Protected Fe valence in quasi-two-dimensional α-FeSi2

We report the first comprehensive study of the high temperature form (a-phase) of iron disilicide. Measurements of the magnetic susceptibility, magnetization, heat capacity and resistivity were performed on well characterized single crystals. With a nominal iron d(6) configuration and a quasi-two-dimensional crystal structure that strongly resembles that of LiFeAs, alpha-FeSi2 is a potential candidate for unconventional superconductivity. Akin to LiFeAs, alpha-FeSi2 does not develop any magnetic order and we confirm its metallic state down to the lowest temperatures (T = 1.8 K). However, our experiments reveal that paramagnetism and electronic correlation effects in alpha-FeSi2 are considerably weaker than in the pnictides. Band theory calculations yield small Sommerfeld coefficients of the electronic specific heat gamma = Ce/T that are in excellent agreement with experiment. Additionally, realistic many-body calculations further corroborate that quasi-particle mass enhancements are only modest in alpha-FeSi2. Remarkably, we find that the natural tendency to vacancy formation in the iron sublattice has little influence on the iron valence and the density of states at the Fermi level. Moreover, Mn doping does not significantly change the electronic state of the Fe ion. This suggests that the iron valence is protected against hole doping and indeed the substitution of Co for Fe causes a rigid-band like response of the electronic properties. As a key difference from the pnictides, we identify the smaller inter-iron layer spacing, which causes the active orbitals near the Fermi level to be of a different symmetry in alpha-FeSi2. This change in orbital character might be responsible for the lack of superconductivity in this system, providing constraints on pairing theories in the iron based pnictides and chalcogenides.