which experimentally have been identified as superconductors. The experimental trend in Tc ?> is well reproduced, and reasonable quantitative agreement is obtained. The calculated Tc ?> of Ni2VAl is larger than either calculated or observed Tc ?>s of any of the Nb compounds. The Fermi surfaces of Ni2NbAl and Ni2NbGa consist of only a single electron pocket around the X point, however under compression a second electron pocket similar to that of Ni2VAl emerges Ni2NbAl and the Tc ?> increases non-monotonically in all the compounds. Fermi surface nesting and associated Kohn anomalies are a common feature of all four compounds, albeit weakest in Ni2VAl.