SPring-8, the large synchrotron radiation facility

Release Date

13 Apr, 2009

Japan Atomic Energy Agency
Tohoku University
Argonne National Laboratory (US)

Shuichi Wakimoto, a deputy principal researcher, and his colleagues at the Quantum Beam Science Directorate of Japan Atomic Energy Agency (JAEA) (Toshio Okazaki, President) succeeded in observing, for the first time in the world, the collective excitation (collective fluctuation)^*1 of electrons that strongly interact with each other in a material^*2, in cooperation with researchers from Tohoku University (Akihisa Inoue, President) and Argonne National Laboratory (Robert Rosner, Director). Although the behavior of electrons that do not interact between them, such as conduction electrons in metals, are well understood in the scheme of today's physics, the behavior of electrons that behave collectively as a result of the strong interaction between them is not well understood because of the difficulty of theoretical interpretation. However, materials with strongly interacting electrons are known to exhibit several new and useful properties, notably high-temperature superconductivity.

In this study, the joint research group investigated the collective behavior of electrons of a high-temperature cuprate superconductor and a related material, nickelate, which are typical examples of materials that exhibit strong interactions between electrons. In these materials, electrons are ordered in a stripe pattern owing to the strong interactions. The research group succeeded in directly observing periodic collective changes of the positions of the electrons aligned in a stripe pattern, referred to as the collective excitation of electrons, in the cuprate and nickelate. This was achieved by improving the conventional method of resonant inelastic X-ray scattering^*3, which enables the observation of the period of the spatial and temporal periodic movement of electrons, by our unique modification^*4 using the facilities of SPring-8 and the Advanced Photon Source at Argonne National Laboratory. This success will open up a new research field in materials science for exploration, namely, the collective excitation of electrons. It is highly expected that the mechanism behind the high-temperature superconductivity of target materials will be clarified by further research on collective excitation.

The research results were published online on 14 April 2009 in Physics Review Letters, published by the American Physical Society.

Publication:
"Charge Excitations in the Stripe-Ordered La_5/3Sr_1/3NiO₄ and La_2-x(Ba,Sr)_xCuO₄ Superconducting Compounds"
S. Wakimoto, H. Kimura, K. Ishii, K. Ikeuchi, T. Adachi, M. Fujita, K. Kakurai, Y. Koike, J. Mizuki, Y. Noda, K. Yamada, A. H. Said, and Yu. Shvyd'ko
Physical Review Letters 102, 157001 (2009), published 14 April 2009

<Figures>

Fig. 1 Doped transition-metal oxide (example: cuprate)
By doping holes, the cuprate changes from an insulator to a high-temperature superconductor. The arrows in the figure indicate the magnetic moment of electrons. The electrons of the cuprate are ordered in a stripe pattern. By inducing fluctuations in these electrons using X-rays, the level of interaction between the electrons can be examined.

Fig. 2 Outline of resonant inelastic X-ray scattering experiment.
By adjusting the incident X-ray energy to the energy level of the inner shell electron of copper, the charge excitation intensity is significantly increased.

Fig. 3 1) Resonant inelastic X-ray scattering spectrum (shown in blue) with momentum transfer (Q_s) corresponding to the periodicity of the electron stripes and 2) resonant inelastic X-ray scattering spectrum (shown in red) with momentum transfer independent of the periodicity of the electron stripes.
For all samples, signals corresponding to collective electron excitation were observed in the spectrum with Q_s corresponding to the periodicity of the electron stripes.

<Glossary>

*1 Collective excitation (collective fluctuation)
Collective excitation is a phenomenon in which periodically ordered electrons fluctuate and vibrate spatially and temporally over the entire crystal. This is considered different from the individual fluctuation of a single electron.

*2 Strongly interacting electrons
For oxides containing transition metals such as copper (Cu) and nickel (Ni), electrons are ordered periodically in a stripe pattern or checkerboard design as a result of the strong interactions between them. La_2-xBa_xCuO₄ (x=1/8) and La_2-xSr_xNiO₄ (x=1/3) are typical examples of materials in which electrons are ordered in a stripe pattern owing to their strong interaction.

*3 Resonant inelastic X-ray scattering
This is a new experimental method that became possible only after X-rays from a large-scale synchrotron radiation facility such as SPring-8 became applicable. Scattering in which the total mechanical energy remains the same before and after collision is called elastic scattering, whereas that in which the total mechanical energy changes after a collision is called inelastic scattering. When synchrotron X-rays are irradiated onto a sample, inelastic X-ray scattering is induced, and X-rays with scattering energy slightly different from the incident energy are generated. By measuring such X-rays, it is possible to obtain the spatial and temporal periods of the fluctuating electrons. When inducing electron resonance by controlling the amount of incident energy, the intensity of scattering X-rays is amplified. In this study, we observed the electron fluctuations of copper atoms (the CuO₂ plane) by resonating the X-ray energy with the energy level of the inner shell electron on an orbit close to the nucleus of the copper atoms.

*4 Unique modification
When the energy difference between the incident X-ray and the scattered X-ray is in the region of 1 eV or less, the collective fluctuation of stripe-ordered electrons appears. However, in general, it is difficult to observe the difference of 1 eV or less, because inelastic scattering overlaps with very strong elastic scattering in the region. In this study, the observation was successfully carried out by adjusting the configuration of the synchrotron X-rays and the measurement apparatus so that elastic scattering is suppressed to a minimum.