First-principles Study On The Electronic Structure Of Silver Selenide

As the development of the information storage technology, magnetoresistancematerials have become a new research hotspot recently. The high temperature phase ofsilver selenide (Ag2Se) is a superionic conductor, and the low temperature phase(stablebelow133℃) is a kind of nawrrow-gap semiconductor with the energy gap ranges fromabout0.03to0.11eV. The stoichiometric Ag2Se exhibit only the ordinarymagnetoresistance, but a slight deviation from stoichiometry (a little excess or deficient)can generate a very large linear magnetoresistance(LMR) with no sign of saturating.Recently our team found, Co doped Ag2Se possess the large LMR similarly. Theunderlying mechanism of the LMR is not clear at present, and a well accepted theorymodel is still to be constructed. Especially, though Ag2Se was predicted theoretically to bea topological non-trivial insulator, the effect of doping on it is still unknown. Utilizing thefirst principle calculation based on density function theory, we studied the geometrystructure, band structure and the density of states (DOS) of the stoichiometric Ag2Se,self-doped Ag2+δSe and Co doped Ag2Se respectively.First we caculated the electronic structure and the DOS of β-Ag2Se by geometricaloptimization of the lattice structure, the results demonstrated that β-Ag2Se is adirect-band-gap semiconductor and the gap is0.031eV. According to the band structureand the DOS, the top of valence band was mainly contributed by Ag4d and Se4pelectrons, and the bottom of conduction band mainly by Ag5s and Se4p electrons.This work reported the band structure and DOS and magnetic properties of self dopedAg2Se by the first principle calculation. The results showed that both substitutionallydoping and the substitutional doping make cell volume increases, and band structure has asignificant change near the Fermi level. The valence band of the substitutionally dopedAg2Se crosses the Fermi level, but both the valance band and the conduction band of the interstitially doped Ag2Se through the Fermi level. the Fermi energy enters into theconduction band and Ag2+δSe exhibits metallic, because DOS move towards the lowerenergy direction.We use first-principles methods to calculate the Co-doped Ag2Se material geometry,band structure and density of states for ferromagnetic and antiferromagnetic states, takinginto account the three different distances of Co-ions. Because of the incorporation of Coions, the cell structure of β-Ag2Se was a serious distortion and the unit cell volumeincreases. For the same kinds of Co ions spacing, the total energy of antiferromagneticstate system is lower than the ferromagnetic state system. The net magnetic moment is6.00001Bfor ferromagnetic state system and the antiferromagnetic state system is zero.The total DOS of the ferromagnetic exhibits asymmetry in the vicinity of the Fermi level,but the total DOS of antiferromagnetic is very good symmetry in the whole energy range.