The electronic pseudo band gap states and electronic transport of the full-Heusler compound Fe₂VAl

Abstract

For Fe₂VAl the temperature-dependent Seebeck coefficient S(T) and electrical resistivity ρ(T) were calculated within the framework of density functional theory (DFT). The DFT calculations were extended in terms of a DFT/LDA+U approach with U − J values attributed to Fe-d-like states. For simulating the general features of the measured data, a large range of U − J values was scanned with U − J = 2.145 eV as the recommended value. For this value a very small negative indirect gap of E(X) − E(Γ) = −0.0093 eV is found, which is significantly reduced as compared to the DFT-GGA value of −0.164 eV. Charge transfer was derived by Bader's approach, resulting in a significant transfer of 0.75 electronic charges to each Fe atom from Al (1.03) and V (0.48). The pseudogap states around the Fermi energy were analyzed in detail in terms of density of states, band structures and charge density contours. These states almost exclusively govern S(T) and ρ(T). They have large dispersions and are centered at Γ and X. They consist of tails of localized V and Fe states dangling to the Al site. The dispersion of the band along the k space direction X–Γ was modelled in terms of a tight-binding ansatz, resulting in k-dependent matrix elements. From our DFT study, based on the findings for S(T) and ρ(T), it appears that Fe₂VAl has a very small negative indirect gap in the electronic structure. By fitting the temperature-dependent Seebeck coefficient within a parabolic band model, a tiny positive band gap of around 0.003 eV is revealed which qualitatively agrees with the DFT results.