The electronic band structure of a B2 intermetallic compound: localised states in CoGa

The results of a band structure calculation for the equiatomic B2 (CsCl) CoGa alloy are presented and discussed with reference to data obtained from electrical resistivity, magnetoresistance and Hall effect measurements on Co_xGa_100-x (40<or=x<65) alloys. The band structure calculations reveal a deep pseudo-gap in the density of states near the Fermi energy, around which the electronic wavefunctions are found to contain about 75% d tight-binding character. It is suggested that this partial localisation is responsible for the anomalously high resistivity (about 250 mu Omega cm) and an exponential rise in the electrical resistivity towards low temperatures found in these alloys. The high resistivity in turn necessitates a modification to the general expression for resistivity in which Mattheissen's rule is multiplied by an average temperature-dependent Debye-Waller factor. This modification adequately reproduces the overall negative temperature coefficient found for Co_xGa_100-x alloys.