Press Release

A key to interfacial conductivity revealed

July 21,2011

Osaka University
High Energy Accelerator Research Organization (KEK)

Physical properties that emerge only at interfaces have long been the focus of great interest. A classic example is the semiconductor diode, and scientists are still examining interfaces to find interesting behavior that might lead to more new types of useful devices. The interfaces between transition metal oxides appear promising because of the wide variety of properties of metal oxides and the availability of highly precise film fabrication techniques.

In 2004, the interface between the insulators SrTiO₃ and LaAlO₃ produced a big surprise. Although both materials are insulators, their interface can be conductive. Moreover, one can control the conductivity by changing the thickness of the LaAlO₃ (a conductive interface requires at least four-unit-cell-thick LaAlO₃) or the surface termination of the SrTiO₃ (only films made with TiO₂-terminated SrTiO₃ substrates show metallic conductivity). Since the anomalous conductivity is controlled by the interfacial structure, clarifying that structure is the key to understanding the conductivity. X-ray diffraction is a very useful tool to determine the interfacial structure with extremely high resolution, and the structures of films with various thicknesses of LaAlO₃ have just recently been studied using the Swiss Light Source[1].

Scientists from universities in Japan and the US (Osaka University, Tokyo University, and Stanford) compared the structures of a metallic interface, an interface made with TiO₂-terminated SrTiO₃, and an insulating interface made with SrO-terminated SrTiO₃, to clarify the key features of metallic conduction[2]. Using the synchrotron BL-3A at the Photon Factory, Japan, the atomic displacements were examined in great detail with modern holographic analysis.

Significant polarization was found only in the SrTiO₃ in the conductive sample. In addition, based on the observed suppression of polarization in the insulating sample, a significant cation deficiency was derived. This cation deficiency can be a factor that affects conductivity. These structural differences were found to be the keys to conductivity control. Such knowledge will help in the design of metal-oxide based electronic devices.

[ References ]
[1] S.A. Pauli, S.J. Leake, B. Delley, M. Bjorck, C.W. Schneider, C.M. Schleputz, D. Martoccia, S. Paetel, J. Mannhart, and P.R. Willmott, Phys. Rev. Lett. 106, 036101 (2011).
[2] R.Yamamoto, C. Bell, Y. Hikita, H.Y. Hwang, H. Nakamura, T.Kimura, and Y. Wakabayashi, Phys. Rev. Lett. 107, 036104 (2011).