Spin Blockade, Orbital Occupation, and Charge Ordering in La_1.5Sr_0.5CoO₄

Using Co-L_2,3 and O-K x-ray absorption spectroscopy, we reveal that the charge ordering in La_1.5Sr_0.5CoO₄ involves high spin (S=3/2) Co²⁺ and low spin (S=0) Co³⁺ ions. This provides evidence for the spin-blockade phenomenon as a source for the extremely insulating nature of the La_2-xSr_xCoO₄ series. The associated e_g² and e_g⁰ 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) Co³⁺ ions. From the identification of the bands we infer that La_1.5Sr_0.5CoO₄ is a narrow band material.