Spin and charge states of Co in half-doped layered cobaltates La1.5Ca0.5CoO4and La1.5Sr0.5CoO4

Abstract

We have investigated the charge dynamics and local spin state in half-doped layered cobaltates La${}_{1.5}$Ca${}_{0.5}$CoO${}_{4}$ and La${}_{1.5}$Sr${}_{0.5}$CoO${}_{4}$. By examining temperature-dependent infrared-active phonon modes, we found clear signatures of the structural distortion related to the spin-state transition around ${T}_{S}=400--600$ K. While the kinetic energy of the electronic system, estimated by the low-energy spectral weight in the optical conductivity spectrum, as well as the optical gap energy is strongly affected by the charge-order instability, it shows a noticeable enhancement upon the spin-state transition which is attributed to the release of the spin blockade of the electron hopping between the low-spin (Co${}^{3+}$) and high-spin (Co${}^{2+}$) states. These understandings of the spin state are further evidenced by the reexamination of high-temperature Seebeck coefficient and magnetic susceptibility.