Corralling electrons in 2D Quantum confinement of strongly correlated electrons in transition metal oxide artificial structures
The wide range of properties exhibited by transition metal oxides, including high-temperature superconductivity and photocatalysis, makes them one of the most interesting groups of materials in quantum nanoscience research today. Yet their extreme sensitivity to changes in temperature, pressure and doping makes them unpredictable and difficult to control: “like a bunch of unruly child prodigies” in the words of Assistant Professor Kumigashira of the Graduate School of Engineering (currently Professor at the Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK)).
The properties of these oxides arise from the state of their electrons. The mobile electrons enabling conductivity are bounded in the narrow 3d orbitals of transition metals. As a result, they strongly influence each other and cannot be regarded as free electrons: they are said to be “strongly correlated electrons.” Quantum confinement, or artificially trapping these electrons in two dimensions in what is called a quantum well, would give greater control over their properties.
Kumigashira’s group is the first to succeed in the quantum confinement of strongly correlated electrons in an oxide artificial structure.
They grew a film composed of monolayers of SrVO3, which shows metallic conductivity arising from the correlated interaction of its electrons, on a Nb:SrTiO3 substrate. They then probed its electronic structure with angle-resolved photoemission spectroscopy using synchrotron radiation at the KEK Photon Factory.
A set of subbands was observed corresponding to the different Vanadium 3d orbitals near the Fermi level, the properties of which changed with film thickness. This demonstrated that electrons were in fact trapped in two-dimensions in the film. Using this quantum well structure it is possible to alter the spin, charge, and orbital degrees of freedom of strongly correlated electrons, and could lead to the expression of new physical properties.
This successful creation and control of metallic quantum well states in artificial structures of a strongly correlated oxide is a first significant step toward creating new physical phenomena and controlling the novel physical properties of such materials, including high-temperature superconductivity. Kumigashira says that it is material scientists’ dream to tame these unruly prodigies. They won’t be appearing in consumer goods for some time, but having corralled these strongly correlated electrons is a first important step into a new world of electronics.
Press release (Japanese)
K. Yoshimatsu, K. Horiba, H. Kumigashira, T. Yoshida, A. Fujimori, and M. Oshima,
“Metallic Quantum Well States in Artificial Structures of Strongly Correlated Oxide,”
Science?333 (2011): 319-322. doi: 10.1126/science.1205771
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