Press Release

Quantum confinement of strongly correlated electrons in artificial oxide structures

October 28, 2011

The University of Tokyo, Japan
High Energy Accelerator Research Organization (KEK), Japan
Japan Science and Technology Agency (JST)

Researchers from the University of Tokyo have succeeded in creating a quantum well structure^*1 using strongly correlated oxides, and have for the first time observed the quantized states that demonstrate the confinement of strongly correlated electrons^*2 in two-dimensional space. The critical factors for success with quantum confinement were (1) precise growth control of a conductive oxide on an atomic scale using laser molecular beam epitaxy^*3 and (2) state-of-the-art spectroscopy using the synchrotron radiation available at the Photon Factory, KEK. Utilizing these growth and characterization techniques, the researchers confirmed that strongly correlated electrons were confined in the quantum well of an artificial oxide structure.
The present result means that the behavior of strongly correlated electrons can be controlled in an artificial two-dimensional space. Strongly correlated electrons confined in two-dimensional space are essential for the emergence of high-temperature superconductivity. Thus, this controllability should allow researchers to develop new superconductors with much higher critical temperatures, hopefully above room temperature, as well as materials having extraordinary functionalities. The successful creation and control of quantum well states in the present study is a first significant step toward creating new electronic devices, as well as controlling the novel physical properties of strongly correlated oxides.

The original report of this work was published on 15 July 2011 in Science.

1. Background

The wide range of properties exhibited by strongly correlated oxides makes them one of the most interesting groups of materials today. High-temperature superconductivity in cuprates is a representative example of the functionality of strongly correlated oxides. Their properties originate in stacked conductive layers, in which strongly correlated electrons are sandwiched by blocking insulating layers (left side of Fig 1). Thus, controlling the behavior of strongly correlated electrons confined in the conductive layers is the key to controlling the extraordinary functionalities of these materials. In fact, it is known that the transition temperature in high-temperature cuprate superconductors increases in proportion to the number of conductive layers sandwiched between two insulating layers. Therefore, it is highly desirable to develop techniques to control the behavior of strongly correlated electrons by using artificial structures.

2. Research Methods and Results

The researchers used laser molecular beam epitaxy to fabricate a quantum well structure based on a strongly correlated conductive oxide of strontium vanadate (SrVO₃). The quantum well structure is structurally similar to that of layered complex oxides, as shown in Fig. 1. In the quantum well structure, however, the number of conductive layers can be controlled freely (Fig. 2). The electronic structures of the fabricated quantum well structures were investigated by angle-resolved photoemission spectroscopy^*4 using synchrotron radiation at the KEK Photon Factory. The results are shown in Fig. 3. A set of quantization states is observed, providing the evidence for the confinement of strongly correlated electrons in two-dimensional space (conductive oxide layers). As can be seen in Fig. 3(a), the energy positions of the several quasiperiodic peaks evolves as a function of conductive layer thickness: With increasing thickness, one additional peak appears after another, and the peak positions shift to higher binding energies. The spectral behavior is summarized in Fig. 3(b), where the experimental results are compared with theoretical calculations. Fig. 3(b) shows that the experimental data are in good agreement with theoretical predictions, indicating the successful creation and control of quantum well states in artificial oxide structures. Furthermore, additional investigation revealed the existence of other interesting phenomena in the quantum well (Fig. 4): 1) oribital-selective quantization originating from the anisotropic orbital character of the V 3d states and 2) unusual enhancement of the effective mass of the quantized electrons, reflecting complex interactions in the quantum well, neither of which has previously been reported in the conventional quantum well states in noble metals.

3. Research Methods and Results

Using this quantum well structure, it is possible to alter the spin, charge, and orbital degree of freedom of strongly correlated electrons, which could lead to the expression of new physical properties. The successful creation and control of metallic quantum well states in an artificial oxide structure is a first significant step toward creating new physical phenomena and controlling the novel physical properties of such materials, including high-temperature superconductivity. Moreover, we can now look forward to the development of electronics based on strongly correlated oxides, such as superconducting devices and ultrafast optical switching devices.

This work was supported by a Grant-in-Aid for Scientific Research (A19684010 and S22224005) from the Japan Society for the Promotion of Science (JSPS) and the JST PRESTO program.  This work at KEK-PF was done under the approval of the Program Advisory Committee (Proposals No. 08S2-003 and No. 09S2-005) at the Institute of Materials Structure Science, KEK.

Publication Title and Authors
“Metallic Quantum Well States in Artificial Structures of Strongly correlated Oxide.”
K. Yoshimatsu, K. Horiba, H. Kumigashira, T. Yoshida, A. Fujimori, and M. Oshima
Science 333, 319 (2011).

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