Topological flat bands in frustrated kagome lattice CoSn (2002.01452v1)

Abstract: Electronic flat bands in momentum space, arising from strong localization of electrons in real space, are an ideal stage to realize strong correlation phenomena. In certain lattices with built-in geometrical frustration, electronic confinement and flat bands can naturally arise from the destructive interference of electronic hopping pathways. Such lattice-borne flat bands are often endowed with nontrivial topology if combined with spin-orbit coupling, while their experimental realization in condensed matter system has been elusive so far. Here, we report the direct observation of topological flat bands in the vicinity of the Fermi level in frustrated kagome system CoSn, using angle-resolved photoemission spectroscopy and band structure calculations. The flat band manifests itself as a dispersionless electronic excitation along the G-M high symmetry direction, with an order of magnitude lower bandwidth (below 150 meV) compared to the Dirac bands originating from the same orbitals. The frustration-driven nature of the flat band is directly confirmed by the real-space chiral d-orbital texture of the corresponding effective Wannier wave functions. Spin-orbit coupling opens a large gap of 80 meV at the quadratic band touching point between the Dirac and flat bands, endowing a nonzero Z2 topological invariant to the flat band in the two-dimensional Brillouin zone. Our observation of lattice-driven topological flat band opens a promising route to engineer novel emergent phases of matter at the crossroad between strong correlation physics and electronic topology.

• The paper demonstrates that flat bands in CoSn emerge with suppressed dispersion (≤150 meV) due to quantum interference in its frustrated kagome lattice.
• Utilizing ARPES and DFT, the study confirms a nontrivial Z2 invariant with an 80 meV SOC-induced band gap at the quadratic touching point with the Dirac band.
• Density functional analysis uncovers distinct d-orbital interactions with rapid charge density decay, emphasizing localization and potential for exotic quantum phases.

Analyzing Topological Flat Bands in Frustrated Kagome Lattice CoSn

The paper "Topological flat bands in frustrated kagome lattice CoSn" offers a comprehensive exploration of the existence and characteristics of topological flat bands within the kagome lattice structure of CoSn. Utilizing a combination of angle-resolved photoemission spectroscopy (ARPES) and band structure calculations, the authors present significant findings that may have far-reaching implications in the paper of correlated electron phenomena and topological insulators.

The paper focuses on CoSn, a binary kagome metal, as an ideal candidate for exploring flat bands due to its frustrated kagome geometry comprising transition metals. Notably, the researchers observed the presence of flat bands with considerably suppressed dispersion near the Fermi level along all momentum space directions. This suppression, with the bandwidth not exceeding 150 meV, is largely attributed to quantum interference effects inherent in the kagome lattice, effectively preventing electron delocalization and thus enhancing many-body interactions.

A defining aspect of this work is the confirmation of a topological nature associated with the kagome lattice bands. Through the inclusion of spin-orbit coupling (SOC) in their theoretical models, the authors identify a large SOC-induced band gap of 80 meV at the quadratic touching point with the Dirac band. This endows the flat bands with a nontrivial Z2 topological invariant—an essential characteristic for the realization of novel electronic phases such as fractional quantum Hall states.

The research also explores the orbital characteristics using density functional theory (DFT). The two observed flat bands originate from distinct d-orbital interactions, with unique chiral textures confirming the flat band's frustration-driven nature. This meticulous analysis reveals that the flat band Wannier functions exhibit rapid charge density decay from a central hexagon, emphasizing localization and the potential for emergent quantum phases.

From a fundamental science perspective, understanding the localization and the interplay of competing interactions in such architecturally intriguing structures can pave the way for theoretical and experimental advancements in realizing exotic states of matter like superconductivity and magnetism. The implications of this paper reach into the field of material science with potential applications in quantum computing and electronic devices relying on topologically protected states.

Looking ahead, the paper suggests potential methods to further investigate these phenomena through experimental adjustments such as doping or strain application. Such routes could further manipulate electronic properties, bringing lattice-borne correlated topological phases closer to practical realization.

In conclusion, this paper not only confirms the theoretical predictions of topological flat bands in kagome lattice systems but also bridges the gap between theoretical physics and experimental observation, providing a fertile ground for future studies on topological materials and correlated electron systems.