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Successful Visualization of Distortion in Right- and Left-Handed Helical Orientation of Electron Cloud (Press Release)

Release Date
07 Apr, 2014
  • BL17SU (RIKEN Coherent Soft X-ray Spectroscopy)
– Proposing and verifying concept of “enantiomerism” in electron orbit -

Osaka University

Key Points
• Verification of the helix chirality of electric quadrupole orientations
• Observation of the spatial distribution of right- and left-handed structures coexisting in the helical electric quadrupoles
• Progress to the development of novel optical materials

RIKEN (President, Ryoji Noyori) has proposed and verified the helix chirality of electric quadrupole[1] orientations, namely, the distortion of an electron cloud (chiral substances have an enantiomorphic structure[2] as shown in human hands; the mirror image of the left hand is not superposable to that of the right hand). This was achieved by a joint research group led by Yoshikazu Tanaka (senior research scientist) of the Excitation Order Research Team, Quantum Order Research Group, RIKEN SPring-8 Center (Director, Tetsuya Ishikawa), and Tsuyoshi Kimura (professor) of the Graduate School of Engineering Science, Osaka University.
An electron orbit is a quantum-mechanical model of an electron cloud surrounding an element. An electron orbit is not always spherical but can be distorted. A type of electron orbit (electron cloud) with a distorted shape is called an electric quadrupole. It is known that the shape and orientation of electric quadrupoles in 3d transition metal compounds[3] and 4f rare-earth metal compounds[4] play a crucial role in electric and magnetic properties such as superconductivity and collosal magnetoresistance, as well as in the optical properties of these substances. Therefore, clues to the electric, magnetic, and optical properties of a substance can be found by examining the state of electric quadrupoles. Thus far, the synchrotron-radiation-based resonant X-ray diffraction[5] performed on electric quadrupoles has revealed the electronic state occurring in some 3d transition metal compounds and 4f rare-earth metal compounds, such as the ferroic order in which all electric quadrupoles are oriented in the same direction or the antiferroic order in which electric quadrupoles are oriented alternately.
The joint research group proposed and verified the enantiomerism in electric quadrupole orientations that had remained untouched because of the difficulty in experimental verification. They succeeded in visualizing the distortion of electron cloud orientations by using a special method based on circularly polarized[7] resonant soft X-ray diffraction, and the synchrotron radiation produced at the large synchrotron radiation facility, SPring-8.[6] They also successfully obtained microscopic images (domain images) of the electric quadrupole orientations that showed the coexistence of an enantiomer pair consisting of the right- and left-handed structures. If the helix chirality of electric quadrupole orientations proposed and verified in this study and its domain structure can be fully controlled by external perturbation such as circular polarization of light, they can be applied to the development of novel optical memory materials for long-term storage.

These achievements were published online in the British scientific journal Nature Materials on 6 April 2014 prior to publication in the printed version of the June issue.

"Observation of quadrupole helix chirality and its domain structure in DyFe3(BO3)4"
T. Usui, Y. Tanaka, H. Nakajima, M. Taguchi, A. Chainani, M. Oura, S. Shin, N. Katayama, H. Sawa, Y. Wakabayashi, and T. Kimura.
Nature Materials, 2014, doi: 10.1038/NMAT3942

Fig. 1	Examples of electric quadrupoles
Fig. 1 Examples of electric quadrupoles

Red and blue indicate regions of positive and negative charge distributions, respectively.

Fig. 2	Mirror-image structures of helical electric quadrupoles
Fig. 2 Mirror-image structures of helical electric quadrupoles


Fig. 3	Crystal structures and enantiomers in DyFe3(BO3)4 electric quadrupoles observed in this study
Fig. 3 Crystal structures and enantiomers in DyFe3(BO3)4 electric quadrupoles observed in this study

Top and bottom are views projected from the direction parallel to and almost perpendicular to the crystal c axis, respectively. The right- or left-handed helical structure of Dy 4f electric quadrupoles was verified by resonant X-ray diffraction using special X-rays called circularly polarized soft X-rays.

*1 Electric quadrupole

A type of electron orbit state. A dipole is a pair of positive and negative charge distributions separated by a distance in space. A compass is a simple example of a magnetic dipole. On the other hand, a quadrupole is a set of four charge distributions, two are positive and two are negative, which are combined in space.

*2 Enantiomerphic structure and enantiomer
A pair of substances whose crystal or molecular structures have the same relation as human hands. For example, our right and left hands are not superposable no matter how we rotate or move them unless they are reflected in a mirror. The right- and left-handed structures have the same physical properties but the plane of polarization of light transmitted through these structures rotates in opposite directions; that is why they are called enantiomers or optical isomers. All amino acids that constitute our body are left-handed. Enantiomers are known to show optical rotation, a property where the plane of polarization of transmitted light rotates in opposite directions, but the structures of enantiomer pairs are distinguished from each other.

*3 3d transition metal compounds
Compounds containing elements from scandium (Sc), atomic number 21, to copper (Cu), atomic number 29. Because of their various chemical binding forms, magnetic structures, and orbital states, these compounds have been utilized in research on a wide variety of functional materials such as high-temperature superconductors.

*4 4f rare-earth metal compounds
Compounds containing elements from lanthanum (La), atomic number 57, to lutetium (Lu), atomic number 71. They are used in various industrial products such as permanent magnets, liquid crystals, and catalysts.

*5 Resonant X-ray diffraction
A diffraction method using synchrotron radiation X-rays. Information about the symmetry, direction, and relative size of a specific electron orbit of an atom can be obtained by irradiating a sample with X-rays with an energy that equals the atom-specific resonant state.

*6 Large synchrotron radiation facility, SPring-8
A facility that generates the world’s highest-performance synchrotron radiation. It is located in Harima Science Garden City in Hyogo prefecture and is owned by RIKEN. Japan Synchrotron Radiation Research Institute (JASRI) is responsible for its operation, management, and support of users. The name SPring-8 is derived from Super Photon ring-8 GeV. Synchrotron radiation refers to the strong electromagnetic waves generated when the orbit of electrons, accelerated to a near-light speed, is bent by magnetic field. Research on various fields from fundamental science and industrial applications is conducted at SPring-8.

*7 Circularly polarized
Light and X-rays are transverse waves in which electric and magnetic fields travel while oscillating. Circularly polarized light refers to the light whose electric field rotates around the axis of propagation of the light while electric and magnetic fields travel a distance of one period. Right circularly polarized light is the light whose electric field rotates counterclockwise with time for an observer who looks at the light in the direction of the light source. The spatial trajectory of right circularly polarized light resembles a left-handed screw when viewed from both directions.

For more information, please contact:
 Yoshikazu Tanaka (RIKEN)

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