Electron chirality found in a symmetric material: JAEA discovers novel state in a uranium compound

A research group including Group Leader Yo Tokunaga and Postdoctoral Researcher Takayuki Ishitobi (currently a postdoctoral researcher at the Graduate School of Science, Tohoku University) of the Research Group for Strongly Correlated Actinide Science, Advanced Science Research Center, Nuclear Science Research Institute, Japan Atomic Energy Agency, together with Assistant Professor Yusei Shimizu (currently an assistant professor at the Institute for Solid State Physics, the University of Tokyo) and Professor Dai Aoki of the Institute for Materials Research, Tohoku University, announced on July 28 that they had discovered an electronic state in the uranium compound URhSn in which only the arrangement of electrons possesses chirality, despite the crystal structure itself being left-right symmetric. The finding is expected to lead to the development of new optical functional materials and spintronic materials. The results were published in the July 20 issue of Physical Review Letters.

The property in which a mirror image cannot be superimposed on the original object, as in the case of left and right hands, is known as "chirality." It is involved in a wide range of phenomena, including differences in the efficacy of pharmaceuticals as well as differences in aroma and taste among compounds composed of the same atoms, and is an important concept underpinning the life sciences, food science, and materials chemistry.

Conventionally, chirality was thought to require a left-right asymmetric arrangement of atoms. In February this year, the research group theoretically discovered the existence of "purely electronic chirality," in which chirality arises from the motion (arrangement) of electrons even when the atomic arrangement itself is left-right symmetric. Purely electronic chirality is a concept in which electron arrangements spontaneously select either a right-handed or left-handed configuration, thereby generating chirality, and the group has since been searching for candidate materials.

The researchers therefore searched for candidate materials under conditions previously identified as giving rise to purely electronic chirality. Using crystal samples of URhSn, a compound consisting of uranium (U), rhodium (Rh), and tin (Sn) that met those conditions, they investigated its detailed electronic state using nuclear magnetic resonance (NMR). NMR is a measurement technique that utilizes the resonance phenomenon exhibited by atomic nuclei in a magnetic field. Based on the same principle as MRI, it enables highly sensitive investigation of electronic states, magnetic properties, and the local environment around atoms within materials.

It was already known that the crystal structure of the compound itself does not possess chirality. However, when cooled below minus 219℃ (54 K), the symmetry of the electrons within the material changed.

The researchers found that the distribution of electrons around uranium atoms became directional, and that the overall arrangement formed a new electronic state possessing chirality, known as chiral antiferroquadrupolar order.

Antiferroquadrupolar order is a state in which the orientations and distortions of electron charge distributions are regularly arranged in opposite directions at neighboring sites, and it has been reported in rare-earth and actinide compounds.

There had previously been no reports of materials in which chirality arises solely from changes in electronic states, overturning conventional wisdom and providing proof of both the theory and its physical realization.

Tokunaga commented: "Going forward, we believe it is necessary to clarify what kinds of materials and conditions give rise to this phenomenon, using the findings of this study as a starting point. By clarifying this, our first goal is to realize the phenomenon at room temperature. In the future, we hope that applying this knowledge to technologies capable of switching chirality with low energy consumption will lead to the development of devices with greater flexibility and functionality. Because uranium compounds are difficult to handle and study, many mysteries remain, making them a treasure trove of new quantum states. We believe this achievement demonstrates their tremendous potential."

Journal Information
Publication: Physical Review Letters
Title: NMR Evidence for Spontaneous Electronic Chiral Order in URhSn
DOI: 10.1103/1vjx-4jfv