Spin blockade, orbital occupation and charge ordering in La_(1.5)Sr_(0.5)CoO4

TL;DR

This study uses x-ray absorption spectroscopy to uncover charge and spin states in La_(1.5)Sr_(0.5)CoO4, revealing spin blockade as a key factor in its insulating behavior and detailing orbital occupations and magnetic properties.

Contribution

The paper provides direct spectroscopic evidence for high spin Co^2+ and low spin Co^3+ ions, elucidating the charge ordering mechanism and its impact on electronic and magnetic properties.

Findings

Charge ordering involves high spin Co^2+ and low spin Co^3+ ions.

Spin blockade explains the material's high insulating nature.

Orbital occupation differences account for bond length variations.

Abstract

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)CoO4 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 non-magnetic (S=0) Co^3+ ions. From the identification of the bands we infer that La_(1.5)Sr_(0.5)CoO4 is a narrow band material.