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Spin polarization recognized in a chiral superconductor: Group led by IMS demonstrates the appearance of a pair of antiparallel spins


A research team led by Professor Hiroshi Yamamoto, consisting of Assistant Professor Daichi Hirobe and Graduate Student Ryota Nakajima (Graduate University for Advanced Studies) of the Research Center of Integrative Molecular Systems at the Institute for Molecular Science, in collaboration with Professor Hiromi Okamoto and Assistant Professor Tetsuya Narushima of the Center for Mesoscopic Sciences at the same institute, has successfully observed spin accumulation in chiral superconductors by applying the latest technology for electronic devices based on organic chiral superconductors. The research team identified the relationship between the chiral crystal structure and spin accumulation, which had been ambiguous, and demonstrated that chirality can indeed be recognized on the surface of a magnet.

The research group attached electrodes to a chiral organic superconductor called (BEDT‐TTF), κ‐(BEDT‐TTF)2Cu(NCS)2 and started experiments in which alternating current was applied. At that time, the top and bottom electrodes were made of magnets so that the relationship between the north and south poles and the up and down spins coming out of the superconductor could be monitored as a voltage. Alternating current was also used to reproduce the state in which the electrons in the solution are shaken by the free movement of the molecules in the solution. When the materials became superconducting at low temperatures, 'two antiparallel spins' appeared at the top and bottom of the chiral superconductor crystal.

Furthermore, when the right/left structures of the crystal are reversed, the inward and outward directions of the spin pairs are also reversed. This proves that the chirality of a crystal can indeed be determined by observing the spins of a chiral superconductor from the outside with a magnet.

The 'two antiparallel spins' found in this study express a strange chirality, in which the left/right handedness can be also reversed by time reversal. This would never happen in the chirality of a molecule or crystal. This is because right‐handed molecules remain right‐handed and left‐handed molecules remain left‐handed even if the direction of time is reversed. This is known as 'time reversal symmetry' in physics.

It was generally thought that the chirality of the spin, which is turned over by such time reversal, and the chirality of the molecule were largely unrelated. However, this experiment found that the chirality of the spin that switches sides in a time reversal can be used to distinguish the chirality of the molecule from the outside. The research group named this 'chirality with odd time reversal (T‐odd chirality)', emphasizing that the time reversal for this chirality is not even.

Yamamoto said, "With these results, we now have a qualitative understanding of the phenomena occurring in chiral molecules interacting with magnets. In the future, a quantitative theory using mathematical formulas will be developed, and we expect to achieve higher efficiency in chiral molecule fractionation using magnets, which will accelerate research in drug discovery and the development of functional molecules. In addition, development can be expected in the field of superconducting spintronics, including quantum computation.

■ CISS effect: An effect when an electron passes through a chiral molecule, its spin is aligned parallel (in the same direction) or anti‐parallel (in the opposite direction) to the direction in which the electron is travelling.

This article has been translated by JST with permission from The Science News Ltd. ( Unauthorized reproduction of the article and photographs is prohibited.

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