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

We establish with angle-resolved photoemission spectroscopy (ARPES) that the electronic structure near the Fermi level of the "misfit" cobaltate [Bi2 Ba2 O4]·[Co O2] ∼2 is similar to that of Na cobaltates Nax Co O2. 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 EF 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.