Electronic structure and polar catastrophe at the surface of LixCoO2 studied by angle-resolved photoemission spectroscopy

Abstract

We report an angle-resolved photoemission spectroscopy (ARPES) study of ${\mathrm{Li}}_{x}{\mathrm{CoO}}_{2}$ single crystals which have a hole-doped ${\mathrm{CoO}}_{2}$ triangular lattice. Similar to ${\mathrm{Na}}_{x}{\mathrm{CoO}}_{2}$, the Co $3d\phantom{\rule{4pt}{0ex}}{a}_{1g}$ band crosses the Fermi level with strongly renormalized band dispersion while the Co $3d\phantom{\rule{4pt}{0ex}}{e}_{g}^{\ensuremath{'}}$ bands are fully occupied in ${\mathrm{Li}}_{x}{\mathrm{CoO}}_{2}$ ($x=0.46$ and 0.71). At $x=0.46$, the Fermi surface area is consistent with the bulk hole concentration indicating that the ARPES result represents the bulk electronic structure. On the other hand, at $x=0.71$, the Fermi surface area is larger than the expectation which can be associated with the inhomogeneous distribution of Li reported in the previous scanning tunneling microscopy study by Iwaya et al. [Phys. Rev. Lett. 111, 126104 (2013)]. However, the Co $3d$ peak is systematically shifted towards the Fermi level with hole doping excluding phase separation between hole rich and hole poor regions in the bulk. Therefore, the deviation of the Fermi surface area at $x=0.71$ can be attributed to hole redistribution at the surface avoiding polar catastrophe. The bulk Fermi surface of Co $3d\phantom{\rule{4pt}{0ex}}{a}_{1g}$ is very robust around $x=0.5$ even in the topmost ${\mathrm{CoO}}_{2}$ layer due to the absence of the polar catastrophe.