Seemingly “Defying the Laws of Physics” – Distinguishing Between Right and Left With Magnets

A Connection Between Spin Chirality and Structural Chirality

The handedness of a spiral structure is inverted by taking the mirror image. A spin chirality, which is represented by an antiparallel-spin pair, is inverted not only by taking the mirror image but also by running time backward, which is not the case with a spiral structure. Such a spin chirality was shown to be connected to a structural chirality by measurements on a chiral superconductor under alternating current excitation. Credit: Hiroshi Yamamoto, Institute for Molecular Science

Electron spins reflect chiral molecules without the mirror.

The control of the chirality of molecules and crystals is crucial in various areas of research such as drug discovery and display technology. In recent times, innovative efforts have been made to utilize magnets to differentiate the handedness (left or right) of chiral molecules, offering an alternative to conventional chemical methods. However, the physics behind this new approach has been a topic of debate and its mechanism is yet to be confirmed. Verifying this hypothesis is important for the improvement of magnet-based chirality control and its application to a broader range of substances.

“To test the hypothesis, you must map electron spins in a single chiral molecule. A big difficulty is that chiral molecules are so small that you cannot do so.” explains team leader Prof. Hiroshi Yamamoto. He adds “Instead, we took up an organic chiral superconductor as a giant chiral molecule.”

Stabilizing electron spins requires that the electrons be correlated, or interfere with each other, over the entire system. In a superconducting state, the interference persists via prolonged coherence of electrons; many electrons collapse into a single quantum-mechanical wave and jointly keep the capability of interference over a long distance. This feature may enable the emulation of the spin distribution in an organic chiral superconductor in a much larger length scale than chiral molecules.

“Through a combination of the latest techniques, we finally detected spin polarizations,” says Ph.D. candidate Ryota Nakajima, who is the leading author. “We are surprised by an excellent correspondence with the proposed hypothesis. We found different spin distributions for different handedness of chiral superconductors.”

“The observed spin distribution is relevant to chiral recognition” elaborates Assistant Prof. Daichi Hirobe, who is one of the two corresponding authors. “Depending on the handedness, two spin polarizations sit face-to-face or back-to-back at both edges of a superconductor. This unique configuration has been hypothesized for chiral molecules, but it has not been verified.” Such spin configuration is unchanged under any rotation of a chiral crystal structure, which is key to magnet-based chiral recognition in a liquid reported previously.

Prof. Hiroshi Yamamoto sees the team’s achievement as a big advance in the understanding of a delicate connection between chirality and magnetism. “Chirality made from spin distributions enables recognition of molecular/crystal chirality from outside the system. This appears to defy the law of physics without careful consideration.”

The research team coined “T-odd chirality” for the spin-related chirality, noting the fact that the spins are reversed by time-reversal operation “T”. Their finding is also expected to find application in future superconducting spintronics.

Reference: “Giant spin polarization and a pair of antiparallel spins in a chiral superconductor” by R. Nakajima, D. Hirobe, G. Kawaguchi, Y. Nabei, T. Sato, T. Narushima, H. Okamoto and H. M. Yamamoto, 18 January 2023, Nature.
DOI: 10.1038/s41586-022-05589-x

The study was funded by the Grants-in-Aid for Scientific Research, from JSPS KAKENHI, Japan, PRESTO JST, Japan, and ERATO JST, Japan.

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