SPring-8, the large synchrotron radiation facility

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Novel Interaction between Electrons Acting as Micromagnets (Press Release)

Release Date
25 May, 2013
  • BL02B1 (Single Crystal Structure Analysis)
  • BL19LXU (RIKEN SR Physics)
- Discovery of interaction applicable to quantum computers -

The University of Tokyo
RIKEN
Japan Synchrotron Radiation Research Institute (JASRI)

Main findings
• The arrangement pattern of micromagnets has been clarified using the beamlines at SPring-8, a large synchrotron radiation facility.
• The “compass” interaction between micromagnets has been demonstrated.
• The results of this study will contribute to the realization of Kitaev spin liquids that can be used in quantum computers.

Individual electrons can serve as micromagnets. For example, when the directions of enormous numbers of such micromagnets in a material are aligned, the entire material behaves as a magnet, which can be used in various applications such as motors and hard discs. When two compasses, which are visible to humans, are brought close to each other, they rotate so that the N pole of one compass becomes aligned with the S pole of the other compass. Therefore, the direction of the magnets will become parallel to the line connecting the two compasses. This is called compass interaction. For electrons acting as micromagnets, however, their direction was previously considered irrespective of their positional relationship.

A joint research group of the University of Tokyo, RIKEN, and JASRI has clarified that compass interaction occurs between the electrons acting as micromagnets in CaIrO3. This is the world’s first experimental demonstration of compass interaction for such magnets. Theoretically, arranging the electrons that act as magnets and are subject to compass interaction is expected to enable the realization of spin liquids*1, which can be used in quantum computers (next-generation computers with processing speed much higher than that of supercomputers). The achievement of this research is expected to markedly increase the likelihood of realizing spin liquids.

The results were published in Physical Review Letters on 24 May 2013.

Publication:
"Resonant X-ray Diffraction Study of the Strongly Spin-Orbit-Coupled Mott Insulator CaIrO3"
Kenya Ohgushi, Jun-ichi Yamaura, Hiroyuki Ohsumi, Kunihisa Sugimoto, Soshi Takeshita, Akihisa Tokuda, Hidenori Takagi, Masaki Takata, and Taka-hisa Arima
Physical Review Letter 110 217212, published 24 May 2013.

<<Figures>>

Fig. 1 Schematic of quantum compass model
Fig. 1 Schematic of quantum compass model

When a number of compasses are placed in a line, they interact with each other, resulting in a stable state in which the N pole of a compass faces the S pole of the adjacent compass. The state in which the needles of the compasses point horizontally [Fig. 1(a)] is more stable in terms of energy than the state in which they point vertically [Fig. 1(b)]. This anisotropic interaction is called compass interaction. In the Heisenberg model, arrangement pattern (a) has the same energy as arrangement pattern (b).

Fig. 2 Interaction between magnetic moments via O atom
Fig. 2 Interaction between magnetic moments via O atom

(a) When the Ir‒O‒Ir bond angle is 180°, two processes contributing to the interaction exist, causing a Heisenberg interaction, which is the sum of the two processes. (b) When the Ir‒O‒Ir bond angle is 90°, the two processes mutually interfere and cancel each other, and no Heisenberg interaction occurs. Lower-order processes cause the quantum compass interaction.

Fig. 3 (a) Magnetic moment of Ir ion in which spin and orbital angular moments are coupled as a result of the spin-orbit interaction. (b) Magnetic structure of post-perovskite-type compound CaIrO3.
Fig. 3 (a) Magnetic moment of Ir ion in which spin and orbital angular moments
are coupled as a result of the spin-orbit interaction.
(b) Magnetic structure of post-perovskite-type compound CaIrO3.

The magnetic moment is ordered in an antiparallel arrangement by the Heisenberg interaction denoted as J1 and ordered in a parallel arrangement by the quantum compass interaction denoted as J2. The magnetic moment is slightly tilted, which is caused by the anisotropy of the quantum compass interaction.

<<Glossary>>
*1　Spin liquid
A material whose state does not exhibit static order down to absolute zero despite the interaction between its magnetic moments.


For more information, please contact:
  Project Associate Prof. Kenya Ohgushi
  (The University of Tokyo, The Institute for Solid State Physics)
    E-mail : mail1

  Ph.D. Takahisa Arima (RIKEN)
    E-mail : mail2

  Dr. Hiroyuki Ohsumi (RIKEN)
    E-mail : mail3