Microscopic model for the semiconductor-to-ferromagnetic-metal transition in FeSi$_{1-x}$Ge$_{x}$ Alloys

TL;DR

This paper presents a microscopic model explaining the transition from a singlet semiconductor to a ferromagnetic metal in FeSi$_{1-x}$Ge$_x$ alloys, capturing key experimental features and phase behavior.

Contribution

It introduces a simplified chain model with hybridization and onsite interactions that reproduces the phase transition and magnetic properties observed in the alloys.

Findings

First order phase transition from semiconductor to ferromagnetic metal

Reproduction of the rapid increase in spin susceptibility

Good agreement between mean field and DMRG calculations at zero temperature

Abstract

The simplified bandstructure introduced by Mazurenko et al to model FeSi is used to analyze the singlet semiconductor to ferromagnetic metal transition in the isoelectronic isostructural alloys, FeSi$_{1-x}$Ge$_x$. The complex bandstructure of the alloy is replaced by an alternating chain of doubly and singly degenerate atoms to represent Fe and Si/Ge respectively. The former(latter) form narrow(broad) bands with a substantial hybridization between them. A substantial onsite repulsion including a Hund's rule coupling is introduced on the Fe sites. The mean field phase diagram contains a first order phase transition from the singlet semiconductor to a ferromagnetic metal with increasing temperature and interaction strength similar to the alloys. The analysis also reproduces the rapid rise of the spin susceptibility in the semiconductor with a crossover to a Curie-Weiss form at higher temperatures. Good agreement is found at zero temperature between the mean field and accurate DMRG calculations.