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Scientists have discovered that a chiral crystal, which exhibits no magnetism, works as a polarizer of electron spins when the charge current is applied at room temperature in the absence of magnetic field. This phenomenon is likely to be originated from the nature that the crystal has a chiral structure. The present work makes a fundamental contribution in revealing universal properties that a wide variety of chiral materials should exhibit.

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Scientists at Osaka Prefecture University together with collaborators from Institute for Molecular Science, The Open University of Japan, and Toho University have discovered that chiral crystals generate a spin-polarized current. In spite of no magnetism in the crystals, the direction of spins orients in the same direction when the charge current is applied and such polarized spins propagate throughout the crystal. This phenomenon can be induced and detected electrically without using magnets or magnetic fields. This effect is likely to be originated from the fact that crystals have chiral structures. The present work makes a fundamental contribution in revealing universal properties that a wide variety of chiral materials should exhibit. The work is published in Physical Review Letters, issued by American Physical Society.
 

Background

When looking at a left hand in the mirror, it looks like a right hand. However, the left hand never overlaps the right hand. Such a geometrical relationship that an image of the object in a plane mirror cannot be brought to coincide with itself is called chirality. A clockwise or counterclockwise helical staircase is an example of a chiral structure. On the other hand, a round ball is not chiral since its image in a mirror overlaps the original shape.

The structure of a material which consists of atoms and/or molecules may exhibit chirality. It is known that chiral molecules or chiral crystals show a chiral structure, as exemplified by deoxyribonucleic acid (DNA), amino acids, and sugars. Such chiral substances are inevitable in living activity. However, their electrical and magnetic characteristics have attracted little attention so far.

Recently, it was found by Ron Naaman et al. in Israel that electron spins orient in the same direction when electrons pass through chiral molecules such as DNA. Because the direction of spins depends on the handedness of a chiral molecule structure, this phenomenon is called chirality-induced spin selectivity (CISS).

Here, spin is an elementary characteristic of an electron as well as its charge that represent electricity. Spin is a source of magnetism and expected to work as one of information carrier at ultimately small length scale. Spin is a product of quantum mechanism that describes the tiny microscopic world. Our understanding of its characteristics and challenge of controlling and manipulating spins via macroscopic experimental method are still under investigation. For example, magnets exhibit magnetic properties since the direction of electron spins orient in a particular direction. The state in which electron spins direct in the same orientation is called a spin-polarized state. On the contrary, non-magnetic materials contain electron spins, the directions of which are distributed randomly. To produce a spin-polarized state in a non-magnetic material is an important research target to promote the next generation electronics and quantum science.

CISS research clarified that non-magnetic chiral molecules make electrons spin polarized. This phenomenon is very puzzling and its mechanism remains to be clarified. It is very interesting to investigate how universal the spin-polarized phenomena are in chiral materials widely found in molecules and crystals.
 

Results

In this study, the researchers focused not a chiral molecule but a ‘chiral crystal’. The chiral crystal that the researchers have investigated possesses a helical arrangement of atoms twisting in one direction. Such a helical structure appears all over the chiral crystal, as shown in upper panel of Figure. Although molecule is a very tiny invisible entity, crystals are large enough to be handled. The crystals also exhibit fabrication feasibility and material stability in usual cases.

A chiral crystal CrNb₃S₆ used in this study is a metal that conducts electricity well but exhibits no magnetism at room temperature in the absence of magnetic field. However, the experiments clarified that electrons flowing in CrNb₃S₆ are spin polarized. Namely, the chiral crystal plays a role to make the spins of flowing electrons aligned in the same direction spontaneously.

Applying an electrical current into the coil found in our daily life induces magnetic fields in the coil. This is a macroscopic electromagnet generating macroscopic magnetic fields. A helical atomic configuration found in the crystals plays a role to generate polarized spins. Namely, it behaves as a ‘microscopic’ electromagnet generating polarized spin in the microscopic tiny world.

More precisely, the researchers succeeded in detecting spin-polarized current generated by chiral crystals electrically without using magnets or magnetic fields. They demonstrated that voltage application induces the flow of spin-polarized carriers in the device made of chiral crystals CrNb₃S₆ with a spin detection electrode, which is consistent with the CISS phenomena. They also found that voltage can be extracted from the chiral crystal by injecting spin-polarized current from the electrode. This corresponds to the inverse effect of the CISS phenomena and indicates that the CISS effect satisfies the reciprocal theorem. Furthermore, spin-polarized state electrically generated in the crystal retains robustly over the whole crystal and propagates even in a region where the current does not flow. Moreover, a handedness of chiral crystal structures is distinguishable by using this phenomenon.

This study clarified that spin-polarized phenomena initially found in chiral molecules occur even in chiral solid crystals, suggesting that such phenomena appear universally in a wide range of chiral materials from chiral molecules to chiral crystals. The study made a fundamental and significant contribution to the research field of spin manipulation and detection in combination with chiral systems.
 

Information of the paper

Authors: A. Inui, R. Aoki, Y. Nishiue, K. Shiota, Y. Kousaka, H. Shishido, D. Hirobe, M. Suda, J. Ohe, J. Kishine, H. M. Yamamoto, and Y. Togawa

Journal Name: Physical Review Letters

Journal Title: “Chirality-Induced Spin-Polarized State of a Chiral Crystal CrNb₃S₆”

DOI: 10.1103/PhysRevLett.124.166602
 

Financial Supports

Grants-in-Aid for Scientific Research, Research Grant of Specially Promoted Research (No. 17H02767, No. 17H02923, No. 19K03751, and No. 19H00891.)
Specially Promoted Research Program by Toyota Physical and Chemical Research Institute (Toyota RIKEN)
 

Contact Person

Prof. Yoshihiko TOGAWA, Osaka Prefecture University
E-mail: y-togawa_at_pe.osakafu-u.ac.jp

Prof. Hiroshi M. YAMAMOTO, Institute for Molecular Science
E-mail: yhiroshi_at_ims.ac.jp

Prof. Jun-ichiro KISHINE, The Open University of Japan
E-mail: kishine_at_ouj.ac.jp

Prof. Jun-ichiro OHE, Toho University
E-mail: junichirou.ohe_at_sci.toho-u.ac.jp

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