Block Spin Ground State and 3-Dimensionality of (K,Tl)Fe$_{1.6}$Se$_2$

TL;DR

This study uses first-principles calculations to reveal the magnetic ground state, electronic structure, and three-dimensionality of (K,Tl)Fe1.6Se2, highlighting the importance of Fe-vacancy ordering in its properties.

Contribution

It provides a detailed first-principles analysis of the magnetic and electronic ground states, including the role of Fe vacancies and the 3D electronic structure, which was not previously understood.

Findings

Ground state is checkerboard antiferromagnetic with large block spin moment.

Yields a metallic ground state except at y=0.8 where an insulator forms.

Electronic structure is highly three-dimensional with unique Fermi surface topology.

Abstract

The magnetic properties and electronic structure of (K,Tl)y Fe1.6 Se2 is studied using first-principles calculations. The ground state is checkerboard antiferromagnetically coupled blocks of the minimal Fe4 squares, with a large block spin moment ~11.2{\mu}B . The magnetic interactions could be modelled with a simple spin model involving both the inter- and intra-block, as well as the n.n. and n.n.n. couplings. The calculations also suggest a metallic ground state except for y = 0.8 where a band gap ~400 - 550 meV opens, showing an antiferromagnetic insulator ground state for (K,Tl)0.8 Fe1.6 Se2 . The electronic structure of the metallic (K,Tl)y Fe1.6 Se2 is highly 3-dimensional with unique Fermi surface structure and topology. These features indicate that the Fe-vacancy ordering is crucial to the physical properties of (K,Tl)y Fe2-x Se2 .