$4f$ electron temperature driven ultrafast electron localization (2024)

Valence transitions in strongly correlated electron systems are caused by orbital hybridization and Coulomb interactions between localized and delocalized electrons. The transition can be triggered by changes in the electronic structure and is sensitive to temperature variations, applications of magnetic fields, and physical or chemical pressure. Launching the transition by photoelectric fields can directly excite the electronic states and thus provides an ideal platform to study the correlation among electrons on ultrafast timescales. The $\mathrm{EuN}{\mathrm{i}}_2{\left({\mathrm{Si}}_{0.21}{\mathrm{Ge}}_{0.79}\right)}_2$ mixed-valence metal is an ideal material to investigate the valence transition of the Eu ions via the amplified orbital hybridization by the photoelectric field on subpicosecond timescales. A direct view on the $4f$ electron occupancy of the Eu ions is required to understand the microscopic origin of the transition. Here we probe the $4f$ electron states of $\mathrm{EuN}{\mathrm{i}}_2{\left({\mathrm{Si}}_{0.21}{\mathrm{Ge}}_{0.79}\right)}_2$ at the sub-ps timescale after photoexcitation by x-ray absorption spectroscopy across the Eu $M_5$-absorption edge. The observed spectral changes due to the excitation indicate a population change of total angular momentum multiplet states $J$ = 0, 1, 2, and 3 of $\mathrm{E}{\mathrm{u}}^{3+}$ and the $\mathrm{E}{\mathrm{u}}^{2+}J$ = 7/2 multiplet state caused by an increase in $4f$ electron temperature that results in a $4f$ localization process. This electronic temperature increases combined with fluence-dependent screening accounts for the strongly nonlinear effective valence change. The data allow us to extract a time-dependent determination of an effective temperature of the $4f$ shell, which is also of great relevance in the understanding of metallic systems' properties, such as the ultrafast demagnetization of ferromagnetic rare-earth intermetallic and their all-optical magnetization switching. In addition, our results elucidate the energetics of charge fluctuations in valence-mixed electronic systems, which provide enriched knowledge regarding the role of valence transitions and orbital hybridization for quantum critical phenomena.

• Received 27 October 2023
• Revised 9 February 2024
• Accepted 4 March 2024

DOI:https://doi.org/10.1103/PhysRevResearch.6.023099

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Condensed Matter, Materials & Applied PhysicsNonlinear Dynamics

Kohei Yamagami^1,*, Hiroki Ueda², Urs Staub³, Yujun Zhang^4,†, Kohei Yamamoto^5,‡, Sang Han Park⁶, Soonnam Kwon⁶, Akihiro Mitsuda⁷, Hirofumi Wada⁷, Takayuki Uozumi⁸, Kojiro Mimura⁸, and Hiroki Wadati^4,9

• ¹Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan
• ²SwissFEL, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland,
• ³Swiss Light Source, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland
• ⁴Department of Material Science, Graduate School of Science, University of Hyogo, 3-2-1 Kouto, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan
• ⁵Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444-8585, Japan
• ⁶PAL-XFEL, Pohang Accelerator Laboratory, 77 Cheongam-Ro, Nam-Gu, Pohang, Gyeongbuk 37673, Republic of Korea
• ⁷Department of Physics, Kyushu University, Motooka 744, Nishi-ku f*ckuoka 819-0395, Japan
• ⁸Department of Physics and Electronics, Osaka Metropolitan University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
• ⁹Institute of Laser Engineering, Osaka University, 2–6 Yamadaoka, Suita, Osaka 565-0871, Japan

• ^*Present address: Japan Synchrotron Radiation Research Institute, 1-1-1, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan.
• ^†Present address: Institute of High Energy Physics, Chinese Academy of Sciences, Yuquan Road 19B, Shijingshan District, Beijing 100049, China.
• ^‡Present address: National Institutes for Quantum Science and Technology, 6-6-11-901 Aramaki, Sendai Aoba-ku, Miyagi 980-8579, Japan.