KIAS- APCTP Workshop on "Quantum Materials"

at Korea Institute for Advanced Study (KIAS), Seoul, Korea

• Sung-Seok A. Seo
  
  Seoul National University
  
  

  Title

  Optical spectroscopic investigations on superlattices of a Mott insulator (LaTiO3) and a band insulator (SrTiO3)

  Abstract

  Recently, artificial superlattices of oxide materials have been attracting a lot of attention due to noble physical properties at their interfaces, which are unattainable in single-phase materials. Ohtomo et al. demonstrated an atomically abrupt interface between a 3d1 Mott insulator (LaTiO3) and a 3d0 band insulator (SrTiO3) experimentally [1]. The superlattice of LaTiO3 and SrTiO3 is an ideal model system to investigate interface properties, because there is a modulation of A-site ions without disturbing a network of transition metal ions and oxygen. Okamoto and Millis theoretically investigated the electronic state of the LaTiO3/SrTiO3 interface, and found that it is quite different from the bulk electronic state. In order to explain this difference, they proposed Ôthe electronic reconstructionÕ scheme at the interface [2]. However, since the interface is buried inside a material by its own definition, there have been little experimental studies to probe physical properties just coming from the interface. Especially, we need experimental methods, which can probe enough penetration depth to approach the interface effectively with a delicate analysis method to separate a signal from bulk states. Here, we used optical spectroscopic methods to investigate the oxide superlattices of (LaTiO3)n(SrTiO3)m (n,m: the number of layers), which were epitaxially grown by pulsed laser deposition equipped with reflection high energy electron diffraction for in-situ surface/interface monitoring. Optical spectroscopy is an indispensable tool to investigate physical properties of strongly correlated electron materials; here we extend its capability to investigate the interface states of the (LaTiO3)n(SrTiO3)m superlattice. We used both optical near-normal transmittance/reflectance techniques and infrared ellipsometry to obtain the optical spectra, and try to isolate the effects coming from the interface state. We observed high conducting states at an interface, in spite of the back that the LaTiO3 and SrTiO3 bulks are insulators. The conducting carrier concentration was also proportional to the number of the interfaces not to the La/Sr ionic ratio. It revealed that the conducting state was from the interface and not to the bulk, which partially supports the proposal of electronic reconstruction at the interface. We also investigated electrodynamics of the free carriers in the interface conducting states by measuring temperature-dependent spectra by infrared ellipsometry. We found out an unusual metallic behaviour that the conducting carrier density increased. This intriguing phenomenon could be attributed to the reduced screening of electrostatic potentials by increasing dielectric permittivity of SrTiO3 at low temperature. Other issues on the LaTiO3/SrTiO3 superlattice, such as 2D electron gas, roles of oxygen vacancies, and solid state diffusion of the La and Sr ions, will be also discussed. [1] A. Ohtomo, D. A. Muller, J. L. Grazul, and H. Y. Hwang, Nature 419, 378 (2002). [2] S. Okamoto and A. J. Millis, Nature 428, 630 (2004).