Prominent electron-hole asymmetry in thermoelectric transport of LaCoO3

We have measured the electrical resistivity and the thermopower of the\nelectron-doped perovskite cobaltites LaCo$_{1-y}$Te$_y$O$_3$. In contrast to\nthe hole-doped systems such as metallic ferromagnets La$_{1-x}M_x$CoO$_3$ ($M$\n= Ca, Sr, Ba), the electron-doped samples show an insulating behavior even in a\nheavily doped range due to a spin-state blockade mechanism that an electron\nhopping from high-spin Co$^{2+}$ to low-spin Co$^{3+}$ site is energetically\nsuppressed. We find that, despite the electron doping, the thermopower shows\nrelatively large positive values above $y=0.05$, strikingly distinct from the\nhole-doped case where it comes close to zero with doping. This prominent\nelectron-hole asymmetry seen in the thermopower originates from a bipolar\nconduction which consists of a slight amount of mobile holes and the main\nimmobile electrons, demonstrating an impact of spin-state blockade on\nthermoelectric transport.\n