Chalcogen height dependence of magnetism and Fermiology in FeTexSe1−x

Abstract

The FeTexSe1−x (x = 0, 0.25, 0.50, 0.75 and 1) system has been studied using density functional theory. Our results show that for FeSe, the local density approximation (LDA) seems a better approximation in terms of magnitude of magnetic energy whereas the generalized gradient approximation (GGA) greatly overestimates it. On the other hand for FeTe, GGA is a better approximation that gives the experimentally observed magnetic state. It has been shown that the height of chalcogen atoms above Fe layers has a significant effect on band structure, electronic density of states (DOS) at the Fermi level N(EF) and Fermi surfaces. For FeSe the value of N(EF) is small so as to satisfy Stoner criteria for ferromagnetism, (I × N(EF) ≥ 1) whereas for FeTe, since the value of N(EF) is large, the same is close to being satisfied. Force minimization for FeTexSe1−x using the supercell approach shows that in a disordered system Se and Te do not share the same site and have two distinct z co-ordinates. This has a small effect on magnetic energy but no significant difference in band structure and DOS near EF when calculated using either the relaxed or average value of z for chalcogen atoms. Thus substitution of Se at the Te site decreases the average value of chalcogen height above Fe layers, which in turn affects the magnetism and Fermiology in the system. By using the coherent potential approximation for the disordered system, we found that height of chalcogen atoms above the Fe layer, rather than chalcogen species or disorder in the anion planes, affects the magnetism and shape of Fermi surfaces (FSs), thus significantly altering nesting conditions, which govern antiferromagnetic spin fluctuations in the system.