NSRRC Activity Report 2023

Physics and Materials Science 011 L aCoO 3 has long been considered a prototype of the spin-state transition seen in several transition- metal compounds. It is known to exhibit a paramagnetic insulating state at low temperatures, undergoing an insulator-to-metal transition at around 450–550 K. However, its temperature-dependent electronic structure has remained an open question for more than 50 years. Now, in an international collaboration of researchers from Germany, Japan, and Taiwan, this long-standing question has been carefully answered using the techniques of bulk-sensitive hard X-ray photoelectron spectroscopy (HAXPES) and soft X-ray absorption spectroscopy (XAS) carried out as a function of temperature (T) across the insulator-to-metal transition. 1 In combination with full atomic-multiplet configuration- interaction cluster model charge-transfer calculations, the authors could clearly show that the low temperature insulating nearly pure (90%) low-spin (LS) t 2g 6 state transforms into a mixed spin-state bad metal with gradually increasing high-spin (HS) t 2g 4 e g 2 state admixture ( Fig. 1 ), thus clarifying the electronic structure of this unusual perovskite oxide. The authors first carried out bulk-sensitive T-dependent HAXPES at the Max-Planck-NSRRC HAXPES endstation at the Taiwan undulator beamline SP 12U1 at SPring-8, Japan. Figure 2 shows the low-T (80 K) and high-T (650 K) Co 2 p core level HAXPES spectra compared with full-multiplet configuration-interaction cluster calculations of the LS and HS Co 2 p photoemission spectra of LaCoO 3 . The low-T (80 K) experimental spectrum was successfully reproduced as a sum of the calculated spectra consisting of 90% LS and 10% HS states. Furthermore, the high-T (650 K) experimental spectrum was successfully reproduced as a sum of the calculated spectra consisting of 50% LS and 50% HS states. Next, the authors obtained T-dependent valence band spectra using HAXPES, as shown in Fig. 3 (see sext page), top left panel. From this, it is seen that the Co 3 d states at a 0.8 eV binding energy is systematically suppressed with increasing temperature. Interestingly, the intensity at the Fermi level remains small at all temperatures, including at 650 K, where LaCoO 3 is deep in the metallic phase. This indicates that LaCoO 3 should be classified as a bad metal at high temperatures. As shown in the bottom left panel, the configuration-interaction calculations of the Co 3 d photoemission can be suitably reproduced using the same electronic parameters. LaCoO 3 : A Mixed Spin-State Unusual Paramagnet Using temperature-dependent bulk-sensitive hard X-ray photoelectron and soft X-ray absorption spectroscopies from 80 to 650 K, it is shown that LaCoO 3 transforms from a low-spin insulator to a mixed-spin-state bad metal. Fig. 1 : Local ionic Co 3+ 3d charge density in the CoO 6 octahedron: LS with the t 2g 6 configuration (left) and HS with the t 2g 4 e g 2 (right) configuration. The electron density for t 2g is shown in blue, and that for e g is shown in yellow. The red dots indicate the positions of the oxygens (faint red dots on the right denote the LS positions). [Reproduced from Ref. 1] Fig. 2 : Full-multiplet configuration-interaction cluster calculations of the (a) LS and (f) HS Co 2 p photoemission spectra of LaCoO 3 . Co 2 p HAXPES experimental spectra at (b) 80 K (e) 650 K. Theoretical simulation of the Co 2 p spectra at (c) 80 K and (d) 650 K using an incoherent sum of the calculated LS and HS spectra with a ratio of 90% LS and 10% HS or 50% LS and 50% HS, respectively. The spectra are normalized to the integrated total intensity. The thin lines represent calculations with reduced broadening of the LS and HS components, with overall intensity divided by a factor of 5 for clarity. [Reproduced from Ref. 1]