Thermoelectric Power of ${\mathrm{H}\mathrm{o}\mathrm{B}\mathrm{a}\mathrm{C}\mathrm{o}}_2{\mathrm{O}}_{5.5}$: Possible Evidence of the Spin Blockade in Cobaltites

Thermoelectric power measurements have been performed for an ordered oxygen-deficient perovskite, ${\mathrm{H}\mathrm{o}\mathrm{B}\mathrm{a}\mathrm{C}\mathrm{o}}_2{\mathrm{O}}_{5.5}$, in which the alternative layers of ${\mathrm{C}\mathrm{o}\mathrm{O}}_6$ octahedra and of $[{\mathrm{C}\mathrm{o}\mathrm{O}}_5{]}_2$ bipyramids are occupied by ${\mathrm{C}\mathrm{o}}^{3+}$ species. The $T$-dependent Seebeck coefficient $S$ shows a clear change of the conduction regime at the metal-insulator (MI) transition ($T_{\mathrm{M}\mathrm{I}}\sim 285\mathrm{K}$). The sign change of $S$ from $S<0$ to $S>0$ can be explained assuming that a spin state transition occurs at $T_{\mathrm{M}\mathrm{I}}$. In the metallic state, ${\mathrm{C}\mathrm{o}}^{2+}$ $e_g$ electrons are moving in a broad band on the background of high or intermediate spin ${\mathrm{C}\mathrm{o}}^{3+}$ species. In contrast, the insulating behavior may result from the ${\mathrm{C}\mathrm{o}}^{3+}$ spin state transition to a low-spin ${\mathrm{C}\mathrm{o}}^{3+}$ occurring in the octahedra. In this phase the transport would occur by hopping of the low-spin ${\mathrm{C}\mathrm{o}}^{4+}{t}_{2g}$ holes, whereas the high-spin ${\mathrm{C}\mathrm{o}}^{2+}$ electrons become immobilized due to a spin blockade.

Figure 1

Temperature dependence of the resistivity ($\rho$) for ${\mathrm{H}\mathrm{o}\mathrm{B}\mathrm{a}\mathrm{C}\mathrm{o}}_2{\mathrm{O}}_{5.5}$. The “1” and “2” arrows are for the cooling and warming curves, respectively. Inset: drawing of the structure showing the planes of ${\mathrm{C}\mathrm{o}\mathrm{O}}_6$ octahedra and double rows of ${\mathrm{C}\mathrm{o}\mathrm{O}}_5$ pyramids.

Figure 2

Left axis: $\mathrm{a}\mathrm{c}\mathrm{\text{−}}\chi \left(T\right)$ curve of ${\mathrm{H}\mathrm{o}\mathrm{B}\mathrm{a}\mathrm{C}\mathrm{o}}_2{\mathrm{O}}_{5.5}$ (real part ${\chi }^{\prime }$) cooling curve (solid line); right axis: ${\chi }^{\prime -1}\left(T\right)$ curve obtained from the cooling ${\chi }^{\prime }\left(T\right)$ curve. $H_{\mathrm{a}\mathrm{c}}=10\mathrm{O}\mathrm{e}$ and $f={10}^3\mathrm{H}\mathrm{z}$.

Figure 3

Temperature dependent Seebeck coefficient $S$ measured upon cooling and warming. Inset: comparison of $S\left(T\right)$ curves collected upon cooling in 0 T and then 7 T.

Figure 4

The scheme showing the processes of electron (a),(b) and hole (c) hopping on the background of the HS and IS ${\mathrm{C}\mathrm{o}}^{3+}$ (a) and LS ${\mathrm{C}\mathrm{o}}^{3+}$ (b),(c). (b) illustrates the phenomenon of a spin blockade. (a) applies to the high-temperature metallic phase of ${\mathrm{H}\mathrm{o}\mathrm{B}\mathrm{a}\mathrm{C}\mathrm{o}}_2{\mathrm{O}}_{5.5}$, whereas (c) corresponds to the processes available at the low temperature insulating phase.