Spin state evolution induced by hole doping in La_2-xSr_xCoO₄

Jun Okamoto^1*, Ashish Chainani¹, Zhi-Ying Chen², Hsiao-Yu Huang¹, Amol Singh¹, Wen-Bin Wu¹, Takao Sasagawa³, Chien-Te Chen¹, Di-Jing Huang^1,2

¹National Synchrotron Radiation Research Center, Hsinchu, Taiwan
²Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
³Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama, Kanagawa, Japan

* Presenter:Jun Okamoto, email:okamoto.jun@nsrrc.org.tw

K₂NiF₄-type antiferromagnetic Mott insulators La₂TMO₄ (TM = 3d transition metal) show various novel physical properties by doping holes, like high T_C superconductivity [1]. La_2-xSr_xCoO₄ shows checkerboard-type charge ordering of Co²⁺ and Co³⁺ ions in 1/3 < x ≤ 0.6 and incommensurate antiferromagnetic ordering controlled by antiferromagnetic exchange interaction between Co²⁺ high spin (HS) in 1/3 < x ≤ 0.9 [2]. As doping holes in La_2-xSr_xCoO₄, Co²⁺ HS is replaced by Co³⁺, but Co³⁺ may take low-spin (LS) or HS states. Information about Co³⁺ spin state with hole doping is important to understand the phase diagram of charge and magnetic ordering in La_2-xSr_xCoO₄. We have measured Co L₃-edge resonant inelastic X-ray scattering (RIXS) of La_2-xSr_xCoO₄ (x =0.5, 0.7, and 0.8) to investigate the change of local electronic structures of Co²⁺ and Co³⁺ with different hole-doping. We have anlyzed changes of RIXS structures originated from Co²⁺ HS and Co³⁺ LS states as Sr doping increases from 0.5 to 0.8, and revealed that splitting of t_2g orbitals of Co³⁺ increases while that of Co²⁺ decreases, and HS ratio in Co³⁺ increases at the same time. This result means that Co³⁺ spin-state evolution is induced by hole doping through the increase of t_2g splitting in Co³⁺ electronic structure.

References　
[1]: J. G. Bednorz and K. A. Müller, Z. Phys. B 64, 189 (1986).　
[2]: Z. Li et al., Sci, Rep. 6, 25117 (2016)

Keywords: strongly correlated electron systems, Resonant Inelasitc X-ray Scattering, Cobaltates, Spin state