Direct observation of strong correlations near the band insulator regime of Bi misfit cobaltates

Abstract

We establish with angle-resolved photoemission spectroscopy (ARPES) that the electronic structure near the Fermi level of the ``misfit'' cobaltate $[{\mathrm{Bi}}_{2}{\mathrm{Ba}}_{2}{\mathrm{O}}_{4}]∙{[\mathrm{Co}{\mathrm{O}}_{2}]}_{\ensuremath{\sim}2}$ is similar to that of Na cobaltates ${\mathrm{Na}}_{x}\mathrm{Co}{\mathrm{O}}_{2}$. Both families of compounds contain triangular Co planes but in a different three-dimensional environment. We propose that the peculiar ARPES line shape of all cobaltates is of the ``peak-dip-hump'' type, due to strong many-body effects. We detect a progressive transfer of spectral weight from the quasiparticle feature near ${E}_{F}$ to a broad hump in misfit phases where Ba is replaced by Sr or Ca. This indicates stronger many-body interactions in proximity to the band insulator regime, which we attribute to the presence of unusual magnetic excitations.