A research group, including Associate Professor Hiroaki Anzai at the Graduate School of Engineering of Osaka Prefecture University, Associate Professor Hitoshi Sato at the Hiroshima Synchrotron Radiation Center of Hiroshima University, and Professor Koichi Hiraoka at the Graduate School of Science and Engineering of Ehime University, conducted a high-resolution angle-resolved photoemission spectroscopy experiment (Keyword 1) using high-brilliance synchrotron radiation (Keyword 2) in order to investigate the mechanism of valence transition phenomena (Keyword 3), in which the valence of ytterbium (hereafter referred to as “Yb”) ions in rare earth compounds abruptly changes.

As a result, the group successfully observed how conduction electrons are quantum-mechanically mixed with Yb 4f electrons (Keyword 4). This can be a convincing mechanism that explains valence transition phenomena, as evidence that conduction electrons transfer electrons between Yb ions with different valences. The research result is a significant achievement in fundamental physics and expected to become a guiding principle for the development of new optical materials that are used for lasers and fluorescent materials.

The results of this research were published online in Physical Review Research, an academic journal issued by the American Physical Society, on September 15 at 12:00 a.m. (Japan time).

Paper Information

Title

Abrupt change in hybridization gap at the valence transition of YbInCu₄

• Physical Review Research (American Physical Society Website)

Keywords

Keyword 1: Angle-resolved photoelectron spectroscopy

When a material is irradiated with light, electrons inside the material are emitted externally through the surface due to photoelectric effect. Angle-resolved photoemission spectroscopy (ARPES) determines the energy and momentum of electrons inside the material by measuring the energy and emission angle of photoelectrons and using energy conservation law and momentum conservation law. The relationship between the energy and the momentum of electrons is called band dispersion, and it can be observed by ARPES experiments. The form and energy of band dispersion reflect the nature of the relevant material. Therefore, it is important to observe band dispersion with accuracy (with high resolution). Against this backdrop, the development of electron spectrometer for ARPES has been pursued around the world.

Keyword 2: Synchrotron radiation

When electrons are accelerated to the speed of light and the direction of their movement is magnetically changed, electromagnetic waves are radiated along the direction of the movement. These electromagnetic waves are called synchrotron radiation. Synchrotron radiation is extremely bright light (of high intensity) and characteristically provides a wide range of wavelengths from infrared rays to X-rays. Synchrotron radiation is used in experiments such as material analysis, structural analysis, and electronic state analysis.

Keyword 3: Valence transition phenomena

“Rare earths” is a general term for 17 elements with the following atomic numbers in the periodic table: 21 scandium (Sc), 39 yttrium (Y), and from 57 lanthanum (La) through 71 lutetium (Lu). In cerium (Ce), europium (Eu), and Yb compounds, rare-earth ions have different valence states, and the mean valence changes depending on external factors such as temperature, pressure, and a magnetic field. In general, the valence slowly changes according to changes in the external field. In rare cases, the valence abruptly changes at a particular temperature or pressure. These phenomena are called valence transition phenomena.

Keyword 4: Conduction electrons and 4f electrons

The wave-like properties of electrons (wave function) are characterized by s, p, d, and f orbitals. Electrons in the s and p orbitals move freely in a material and conduct electricity. Meanwhile, Ce and the subsequent rare earth elements have an incomplete 4f shell, which are not filled with electrons, inside the outermost shell. These 4f orbitals are distributed near the atomic nucleus. Rare earth compounds are known to show unusual phenomena, including valence transition and superconductivity, as a result of the mutual interaction between localized 4f electrons and itinerant conduction electrons.

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