Many new types of superconductivity have been discovered in strongly-correlated electron systems, in which many electrons interact strongly with each other. It has been learned that, in these systems, the Coulomb repulsion between the electrons can be avoided by an unconventional pairing, in which the pairing amplitude vanishes when both electrons are on the same site. Many of such unconventional superconductors have an anisotropic superconducting energy gap, and the superconducting gap structure is a key to identifying the pairing mechanism. However, many of these new types of superconductivity have a low transition temperature of 3 K (−270ºC) or less, and difficult experiments were required to study the gap anisotropy.
We have been focusing on developing experimental techniques to identify the anisotropy in the superconducting gap based on how the amount of superconducting electrons changes under a rotating magnetic field. More specifically, we developed a calorimeter that can measure specific heat, which can be used to evaluate the amount of broken electron pairs, at temperatures as low as nearly absolute zero, or 0.04 K (−273.11ºC), and combined it with a vector magnet system that can control the direction of the magnetic field in 3D space to study the gap anisotropy with various superconductors. As a recent result of this research, we presented experimental data showing that superconductor UTe2, which is extraordinarily resistant to magnetic fields, has gap-zero points in the a-axis direction, thereby suggesting that this superconductor has an unusual internal degree of freedom. Some of the research using this system brought about discoveries that revised long-held ideas. For example, it was experimentally demonstrated that CeCu2Si2, the new type of superconductor discovered first, has isotropic superconducting gaps despite it being a strongly-correlated electron system. This suggests the surprising possibility that electrons on the same site can be paired even when they are subject to a strong Coulomb repulsive force. As just described, cryogenic experiments have contributed to revealing new aspects of superconductivity, offering hot topics in the field of condensed matter physics.
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 Homepage of Kittaka group https://www.phys.chuo-u.ac.jp/labs/kittaka/index.html
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 Seimei "Heriumu Risaikuru Shakai wo Mezasite" (Statement, "Toward a Helium Recycling Society") https://www.jps.or.jp/information/2019/12/helium.php