[Paper Review] Intertwining of magnetism and charge ordering in kagome FeGe

Sen Shao, Jia‐Xin Yin+12 more|arXiv (Cornell University)|Jun 24, 2022

Topological Materials and Phenomena4 citations

TL;DR

This study proposes that FeGe exhibits a 2×2×1 charge density wave (CDW) driven by Fermi surface nesting of kagome-derived electronic states, with Ge atoms in honeycomb layers undergoing a generalized Kekulé distortion while Fe kagome layers remain largely intact. The CDW is stabilized by the intertwining of magnetic exchange coupling and electron correlations, enhancing Fe magnetic moments and explaining experimental STM data.

ABSTRACT

Recent experiments report a charge density wave (CDW) in the antiferromagnet FeGe, but the nature of the charge ordering and the associated structural distortion remains elusive. We discuss the structural and electronic properties of FeGe. Our proposed ground state phase accurately captures atomic topographies acquired by scanning tunneling microscopy. We show that the 2$ imes$2$ imes$1 CDW likely results from the Fermi surface nesting of hexagonal-prism-shaped kagome states. FeGe is found to exhibit distortions in the positions of the Ge atoms instead of the Fe atoms in the kagome layers. Using in-depth first-principles calculations and analytical modeling, we demonstrate that this unconventional distortion is driven by the intertwining of magnetic exchange coupling and CDW interactions in this kagome material. Movement of Ge atoms from their pristine positions also enhances the magnetic moment of the Fe kagome layers. Our study indicates that magnetic kagome lattices provide a material candidate for exploring the effects of strong electronic correlations on the ground state and their implications for transport, magnetic, and optical responses in materials.

Motivation & Objective

• To resolve the nature of charge density wave (CDW) order and associated structural distortions in the kagome antiferromagnet FeGe.
• To determine whether the CDW arises from Fermi surface nesting or electron correlation effects.
• To explain the observed atomic topographies in scanning tunneling microscopy (STM) experiments.
• To clarify why Ge atoms, not Fe atoms, are displaced in the CDW phase.
• To explore the role of magnetic exchange coupling in stabilizing the CDW and enhancing magnetic moments.

Proposed method

• Employed the CALYPSO structure search algorithm to identify low-energy antiferromagnetic supercell structures of FeGe.
• Performed first-principles calculations using VASP with PBE exchange-correlation functional and projector-augmented wave method.
• Conducted phonon spectrum and Gibbs free energy calculations via the quasi-harmonic approximation in PHONOPY to assess thermodynamic stability.
• Simulated STM images using a 3-unit-cell thick slab model with vacuum layers, focusing on surface electronic density of states.
• Calculated electronic susceptibility using the Lindhard response function at zero frequency to probe CDW instability via Fermi surface nesting.
• Used VASPKIT for post-processing of band structure and density of states data.

Experimental results

Research questions

• RQ1 What is the origin of the 2×2×1 charge density wave in FeGe, and which atomic sublattice undergoes structural distortion?
• RQ2 How does Fermi surface nesting of kagome states drive the formation of the CDW in FeGe?
• RQ3 Why do Ge atoms in the honeycomb layers displace rather than Fe atoms in the kagome layers during the CDW transition?
• RQ4 How does the magnetic exchange coupling intertwine with CDW order to stabilize the ground state?
• RQ5 What is the impact of the CDW on the magnetic moment of Fe atoms in the kagome lattice?

Key findings

• The 2×2×1 CDW in FeGe arises from Fermi surface nesting of hexagonal-prism-shaped kagome states, driving a generalized Kekulé distortion in the Ge honeycomb layers.
• Structural distortion primarily involves displacement of Ge atoms in the honeycomb layers, not Fe atoms in the kagome layers, consistent with STM observations.
• The CDW phase is stabilized by the intertwining of magnetic exchange coupling and electron correlation effects, with no significant Fe displacement.
• The CDW enhances the magnetic moment of Fe atoms in the kagome layers, indicating strong electronic correlation effects.
• The predicted 2×2×1 CDW structure matches experimental STM data on atomic topography and electronic density of states.
• The study identifies metastable 1×√3×1 and 2×2√3×1 CDW phases, providing theoretical targets for future experimental validation.