Photoinduced valence dynamics in ${\mathrm{EuNi}}_{2}({\mathrm{Si}}_{0.21}{\mathrm{Ge}}_{0.79}{)}_{2}$ studied via time-resolved x-ray absorption spectroscopy

Photoinduced valence dynamics in ${EuNi}_{2} ({Si}_{0.21} {Ge}_{0.79})_{2}$ studied via time-resolved x-ray absorption spectroscopy

Y. Yokoyama, K. Kawakami, Y. Hirata, K. Takubo, K. Yamamoto, K. Abe, A. Mitsuda, H. Wada, T. Uozumi, S. Yamamoto, I. Matsuda, S. Kimura, K. Mimura, and H. Wadati

Phys. Rev. B 100, 115123 – Published 12 September 2019

Abstract

The photoinduced valence dynamics of ${EuNi}_{2} {({Si}_{0.21} {Ge}_{0.79})}_{2}$ were investigated using time-resolved x-ray absorption spectroscopy for Eu $M_{5}$ edge. Through the pump-probe technique with synchrotron x-ray and Ti:sapphire laser pulses, a photoinduced valence transition was observed from ${Eu}^{3 +}$ to ${Eu}^{2 +}$ . Because the lifetime of a photoinduced state can be up to 3 ns, a metastable state is considered to be realized. By comparing the experimental results with the theoretical calculations, the photoinduced valence transition between Eu $4 f$ and conduction electrons was quantitatively evaluated.

Received 10 May 2019
Revised 30 July 2019

DOI:https://doi.org/10.1103/PhysRevB.100.115123

Physics Subject Headings (PhySH)

Electronic structure

Pump-probe spectroscopy X-ray absorption spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Y. Yokoyama ^1,2,*, K. Kawakami³, Y. Hirata^1,2, K. Takubo^1,†, K. Yamamoto^1,2,‡, K. Abe³, A. Mitsuda⁴, H. Wada⁴, T. Uozumi³, S. Yamamoto^1,2, I. Matsuda^1,2, S. Kimura^5,6, K. Mimura³, and H. Wadati^1,2,§

¹Institute for Solid State Physics, University of Tokyo, Chiba 277-8581, Japan
²Department of Physics, University of Tokyo, Tokyo 113-0033, Japan
³Graduate School of Engineering, Osaka Prefecture University, Sakai 599-8531, Japan
⁴Department of Physics, Kyushu University, Fukuoka 819-0395, Japan
⁵Graduate School of Frontier Biosciences, Osaka University, Suita 565-0871, Japan
⁶Department of Physics, Graduate School of Science, Osaka Unviersity, Toyonaka 560-0043, Japan

^*Present address: Japan Synchrotron Radiation Research Institute (JASRI), Sayo 679-5198, Japan; y.yokoyama@spring8.or.jp
^†Present address: Department of Chemistry, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan.
^‡Present address: Department of Materials Molecular Science, Institute for Molecular Science, Myodaiji-cho, Okazaki, Aichi 444-8585, Japan.
^§Present address: Graduate School of Material Science, University of Hyogo, 3-2-1, Koto, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan.

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Vol. 100, Iss. 11 — 15 September 2019

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Figure 1
Eu $M_{5}$ -edge TEY XAS spectra of ${EuNi}_{2} {({Si}_{0.21} {Ge}_{0.79})}_{2}$ at 40 K. Arrows indicate the characteristic peaks of ${Eu}^{2 +}$ at 1129.6 eV and ${Eu}^{3 +}$ at 1131.5 eV.
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Figure 2
Absorption coefficient spectra of ${EuNi}_{2} {({Si}_{0.25} {Ge}_{0.75})}_{2}$ at several temperatures from the Kramers-Kronig analysis of reflectivity spectra reported in Ref. [31]. $T_{v}$ denotes the valence transition temperature.
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Figure 3
(a) Schematic diagram of setup for the pump-probe technique. The incident visible laser and x-ray pulses were overlapped at the sample surface. The delay time was defined as the time difference between the visible pump laser pulse and the probe x-ray pulse. (b) MCP output on the oscilloscope during the time-resolved measurements at a delay time $= 0.07 ns$ . The incident x-ray energy was set to 1131.5 eV ( ${Eu}^{3 +}$ ). Solid and dotted lines correspond to “X-ray: ON, laser: ON” and “X-ray: OFF, laser: ON”, respectively. The arrow indicates the laser position. The results were integrated over several thousand iterations.
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Figure 4
(a) Tr-XAS intensity for ${Eu}^{2 +}$ (1129.6 eV) and ${Eu}^{3 +}$ (1131.5 eV) obtained by the PEY mode. Dots and lines represent the experimental and fitting results. The measurements were performed at 40 K under the 1.5 eV laser irradiation with a fluence of $\sim 8 mJ / {cm}^{2}$ . Delay time was defined as the time difference between the synchrotron x-ray and laser pulse. The zero point of the delay time was determined from the fitting results. The error bars were obtained from the standard deviation of the intensity at negative delay times smaller than $- 0.15 ns$ . (b) Laser fluence dependence of XAS intensity in ${EuNi}_{2} {({Si}_{0.21} {Ge}_{0.79})}_{2}$ probed for ${Eu}^{2 +}$ (1129.6 eV) and ${Eu}^{3 +}$ (1131.5 eV) obtained by the PEY mode. During the measurements, the delay time was fixed at 0.07 ns. Dots represent the experimental results and lines denote the fitting results.
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Figure 5
Eu $M_{5}$ -edge PEY XAS spectra of ${EuNi}_{2} {({Si}_{0.21} {Ge}_{0.79})}_{2}$ at various delay times. The peaks at 1129.6 and 1131.5 eV correspond to the main peaks of ${Eu}^{2 +}$ and ${Eu}^{3 +}$ , respectively. The theoretical spectra were obtained by intra-atomic multiplet calculations of ${Eu}^{2 +}$ and ${Eu}^{3 +}$ from Ref. [18].
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Figure 6
(a) Comparison between the experimental Eu $M_{5}$ -edge PEY XAS spectra of ${EuNi}_{2} {({Si}_{0.21} {Ge}_{0.79})}_{2}$ at various delay times (dotted lines), and the theoretical spectra reproduced by the linear combinations of ${Eu}^{2 +}$ and ${Eu}^{3 +}$ (solid lines). (b) The time-dependent variation of the mean valence of Eu evaluated by the calculations. The error bars were obtained from the variation of the reference bunch spectra in Fig. 5.
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Physical Review B

covering condensed matter and materials physics

Photoinduced valence dynamics in ${EuNi}_{2} ({Si}_{0.21} {Ge}_{0.79})_{2}$ studied via time-resolved x-ray absorption spectroscopy

Y. Yokoyama, K. Kawakami, Y. Hirata, K. Takubo, K. Yamamoto, K. Abe, A. Mitsuda, H. Wada, T. Uozumi, S. Yamamoto, I. Matsuda, S. Kimura, K. Mimura, and H. Wadati

Phys. Rev. B 100, 115123 – Published 12 September 2019

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Physical Review B

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Photoinduced valence dynamics in EuNi2(Si0.21Ge0.79)2 studied via time-resolved x-ray absorption spectroscopy

Y. Yokoyama, K. Kawakami, Y. Hirata, K. Takubo, K. Yamamoto, K. Abe, A. Mitsuda, H. Wada, T. Uozumi, S. Yamamoto, I. Matsuda, S. Kimura, K. Mimura, and H. Wadati

Phys. Rev. B 100, 115123 – Published 12 September 2019

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Photoinduced valence dynamics in ${EuNi}_{2} ({Si}_{0.21} {Ge}_{0.79})_{2}$ studied via time-resolved x-ray absorption spectroscopy