Spin Blockade, Orbital Occupation, and Charge Ordering in ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{CoO}}_4$

Using $\mathrm{Co}\mathrm{\text{−}}{L}_{2,3}$ and $\mathrm{O}\mathrm{\text{−}}K$ x-ray absorption spectroscopy, we reveal that the charge ordering in ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{CoO}}_4$ involves high spin ($S=3/2$) ${\mathrm{Co}}^{2+}$ and low spin ($S=0$) ${\mathrm{Co}}^{3+}$ ions. This provides evidence for the spin-blockade phenomenon as a source for the extremely insulating nature of the ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CoO}}_4$ 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 ($S=0$) ${\mathrm{Co}}^{3+}$ ions. From the identification of the bands we infer that ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{CoO}}_4$ is a narrow band material.

Figure 1

Isotropic $\mathrm{Co}\mathrm{\text{−}}{L}_{2,3}$ XAS spectra of (a) ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{CoO}}_4$, (b) the HS-CoO plus $\mathrm{LS}\mathrm{\text{−}}{\mathrm{EuCoO}}_3$ scenario, (c) the HS-CoO plus $\mathrm{HS}\mathrm{\text{−}}{\mathrm{Sr}}_2{\mathrm{CoO}}_3\mathrm{Cl}$ scenario, (d) ${\mathrm{Sr}}_2{\mathrm{CoO}}_3\mathrm{Cl}$, (e) ${\mathrm{EuCoO}}_3$, and (f) CoO, all taken at 300 K.

Figure 2

(a) Experimental polarization-dependent $\mathrm{Co}\mathrm{\text{−}}{L}_{2,3}$ XAS of ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{CoO}}_4$ for $\mathbf{E}\parallel \mathbf{c}$ (red line) and $\mathbf{E}\perp \mathbf{c}$ (black dotted line); (b), (c), and (d) the experimental linear dichroism (magenta dotted lines) together with the theoretical one (black lines) calculated for scenarios with $\mathrm{HS}\mathrm{\text{−}}{\mathrm{Co}}^{2+}$ and LS-, HS-, or $\mathrm{IS}\mathrm{\text{−}}{\mathrm{Co}}^{3+}$, respectively. The calculated linear dichroism for the $\mathrm{HS}\mathrm{\text{−}}{\mathrm{Co}}^{2+}$, LS-, HS-, and $\mathrm{IS}\mathrm{\text{−}}{\mathrm{Co}}^{3+}$ is also shown individually in (e), (f), (g), and (h).

Figure 3

(a) Isotropic $\mathrm{O}\mathrm{\text{−}}K$ XAS of ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{CoO}}_4$ taken at 18 K (blue dotted line) and 300 K (black solid line), together with those of ${\mathrm{EuCoO}}_3$ (LS) and CoO. (b) Polarization-dependent $\mathrm{O}\mathrm{\text{−}}K$ XAS, taken with $\mathbf{E}\parallel \mathbf{c}$ (solid line) and $\mathbf{E}\perp \mathbf{c}$ (dotted line), together with the $\mathrm{LDA}+U$ unoccupied O $2p$ density of states (blue curves).