# Kondo-Ising and Tight-Binding Models for TmB4

## TL;DR

This paper investigates the magnetic and electronic properties of TmB4 using ab-initio calculations, tight-binding models, and an effective Kondo-Ising model to explain its magnetic behavior and conduction features.

## Contribution

It introduces a comprehensive theoretical framework combining ab-initio, tight-binding, and Kondo-Ising models to understand TmB4's magnetic and electronic properties.

## Key findings

- The Fermi surface features relate to anisotropic conduction.
- A large magnetic moment (~6 μ_B) involves subtle crystal field effects.
- Kondo-Ising interactions explain fractional magnetization plateaus.

## Abstract

In $TmB_4$, localized electrons with a large magnetic moment interact with metallic electrons in boron-derived bands. We examine the nature of $TmB_4$ using full-relativistic ab-initio density functional theory calculations, approximate tight-binding Hamiltonian results, and the development of an effective Kondo-Ising model for this system. Features of the Fermi surface relating to the anisotropic conduction of charge are discussed. The observed magnetic moment $\sim 6 \, \mu_B$ is argued to require a subtle crystal field effect in metallic systems, involving a flipped sign of the effective charges surrounding a Tm ion. The role of on-site quantum dynamics in the resulting Kondo-Ising type "impurity" model are highlighted. From this model, elimination of the conduction electrons will lead to spin-spin (RKKY-type) interaction of Ising character required to understand the observed fractional magnetization plateaus in $TmB_4$.