This is a copy of the slides presented at the meeting but not formally written up for the volume.

MBE growth of transition-metal oxide thin films using the pulsed laser deposition (PLD) technique has opened up new opportunities in the electronic structure studies of correlated systems using photoemission spectroscopy [1]. In this talk, I will present the results of photoemission studies on interfaces between a band insulator and a Mott insulator. In SrTiO3/LaTiO3 superlattices [2], metallic states were observed and systematic spectral weight transfer between the coherent part and incoherent part of the Ti 3d band was found to take place [3] as inferred from the electron-energy-loss spectroscopy study [4] and predicted theoretically [5]. For TiO2/VO2 interfaces, on the contrary, no metallic state was observed when VO2 was in the low-temperature insulating phase. The different behaviours are attributed to the absence of polarity discontinuity at the interface:electronic reconstruction;occurs at the polarity-discontinuous SrTiO3/LaTiO3 interface and does not at the polarity-continuous TiO2/VO2 interface. In entirely polar heterostructures LaAlO3/LaTiO3 [6] and LaAlO3/LaVO3 [7], soft x-ray and hard x-ray core-level photoemission measurements revealed that the Ti and V valences change at the topmost layers of the interfaces, suggesting electronic reconstruction competing with chemical reconstruction. This work was done in collaboration with M. Takizawa, H. Wadati, K. Maekawa, A. Chikamatsu, S. Minohara, A. Maniwa, H. Kumigashira, M. Oshima, M. Kawasaki, H. Koinuma, Y. Takata, K. Horiba, M. Matsunami, S. Shin, M. Yabashi, K. Tamasaku, Y. Nishino, D. Miwa, T. Isikawa, K. Shibuya, M. Lippmaa, Y. Nagao, Y. Muraoka, Z. Hiroi, Y. Hotta, T. Susaki, H.Y. Hwang, G.W.J. Hassink, and G. Rijnders. References [1] K. Horiba et al., Rev. Sci. Instr. 74, 3406 (2003). [2] K. Shibuya et al., Jpn. J. Appl. Phys. 43, L1178 (2004). [3] M. Takizawa et al., Phys. Rev. Lett.; 97, 057601 (2006). [4] A. Ohtomo, D.A. Muller, J.L. Grazul and H.Y. Hwang, Nature 419, 378 (2002). [5] S. Okamoto and A.J. Millis, Nature 428, 630 (2004); Phys. Rev. B 70, 241104 (2004). [6] G.W.J. Hassink et al., to be submitted. [7] Y. Hotta et al., submitted to Appl. Phys. Lett.