[Paper Review] Photoemission Evidence of a Novel Charge Order in Kagome Metal FeGe

Zhisheng Zhao, Tongrui Li+17 more|arXiv (Cornell University)|Aug 16, 2023

Topological Materials and PhenomenaPhysics and Astronomy3 citations

TL;DR

This study uses angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) to demonstrate that charge order (CO) in kagome metal FeGe is driven not by electronic nesting or Fermi surface nesting, but by a first-order structural transition involving large dimerization of Ge1 atoms along the c-axis, which lowers magnetic energy. The key finding is that a sudden upward shift in Ge-4p and Fe-3d bands at the A and K points, respectively, upon CO formation, provides direct evidence for this unconventional magnetic-driven CO mechanism.

ABSTRACT

A charge order has been discovered to emerge deep into the antiferromagnetic phase of the kagome metal FeGe. To study its origin, the evolution of the low-lying electronic structure across the charge order phase transition is investigated with angle-resolved photoemission spectroscopy. We do not find signatures of nesting between Fermi surface sections or van-Hove singularities in zero-frequency joint density of states, and there are no obvious energy gaps at the Fermi level, which exclude the nesting mechanism for the charge order formation in FeGe. However, two obvious changes in the band structure have been detected, i.e., one electron-like band around the K point and another one around the A point move upward in energy position when the charge order forms. These features can be well reproduced by our density-functional theory calculations, where the charge order is primarily driven by magnetic energy saving via large dimerizations of a quarter of Ge1-sites (in the kagome plane) along the c-axis. Our results provide strong support for this novel charge order formation mechanism in FeGe, in contrast to the conventional nesting mechanism.

Motivation & Objective

• To resolve the mechanism behind the 2×2×2 charge order (CO) phase in kagome metal FeGe, which coexists with antiferromagnetism.
• To determine whether the CO is driven by electronic nesting or by structural distortions, particularly in the absence of clear energy gaps or van Hove singularities at the Fermi level.
• To identify the origin of the observed band structure changes across the CO transition using experimental and theoretical methods.
• To clarify the role of Ge1-site dimerization and its impact on magnetic and electronic energy stabilization in FeGe.

Proposed method

• Angle-resolved photoemission spectroscopy (ARPES) was used to map the electronic structure of FeGe across the charge order transition temperature (~100 K).
• Measurements were performed at various temperatures and photon energies to probe the Fermi surface, band dispersion, and joint density of states (JDOS) at zero frequency.
• DFT calculations were performed to model the 2×2×2 CO superstructure, focusing on atomic displacements, particularly Ge1-dimerization along the c-axis.
• Band structures were unfolded into a 1×1×1 primitive Brillouin zone to compare experimental ARPES data with theoretical predictions.
• Theoretical analysis focused on the energetic cost of structural distortions and the resulting spin polarization changes in Fe sites.
• Comparison between experimental ARPES spectra and DFT-predicted band structures was used to validate the proposed mechanism.

Experimental results

Research questions

• RQ1 Is the charge order in FeGe driven by Fermi surface nesting or van Hove singularity effects?
• RQ2 What is the origin of the observed band shifts in the electronic structure upon charge order formation?
• RQ3 Does the absence of a clear energy gap at the Fermi level rule out conventional charge density wave (CDW) mechanisms?
• RQ4 How does Ge1-site dimerization along the c-axis influence the electronic and magnetic ground state of FeGe?
• RQ5 Can the observed first-order-like band evolution be explained by a magnetic energy gain from structural distortion?

Key findings

• No evidence of Fermi surface nesting or van Hove singularities near the Fermi level was found, excluding a conventional CDW mechanism driven by electronic energy gain.
• A sudden upward shift in an electron-like band dominated by Ge-4p orbitals was observed at the A point in the Brillouin zone upon charge order formation, indicating a strong structural and electronic reorganization.
• A smaller but significant upward shift in an Fe-3d-dominated band near the K point was also detected, consistent with minor Fe-site distortions.
• DFT calculations confirmed that large dimerization of Ge1 atoms along the c-axis induces the charge order and lowers the system's magnetic energy, overcoming the cost of structural distortion.
• The observed band evolution is best explained by a first-order structural transition driven primarily by magnetic energy saving, not by electronic correlations or nesting.
• The results support a novel, unconventional charge order mechanism in FeGe, where magnetic energy minimization via Ge1-dimerization is the dominant driving force.