title: Spin blockade, orbital occupation, and charge ordering in La1.5Sr0.5CoO4.
abstract: Using Co-L2,3 and O-K x-ray absorption spectroscopy, we reveal that the charge ordering in La1.5Sr0.5CoO4 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 La2-xSrxCoO4 series. The associated e{g}{2} and e{g}{0} 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 La1.5Sr0.5CoO4 is a narrow band material.
authors.affiliation: II. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany.
authors.forename: C F
authors.initials: CF
authors.lastname: Chang
authors.forename: Z
authors.initials: Z
authors.lastname: Hu
authors.forename: Hua
authors.initials: H
authors.lastname: Wu
authors.forename: T
authors.initials: T
authors.lastname: Burnus
authors.forename: N
authors.initials: N
authors.lastname: Hollmann
authors.forename: M
authors.initials: M
authors.lastname: Benomar
authors.forename: T
authors.initials: T
authors.lastname: Lorenz
authors.forename: A
authors.initials: A
authors.lastname: Tanaka
authors.forename: H-J
authors.initials: HJ
authors.lastname: Lin
authors.forename: H H
authors.initials: HH
authors.lastname: Hsieh
authors.forename: C T
authors.initials: CT
authors.lastname: Chen
authors.forename: L H
authors.initials: LH
authors.lastname: Tjeng