Protected Fe valence in quasi-two dimensional $\alpha$-FeSi$_2$

TL;DR

This study investigates the electronic and magnetic properties of high-temperature $ extalpha$-FeSi$_2$, revealing weak correlations, protected iron valence, and structural differences from iron pnictides that may explain its lack of superconductivity.

Contribution

It provides the first comprehensive experimental and theoretical analysis of $ extalpha$-FeSi$_2$, highlighting its weak electronic correlations and the protected nature of iron valence despite doping.

Findings

Weak electronic correlations compared to pnictides

Iron valence remains stable despite vacancy formation and doping

Structural differences affect orbital symmetry and may inhibit superconductivity

Abstract

We report the first comprehensive study of the high temperature form ($\alpha$-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, $\alpha$-FeSi$_2$ is a potential candidate for unconventional superconductivity. Akin to LiFeAs, $\alpha$-FeSi$_2$ 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 $\alpha$-FeSi$_2$ are considerably weaker than in the pnictides. Band theory calculations yield small Sommerfeld coefficients of the electronic specific heat $\gamma=C_e/T$ that are in excellent agreement with experiment. Additionally, realistic many-body calculations further corroborate that quasi-particle mass enhancements are only modest in $\alpha$-FeSi$_{2}$ . 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 $\alpha$-FeSi$_2$. 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.