[Paper Review] Anomalous quasiparticles in the zone center electron pocket of the kagomé ferromagnet Fe3Sn2

Sandy Adhitia Ekahana, Yeong‐Ah Soh+15 more|arXiv (Cornell University)|Jun 28, 2022

Topological Materials and Phenomena38 references4 citations

TL;DR

This study uses microfocused laser-based angle-resolved photoemission spectroscopy (micro-ARPES) to resolve sharply defined quasiparticles in the zone-center electron pocket of the kagome ferromagnet Fe3Sn2, revealing three-fold symmetric electron pockets and anomalously long quasiparticle mean free paths. The findings indicate strong electron-electron interactions linked to a flat band just above the Fermi level, unaccounted for in existing theories of metallic kagome ferromagnets.

ABSTRACT

One material containing kagome bilayers and featuring both exceptional magnetism and electron transport is the ferromagnetic metal Fe3Sn2. Notwithstanding the widespread interest in Fe3Sn2, crystal twinning, difficulties in distinguishing surface from bulk states, and a large unit cell have until now prevented the synchrotron-based spectroscopic observation of sharply resolved quasiparticles near the Fermi surface which could be responsible for the anomalous properties appearing at low temperatures for the material. Here we report microfocused laser-based angle-resolved photoemission spectroscopy (micro-ARPES), which offers the first look at such quasiparticles. The high spatial resolution allows individual crystal twin domains to be examined in isolation, resulting in the discovery of three-fold symmetric electron pockets at the Brillouin zone (BZ) center, not predicted by early tight-binding descriptions but in agreement with density functional theory (DFT) calculations, which also feature Weyl nodes. The quasiparticles in these pockets have remarkably long mean free paths, and their Fermi surface area is consistent with reported quantum oscillations. At the same time, though, the best-defined Fermi surface is reduced at low temperature, and the quasiparticles generally are marginal in the sense that their wavelength uncertainty is of order the deviation of the quasiparticle wavelength from the Fermi vector. We attribute these manifestations of strong electron-electron interactions to a flat band predicted by our DFT to lie just above the dispersive bands seen in this experiment. Thus, beyond demonstrating the impact of twin averaging for ARPES measurements of band structures, our experiments reveal many-body physics unaccounted for by current theories of metallic kagome ferromagnets.

Motivation & Objective

• To overcome limitations in conventional ARPES caused by crystal twinning and bulk-surface state mixing in Fe3Sn2.
• To resolve the electronic structure of the zone-center electron pocket in Fe3Sn2 with high spatial and energy resolution.
• To identify the origin of anomalous low-temperature properties in this kagome ferromagnet.
• To investigate the role of electron-electron interactions in shaping quasiparticle behavior in a metallic kagome system.

Proposed method

• Employed microfocused laser-based angle-resolved photoemission spectroscopy (micro-ARPES) to achieve high spatial resolution.
• Performed measurements on isolated twin domains to avoid averaging effects from crystal twinning.
• Used density functional theory (DFT) calculations to model the electronic band structure and validate experimental observations.
• Compared experimental Fermi surface area with quantum oscillation data to confirm quasiparticle consistency.
• Analyzed quasiparticle lifetime and spectral weight to assess many-body effects and interaction strength.
• Mapped the momentum-space dispersion and Fermi surface topology to identify three-fold symmetric electron pockets.

Experimental results

Research questions

• RQ1 What is the true electronic structure of the zone-center electron pocket in Fe3Sn2, free from twin-domain averaging?
• RQ2 Why do quasiparticles in Fe3Sn2 exhibit anomalously long mean free paths despite strong electron correlations?
• RQ3 How do the observed Fermi surface features relate to the reported quantum oscillations in this material?
• RQ4 What is the origin of marginal quasiparticle behavior with large wavelength uncertainty relative to the Fermi wavevector?
• RQ5 To what extent do flat bands near the Fermi level mediate strong electron-electron interactions in this kagome ferromagnet?

Key findings

• Three-fold symmetric electron pockets were observed at the Brillouin zone center, not predicted by early tight-binding models but consistent with DFT calculations.
• Quasiparticles in the electron pocket exhibit remarkably long mean free paths, indicating high coherence despite strong correlations.
• The Fermi surface area of the observed pocket matches that inferred from quantum oscillation measurements.
• The most well-defined Fermi surface feature is reduced at low temperatures, indicating a breakdown of quasiparticle coherence.
• Quasiparticles show marginal behavior, with wavelength uncertainty comparable to the deviation from the Fermi wavevector, signaling strong electron-electron interactions.
• A flat band predicted by DFT, located just above the dispersive bands, is identified as the likely origin of the anomalous many-body effects.