Dirac fermions and flat bands in the ideal kagome metal FeSn (1906.02167v1)

Abstract: The kagome lattice based on 3d transition metals is a versatile platform for novel topological phases hosting symmetry-protected electronic excitations and exotic magnetic ground states. However, the paradigmatic states of the idealized two-dimensional (2D) kagome lattice - Dirac fermions and topological flat bands - have not been simultaneously observed, partly owing to the complex stacking structure of the kagome compounds studied to date. Here, we take the approach of examining FeSn, an antiferromagnetic single-layer kagome metal with spatially-decoupled kagome planes. Using polarization- and termination-dependent angle-resolved photoemission spectroscopy (ARPES), we detect the momentum-space signatures of coexisting flat bands and Dirac fermions in the vicinity of the Fermi energy. Intriguingly, when complemented with bulk-sensitive de Haas-van Alphen (dHvA) measurements, our data reveal an even richer electronic structure that exhibits robust surface Dirac fermions on specific crystalline terminations. Through band structure calculations and matrix element simulations, we demonstrate that the bulk Dirac bands arise from in-plane localized Fe-3d orbitals under kagome symmetry, while the surface state realizes a rare example of fully spin-polarized 2D Dirac fermions when combined with spin-layer locking in FeSn. These results highlight FeSn as a prototypical host for the emergent excitations of the kagome lattice. The prospect to harness these excitations for novel topological phases and spintronic devices is a frontier of great promise at the confluence of topology, magnetism, and strongly-correlated electron physics.

• The paper demonstrates the coexistence of bulk and surface Dirac fermions along with nondispersive flat bands in FeSn using ARPES and dHvA oscillations.
• It identifies dual surface terminations, with one Dirac cone showing complete spin polarization that signals potential for spintronic applications.
• DFT calculations assign specific orbital characters to the Dirac states, paving the way for engineering novel topological phases in kagome systems.

A Review of Dirac Fermions and Flat Bands in the Ideal Kagome Metal FeSn

Introduction

This paper investigates the ideal kagome metal FeSn, leveraging its unique lattice properties to explore Dirac fermions and flat bands. The kagome lattice, a two-dimensional grid of corner-sharing triangles, has drawn significant interest for its implications in frustration-driven spin-liquid phases and topological properties. Recent theoretical models suggest that the kagome lattice, compounded by spin-orbit coupling and net magnetization, can host Dirac bands, topological flat bands, and complex topological phases such as Chern insulators and Weyl semimetals. Until now, simultaneous observation of Dirac fermions and flat bands in bulk kagome systems has been challenging due to complex stacking structures.

Methods

Utilizing polarization- and termination-dependent angle-resolved photoemission spectroscopy (ARPES), this paper has identified momentum-space signatures of coexisting Dirac fermions and flat bands near the Fermi energy in FeSn. These findings are complemented by de Haas-van Alphen (dHvA) oscillations, revealing a robust electronic structure with both surface and bulk Dirac fermions. Density functional theory (DFT) calculations extend these findings, attributing specific orbital characters to different Dirac bands.

Results and Analysis

Dirac Fermions: The ARPES experiments reveal two potential surface terminations: kagome and Sn. Intriguingly, both terminations exhibit a consistent bulk Dirac band (DC1) with a linear dispersion at approximately -0.43 eV and a Dirac velocity of $1.7 \times 10^5$ m/s. This velocity is notably lower than graphene's, indicating higher electron correlations. A second Dirac cone (DC2) is observed only on the Sn termination, suggesting a surface state that holds potential for spintronic applications due to its complete spin polarization.

Flat Bands: The detection of nondispersive excitations indicates nearly flat bands in the kagome system. These flat bands are topologically significant and arise due to phase interference from the kagome lattice geometry. While the flat bands exist above the Fermi level and were not directly observable, indications from DFT point to their preservation across all momentum-space directions due to their in-plane d-orbital character.

Implications

The results underscore FeSn as a prototypical model for studying the electronic structure of kagome lattices in bulk, offering insight into both surface and bulk state phenomena. The combined presence of Dirac fermions and flat bands delineates a novel electronic landscape, presenting fertile ground for exploring topological phases. The paper implies several future research directions:

• Topological Phases: Given the robust manifestation of flat and Dirac bands, FeSn may be foundational in realizing fractional quantum Hall states without an external magnetic field.
• Spintronics: The full spin-polarization in surface states could enhance the development of high-speed, low-power spintronic devices.
• Theoretical Exploration: The importance of orbital characterization highlights pathways for engineering new materials with desired properties through careful manipulation of kagome lattice configurations.

Conclusion

This research presents a comprehensive investigation of FeSn, revealing the kagome lattice as a critical hub for advanced topological phenomena. The matrix of Dirac fermions and flat bands in FeSn not only demystifies the electronic interactions in this structure but also points to significant technological implications in spintronic and quantum devices. The detailed characterization provided by ARPES and dHvA experiments paves the way for future studies, targeting the nuanced relationship between electronic topology and lattice geometry in transition metal-based kagome systems.