Spin blockade, orbital occupation, and charge ordering in La1.5Sr0.5CoO4.

Using Co-L 2,3 and O-K x-ray absorption spectroscopy, we reveal that the charge ordering in La 1.5 Sr 0.5 CoO4 involves high spin (S = 3/2) Co 2+ and low spin (S=0) Co 3+ ions. This provides evidence for the spin-blockade phenomenon as a source for the extremely insulating nature of the La 2-x Sr x CoO 4 series. The associated e 2 g and eg orbital occupation accounts for the large contrast in the Co-O bond lengths and, in turn, the high charge ordering temperature. Yet, the low magnetic ordering temperature is naturally explained by the presence of the nonmagnetic (S = 0) Co 3+ ions. From the identification of the bands we infer that La 1.5 Sr 0.5 CoO 4 is a narrow band material.