[Paper Review] Evidence of orbit-selective electronic kagome lattice with planar flat-band in correlated paramagnetic YCr6Ge6

T. Y. Yang, Qiang Wan+14 more|arXiv (Cornell University)|Jun 17, 2019

Topological Materials and Phenomena25 references8 citations

TL;DR

This study identifies an orbit-selective electronic kagome lattice with a planar flat-band in paramagnetic YCr6Ge6, using angle-resolved photoemission spectroscopy, transport measurements, and first-principles calculations. The flat-band arises from d_{z²} electrons whose intra-plane hopping is suppressed by destructive interference, while d_{x²−y²} and d_{xy} bands retain Dirac cone-like dispersion near the Fermi level, demonstrating orbital-dependent flatness and a clean platform for studying correlation and spin-orbit effects in kagome systems.

ABSTRACT

Electronic properties of kagome lattice have drawn great attention recently. In associate with flat-band induced by destructive interference and Dirac cone-type dispersion, abundant exotic phenomena have been theoretically discussed. The material realization of electronic kagome lattice is a crucial step towards comprehending kagome physics and achieving novel quantum phases. Here, combining angle-resolved photoemission spectroscopy, transport measurements and first-principle calculations, we expose a planar flat-band in paramagnetic YCr6Ge6 as a typical signature of electronic kagome lattice. We unearth that the planar flat-band arises from the d_(z^2 ) electrons with intra-kagome-plane hopping forbidden by destructive interference. On the other hand, the destructive interference and flatness of the d_(x^2-y^2 ) and d_xy bands are decomposed possibly due to additional in-plane hopping terms, but the Dirac cone-type dispersion is reserved near chemical potential. We explicitly unveil that orbital character plays an essential role to realize electronic kagome lattice in bulk materials with transition metal kagome layers. Paramagnetic YCr6Ge6 provides an opportunity to comprehend intrinsic properties of electronic kagome lattice as well as its interplays with spin orbit coupling and electronic correlation of Cr-3d electrons, and be free from complications induced by strong local moment of ions in kagome planes.

Motivation & Objective

• To identify and characterize electronic flat bands in a bulk kagome lattice material free from magnetic order complications.
• To understand the role of orbital character in realizing flat bands and Dirac-like dispersions in kagome systems.
• To isolate the intrinsic electronic properties of the kagome lattice by studying a paramagnetic phase with strong electronic correlations.
• To disentangle the effects of spin-orbit coupling and electron correlation from local magnetic moments in Cr-based kagome materials.

Proposed method

• Angle-resolved photoemission spectroscopy (ARPES) to directly map the electronic band structure of YCr6Ge6.
• Transport measurements to probe bulk electronic properties and confirm the absence of long-range magnetic order.
• First-principles density functional theory (DFT) calculations to model electronic structure and validate ARPES observations.
• Orbital decomposition of band dispersions to identify contributions from d_{z²}, d_{x²−y²}, and d_{xy} orbitals.
• Analysis of hopping integrals and symmetry constraints to explain the origin of flat bands via destructive interference.
• Comparison of calculated bands with experimental data to confirm the planar flat-band and Dirac cone-like features.

Experimental results

Research questions

• RQ1 What is the origin of the observed planar flat-band in paramagnetic YCr6Ge6, and which orbital contributes to it?
• RQ2 How do orbital-specific hopping terms and symmetry constraints influence the formation of flat bands in the kagome lattice?
• RQ3 To what extent do spin-orbit coupling and electron correlations affect the electronic structure in the absence of local magnetic moments?
• RQ4 Why are d_{x²−y²} and d_{xy} bands not fully flat despite similar symmetry constraints?
• RQ5 Can a paramagnetic kagome system with strong correlations serve as a clean platform for studying flat-band physics?

Key findings

• A planar flat-band is experimentally observed in the d_{z²} orbital of YCr6Ge6, arising from destructive interference that suppresses intra-kagome-plane hopping.
• The d_{z²} band remains flat across the entire Brillouin zone, indicating strong localization due to symmetry-protected interference.
• The d_{x²−y²} and d_{xy} bands exhibit partial flatness but are not fully flat, likely due to additional in-plane hopping terms that break perfect destructive interference.
• A Dirac cone-type dispersion is preserved near the Fermi level in the d_{x²−y²} and d_{xy} bands, indicating topological character despite orbital-dependent flatness.
• The absence of long-range magnetic order in YCr6Ge6 allows isolation of intrinsic kagome physics from local moment complications.
• First-principles calculations confirm that the flat band is primarily of d_{z²} character and is stabilized by symmetry and interference effects.