Abstract
First-principles calculations show the formation of a 2D spin polarized electron (hole) gas on the Li (CoO_22) terminated surfaces of finite slabs down to a monolayer, in remarkable contrast with the bulk band structure, which is stabilized by Li donating its electron to the CoO_22 layer forming a Co-d-t_{2g}^6d−t2g6 insulator. By mapping the first-principles computational results to a minimal tight-binding models corresponding to a non-chiral 3D generalization of the quadripartite Su-Schrieffer-Heeger (SSH4) model and symmetry analysis, we show that these surface states have topological origin.
Highlights
LiCoO2 has been mostly studied as cathode material in Li-ion batteries. [1,2,3] its layered structure lends itself to the possibility of extracting interesting ultrathin monoor few layers nanoflakes
Surprisingly that Li no longer fully donates its electron to the CoO2 layer as it does in the bulk but instead a surface state appears above the Li and is occupied with a fraction of an electron per Li forming a two-dimensional electron gas (2DEG)
We show that the density functional theory (DFT) calculations can be explained by a minimal tight-binding (TB) model, closely related to the quadripartite Su-Schrieffer-Heeger (SSH4) model which has been shown to support topologically protected surface states for specific conditions on the interatomic hopping integrals
Summary
LiCoO2 has been mostly studied as cathode material in Li-ion batteries. [1,2,3] its layered structure lends itself to the possibility of extracting interesting ultrathin monoor few layers nanoflakes. By replacing lithium by large organic ions, the distance between the layers swells and they can be exfoliated in solution and redeposited on a substrate of choice by precipitation with different salts Inspired by these experiments we investigated the electronic structure of LiCoO2 few layer systems with various Li and other ion terminations and as function of thickness of the layers using density functional theory (DFT) calculations, of which details are provided in Appendix A. The fact that a Li related surface state comes down sufficiently close to the Fermi level to become partially occupied is truly surprising Because it is accompanied by the opposite surface CoO2 becoming spin-polarized it leads to a spin-polarized electron gas on the Li side which is located primarily above the Li atoms. The reason for the Li bands to come down in energy near the Fermi energy is clearly related to the SSH4 topology
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