Ordering Phenomena in Transition Metal Oxides

Electron correlation and electron-phonon interaction in perovskite oxides from ARPES data   

Transition-metal oxides ABO₃ with the perovskite structure show a variety of physical properties such as Fermi-liquid behavior, metal-insulator transition, colossal magneto-resistance and various types of spin-orbital-charge ordering. However, experimental information about their electronic structures has been limited compared to oxides with layered structures such as high-T_c cuprates primarily because angle-resolved photoemission spectroscopy (ARPES) experiment has been difficult to perform due to the lack of cleaved surfaces. Recently, ARPES experiment has become feasible when electrically neutral surfaces are exposed by cleaving or atomically flat surfaces are synthesized by molecular beam epitaxy using pulsed laser deposition technique. In this talk, ARPES results on perovskite-type Ti, V, Mn and Fe oxides are presented and analyzed based on first-principles LDA and empirical tight-binding band-structure calculations. Effects of electron correlation have been observed as a renormalization of the coherent quasi-particle (QP) band in metallic compounds, SrVO₃ (d¹) [1] and La_1-xSr_xMnO₃ (d^4-x) [2]. Not only the coherent part but also the incoherent are found to show significant band dispersion. Bandwidth control has been confirmed for SrVO₃ and CaVO₃ from ARPES data, as well as for La_0.6Sr_0.4MnO₃ and Pr_0.6Ca_0.4MnO₃ [3]. Effects of electron-phonon interaction have been observed as a polaronic shift of energy bands in insulating SrTiO₃ (d⁰) and La_1-xSr_xFeO₃ (d^5-x) [4] and as a broadening and weakening of the QP band near E_F in La_1-xSr_xMnO₃ [2].

This work has been done in collaboration with T. Yoshida, H. Eisaki, Z.-X. Shen, H. Wadati, M. Takizawa, A. Chikamatsu, M. Minohara, H. Kumigashira, M. Oshima, M. Lippmaa, M. Kawasaki and H. Koinuma and supported by JSPS and MEXT, Japan. Experiment has been done at Stanford Synchrotron Radiation Laboratory operated by the DOE’s Office of Basic Science and at Photon Factory, KEK.

[1] T. Yoshida et al., Phys. Rev. Lett. 95, 146404 (2005); M. Takizawa et al.,     submitted.

[2] A. Chikamatsu et al., Phys. Rev. B 73, 195105 (2006); 76, 201103 (2007).

[3] H. Wadati et al., Phys. Rev. Lett. 100, 026402 (2008).

[4] H. Wadati et al., Phys. Rev. B 74, 155114 (2006).