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Discovery of ‘quantum’ supercritical state of electrons induced by valence fluctuation

The University of Tokyo
RIKEN

Overview:

The group by research fellow Kentaro Kuga (affiliated at the time of the research), research associate Yosuke Matsumoto (affiliated at the time of the research), professor Satoru Nakatsuji and others at the Institute for Solid State Physics, the University of Tokyo (Director Masashi Takigawa), in collaboration with RIKEN (President Hiroshi Matsumoto) SPring-8 Center, has discovered a crossover where the valence of the ytterbium ion changes rapidly by adding a very small amount (~ 1.4%) of iron into the ytterbium compound (α-YbAl1-xFexB4). The valence was measured by hard X-ray photoemission spectroscopy using SPring-8 RIKEN beamline BL29XU. The valence is unstable around the crossover region and the large critical valence fluctuation is expected.

The state of ytterbium ions and the change of the valence, volume and the resistivity against the amount of iron substitution x in α-YbAl1-xFexB4. The high valence state and the low valence state are fluctuating at the valence crossover region as illustrated at the top of the figure. The arrows indicate the magnetism. The middle of the figure indicates the sharp valence change at around x = 0.014 at the absolute temperature of 20 Kelvin. The volume shows similar x dependence to the valence. The image plot at the bottom of the figure illustrates the strange metallic state, the normal metallic state and the magnetic ordered state in the yellow and red regions, left side of the blue region and the right side of the blue region, respectively. The image plot is colored by the exponent of the temperature dependence of resistivity. The quantum critical point exists at x = 0.014 and at absolute zero temperature.
The state of ytterbium ions and the change of the valence, volume and the resistivity against the amount of iron substitution x in α-YbAl1-xFexB4.
The high valence state and the low valence state are fluctuating at the valence crossover region as illustrated at the top of the figure. The arrows indicate the magnetism. The middle of the figure indicates the sharp valence change at around x = 0.014 at the absolute temperature of 20 Kelvin. The volume shows similar x dependence to the valence. The image plot at the bottom of the figure illustrates the strange metallic state, the normal metallic state and the magnetic ordered state in the yellow and red regions, left side of the blue region and the right side of the blue region, respectively. The image plot is colored by the exponent of the temperature dependence of resistivity. The quantum critical point exists at x = 0.014 and at absolute zero temperature.

In addition to the valence crossover, the group discovered a strange metallic state that did not show magnetic order down to absolute zero temperature (non-magnetic properties) in spite that magnetization became quite large (magnetic properties). The discovery of both a valence crossover and a strange metallic state indicates the fluctuation between the high valence state exhibiting magnetic properties and the low valence state showing nonmagnetic properties (figure). The detailed analysis of the magnetization also indicated that the quantum critical point exists at the 1.4% of the additive amount of iron and at the absolute zero temperature. This is the first discovery in the world to illustrate the quantum valence critical point and the existence of its related strange metallic state. It is also true that the supercritical fluid that is being widely applied industrially has realized in the form of valence change. It is expected to facilitate discovery of “quantum” supercritical fluids in fluids like water and carbon dioxide.

This research paper was published on the online edition of Science Advances, the U.S. science journal on February 23, 2018.


Reference:

  • Journal: Science Advances (2018)
  • Title: Quantum Valence Criticality in a Correlated Metal
  • Author: Kentaro Kuga+, Yosuke Matsumoto+, Mario Okawa, Shintaro Suzuki, Takahiro Tomita, Keita Sone, Yasuyuki Shimura, Toshiro Sakakibara, Daisuke Nishio-Hamane, Yoshitomo Karaki, Yasutaka Takata, Masaharu Matsunami, Ritsuko Eguchi, Munetaka Taguchi, Ashish Chainani, Shik Shin, Kenji Tamasaku, Yoshinori Nishino, Makina Yabashi, Tetsuya Ishikawa, Satoru Nakatsuji* (+:equal contribution, * Corresponding author)
  • DOI: 10.1126/sciadv.aao3547
(Published on: Tuesday March 20th, 2018)