Surface valence transition in SmS by alkali metal adsorption

Letter

Surface valence transition in SmS by alkali metal adsorption

Takuto Nakamura, Toru Nakaya, Yoshiyuki Ohtsubo, Hiroki Sugihara, Kiyohisa Tanaka, Ryu Yukawa, Miho Kitamura, Hiroshi Kumigashira, Keiichiro Imura, Hiroyuki S. Suzuki, Noriaki K. Sato, and Shin-ichi Kimura

Phys. Rev. B 107, L041102 – Published 5 January 2023

Abstract

The electronic structure changes of SmS surfaces under potassium (K) doping are elucidated using synchrotron-based core-level photoelectron spectroscopy and angle-resolved photoelectron spectroscopy (ARPES). The Sm core-level and ARPES spectra indicate that the Sm mean valence of the surface increased from the nearly divalent to trivalent states, with increasing K deposition. Carrier-induced valence transition (CIVT) from ${Sm}^{2 +}$ to ${Sm}^{3 +}$ exhibits a behavior opposite to that under conventional electron doping. Excess electrons from K atoms are transferred to S sites and the liberated electrons from ${Sm}^{3 +}$ ions due to CIVT at the surface are trapped like local excitons around the ${Sm}^{3 +}$ ions, which is inconsistent with the phase transition from the black insulator with ${Sm}^{2 +}$ to the gold metal with ${Sm}^{3 +}$ under pressure. This CIVT helps to clarify the pressure-induced black-to-golden phase transition in this material, which originates from the Mott transition of excitons.

Received 30 May 2022
Revised 13 December 2022
Accepted 14 December 2022

DOI:https://doi.org/10.1103/PhysRevB.107.L041102

Physics Subject Headings (PhySH)

Electronic structure

Heavy-fermion systems Strongly correlated systems

Angle-resolved photoemission spectroscopy X-ray photoemission electron microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Takuto Nakamura ^1,2,*, Toru Nakaya², Yoshiyuki Ohtsubo ^3,1,2, Hiroki Sugihara², Kiyohisa Tanaka⁴, Ryu Yukawa⁵, Miho Kitamura⁶, Hiroshi Kumigashira ⁷, Keiichiro Imura ^8,†, Hiroyuki S. Suzuki⁹, Noriaki K. Sato^8,10, and Shin-ichi Kimura ^1,2,4,‡

¹Graduate School of Frontier Biosciences, Osaka University, Suita 565-0871, Japan
²Department of Physics, Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan
³National Institutes for Quantum Science and Technology, Sendai 980-8579, Japan
⁴Institute for Molecular Science, Okazaki 444-8585, Japan
⁵Graduate School of Engineering, Osaka University, Suita 565-0871, Japan
⁶Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba 305-0801, Japan
⁷Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, Sendai 980-8577, Japan
⁸Department of Physics, Nagoya University, Nagoya 464-8602, Japan
⁹Institute for Solid State Physics, The University of Tokyo, Kashiwa 277-8581, Japan
¹⁰Center for General Education, Aichi Institute of Technology, Toyota 470-0392, Japan

^*nakamura.takuto.fbs@osaka-u.ac.jp
^†Present address: National Institutes for Institute of Liberal Arts and Sciences, Nagoya University, Nagoya 464-8601, Japan.
^‡kimura.shin-ichi.fbs@osaka-u.ac.jp

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Issue

Vol. 107, Iss. 4 — 15 January 2023

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Images

Figure 1
(a),(b) Sm $3 d$ and (c),(d) Sm $4 d$ core-level spectra of SmS single crystals as a function of the amount of K deposition, with excitation photon energies at (a),(c) 1150 eV and (b),(d) 1500 eV. (e) Schematic of the photoemission detection depth for different kinetic energies. (f) Sm mean valence depending on K deposition, evaluated from the peak area ratio between the ${Sm}^{3 +}$ and ${Sm}^{2 +}$ multiplets after subtracting the Shirley-type background. (g) Sm mean valence as a function of the mean free path of photoelectrons. Dashed lines indicate the fitting curves expected with a two-layer model, shown in Supplemental Material S3. (h) Surface and bulk Sm mean valences as a function of the amount of K deposition, evaluated with the two-layer model.
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Figure 2
(a) Three-dimensional bulk Brillouin zone of SmS. (b) Photon-energy dependence of valence band spectra at normal emission ( $k_{x}$ = $k_{y}$ = 0 $Å^{- 1}$ ). Intensities of the photoelectron are normalized by maximum counts of the valence band spectra at each excitation energy in the measured energy range (binding energy = 0–6.0 eV). Inner potential was set to 14.1 eV, based on the periodicity of the S $3 p$ band around 4.5 eV. (c) ARPES intensity plot and (d) second derivative of the ARPES intensity of the pristine-SmS surface along $Γ$ – $X$ , with 60 eV photons ( $k_{z} \sim 0 Å^{- 1}$ ), respectively. (e),(f) Same as (c),(d) but for the K-adsorbed SmS ( $K / SmS$ ) surface. White dashed lines serve as guides for the band dispersion.
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Figure 3
(a),(b) Second derivative of ARPES intensity maps near the Fermi level along $Γ$ – $X$ , with 60 eV photons of (pristine) SmS and 0.4 ML–K/SmS. $^{6} H$ and $^{6} F$ indicate the multiplet structures of the Sm $4 f^{5}$ final state after photoexcitation. (c) Energy distribution curves (EDCs) at the $Γ$ point (lower panel) and the $X$ point (upper panel) of the SmS surface with the fitting curves. Dots and lines represent the raw data and fitted spectra (dash-dotted line: bulk component; dash-double-dotted line: surface component; solid line: sum of the bulk and surface components). Each component is fitted by the Voigt function after subtracting the Shirley-type background (dashed line). (d) Same as (c) but for the 0.4 ML–K/SmS surface.
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Figure 4
(a) EDCs at the $Γ$ point (black line) and $X$ point (green line) of pristine SmS with 200 eV photons. (b) Same as (a) but for 0.4 ML–K/SmS. S $2 p$ core-level spectra of (c) pristine SmS and (d) K/SmS with 200-eV photons. Dashed lines indicate the Shirley-type background. Blue dots and black lines represent the raw data and fitted curve using the Voigt function and background. (e) Schematic of the phase transition of SmS. Top left and top right images depict the real space image of pristine SmS and K/SmS, respectively. The large (small) sphere indicates the Sm (S) ion. The bottom left figure is an expected model of electron-doped SmS, while retaining the same lattice constant as pristine SmS, in contrast to the golden phase of SmS, by applying pressure, as shown in the bottom right figure. Yellow circles depict the electron-hole pairs (excitons).
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