NSRRC Activity Report 2023

012 NSRRC ACTIVITY REPORT 2023 Finally, as shown in the top right panel, the authors carried out T-dependent XAS at the Dragon beamline TLS 11A1 at the NSRRC in Taiwan. The pre-edge region spectral features of the O-K XAS from 527 to 531 eV are due to transitions from the O 1 s core level to the O 2 p orbitals that are mixed with the unoccupied Co 3 d t 2g and e g states. A clear strong transfer of spectral weight is observed from the higher-energy structure at around 529.3 eV to the lower-energy structure at around 528.4 eV on increasing temperature from 10 to 650 K. These changes are due to a spin-state transition and can be reproduced suitably by configuration-interaction calculations of the inverse photoemission spectra, as shown in the bottom right panel. The authors conclude that at low temperatures, the LS Co 3+ ion has its t 2g shell completely occupied, 2 and only transitions to the higher-lying empty e g states are possible. In contrast, at higher temperatures, with a portion of the Co ions in the HS state, 2 the t 2g states become partially unoccupied, thereby allowing transitions to the lower-lying t 2g states, and this leads to the appearance of the 528.4 eV structure. In summary, the T-dependent photoemission and X-ray absorption spectra for bulk LaCoO 3 exhibit clear changes across the gradual spin-state and insulator-to-metal transitions. Reduction of the band gap was observed with increasing temperature, but the near-Fermi-level intensity remained small even in the metallic phase. The spectra could be explained quantitatively in terms of the sums of the LS and HS Co 3+ spectra. (Reported by Ashish Chainani) This report features the work of Liu-Hao Tjeng and his collaborators published in Phys. Rev. X 13 , 011037 (2023). TLS 11A1 (Dragon) MCD, XAS SP 12U1 HAXPES • HAXPES, XAS • Materials Science, Condensed-matter Physics References 1. D. Takegami, A. Tanaka, S. Agrestini , Z. Hu, J. Weinen, M. Rotter, C. SchÜ ß ler-Langeheine, T. Willers, T. C. Koethe , T. Lorenz , Y. F. Liao, K. D. Tsuei, H.-J. Lin, C. T. Chen, L. H. Tjeng, Phys. Rev. X 13 , 011037 (2023). 2. M. W. Haverkort, Z. Hu, J. C. Cezar, T. Burnus, H. Hartmann, M. Reuther, C. Zobel, T. Lorenz, A. Tanaka, N. B. Brookes, H. H. Hsieh, H.-J. Lin, C. T. Chen, L. H. Tjeng, Phys. Rev. Lett. 97 , 176405 (2006). Fig. 3 : The valence band HAXPES (top left) and the pre-edge region of the XAS (top right) spectra of LaCoO 3 at different temperatures. Bottom: Configuration-interaction calculations of the Co 3 d photoemission (PES, bottom left) and inverse photoemission spectra (IPES, bottom right) using 10 Dq = 0.75 eV for the LS state and 0.40 eV for the HS state. The HS calculations are performed with (solid line) and without (dotted line) the hybridization reduction to model the presence or absence, respectively, of local lattice relaxations. [Reproduced from Ref. 1]