Tokyo Metropolitan University Research Center

首都大学東京 Tokyo Metropolitan University

Superconductivity Science and Engineering

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Academic background

Promotion of the research for superconductivity in strongly correlated electron systems

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Director of research center:
Professor Takashi Hotta,
Dr. Science
Department of Physics,
Graduate School of Science and Engineering

It has been well know that conventional superconductivity originates from the formation of s-wave Cooper pair composed of up- and down-spin electrons due to effective attraction mediated by lattice vibrations. One may worry about the influence of Coulomb repulsion on the pair formation, but it does not work so strongly between pair electrons, since the energy of lattice vibration is generally smaller than the electron bandwidth and its time scale is largely different from that for the Coulomb interaction. Namely, the pair can be stabilized by the weak effective attraction, even if the Coulomb repulsion exists.

In 1979, superconductivity due to the Cooper pair of electrons with large effective mass has been discovered in CeCu2Si2, indicating the emergence of superconductivity even in a system with strong electron interactions. In 1986, cuprate superconductors have been discovered and it has been highly expected that high-temperature superconductivity is found in such a system with strong electron correlation. Previously, magnetism and superconductivity were considered to be incompatible with each other, but in unconventional superconductivity with non-s-wave Cooper pair, magnetic fluctuations have been believed to mediate the pair formation. Recently, in some uranium compounds, ferromagnetism and superconductivity have been found to coexist, suggesting that magnetism is good friend with superconductivity in some materials, in sharp contrast to the previous concept that magnetism is not compatible with superconductivity.

In recent decades, unconventional superconductivity has been attracting considerable attention, since it commonly appears in p-electron systems such as molecular conductors, d-electron systems such as transition metal compounds, and f-electron systems such as rare-earth and actinide compounds.  These materials are collectively referred to as “strongly correlated electron systems” and the research of exotic superconductivity in the vicinity of novel electronic ordered state has been actively performed in the world. From the viewpoint of the basic science, it is important to clarify new quantum critical phenomena. The research of superconductivity in strongly correlated electron systems is also considered to be consistent with the trend of modern material science that reveals physical properties and novel functions of newly synthesized materials.  In this sense, the strongly correlated electron systems widely provide the source of both basic and applied research fields.