Photon Factory Activity Report 2019 #37 (2020)

BL-2A/2016G096

Resonance photoemission spectroscopy study of SmO thin films
Shoya SAKAMOTO1,*, Kenichi KAMINAGA2,3, Daichi OKA4, Ryu YUKAWA5,
Masafumi HORIO1, Yosuke NONAKA1, Keisuke KOSHIISHI1, Masaki KOBAYASHI5,
Hiroshi KUMIGASHIRA5,6, Tomoteru FUKUMURA3,4,7, and Atsushi FUJIMORI1,8
Department of Physics, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
Department of Chemistry, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
3WPI-Advanced Institute for Materials Research and Core Research Cluster, Tohoku University,Sendai, Miyagi 980-8577, Japan
4
Department of Chemistry, Tohoku University, Sendai, Miyagi 980-8578, Japan
5
Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research
Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan
6
Department of Physics, Tohoku University, Sendai, Miyagi 980-8578, Japan
7
Center for Spintronics Research Network, Tohoku University, Sendai, Miyagi 980-8578, Japan
8
Department of Applied Physics, Waseda University, Shinjuku-ku, Tokyo 169-8555, Japan1

23 Results and Discussion
Figure 1(a) shows valence-band photoemission spectraof the SmO thin film taken with the photon energies of1070 eV (off resonance) and 1079 eV (on resonance).
These photon energies correspond to the energies below
and at the Sm M5 absorption edge as indicated by bars on
the x-ray absorption spectrum shown in Fig. 1(c). The
photoemission spectra are normalized to their maxima.

Intensity [arb. unit](b)

SmO/YAlO3
11 K

O 2p + Sm 5d

Sm 4f
Off res.
hν = 1070 eV

AlOx 5 nm
On res.
hν = 1079 eVhν = 1300 eV-10 eV -8-6-4
E - EF [eV]-20-0.8 0 0.8
E - EF [eV]
SmM5(c)XAS [arb. unit]

2 Experiment and Calculation
A 20-nm-thick SmO (001) epitaxial thin film was grown
on YAlO3 (110) substrates by pulsed laser deposition. Thesample was capped with a 2-nm-thick AlOx layer to protect
the sample from oxidation. The photoemission experiment
was performed at the beamline BL-2A of Photon Factory.
The measurements were done at the temperature of 11 K
with the photon energies of 1-1.3 keV, which cover the Sm
M4,5 absorption edge.

(a)Intensity [arb. unit]1 Introduction
Rare-earth materials show intriguing physicalphenomena such as valence fluctuation, Kondo insulatingstate, and heavy-fermion behavior. Samarium compoundswith mixed valence are attracting renewed interest becausesome of them are proposed to have topologically nontrivial band structures [1,2]. Recently, SmO thin films havebeen synthesized and reported to have the mixed valenceof 2.9+ [3], although Sm chalcogenides such as SmS and
SmSe show the integer valence of 2+ at ambient pressure.
This mixed-valence state of SmO consisting of 4f6 (Sm2+)
and 4f55d1 (Sm3+) configurations appears because the smalllattice constant of SmO compared to the Sm chalcogenides
makes the bottom of the 5d conduction bands lie below the4f levels.
In the present study, we have performed resonancephotoemission spectroscopy measurements of SmO thinfilm at the Sm M5 absorption edge to reveal the valenceband structure.

1073 1083hν [eV]

Fig. 1 (a) Resonance valence-band photoemission spectraof the SmO thin film. (b) Magnified plot around the Fermi
level. (c) Sm M5-edge absorption spectrum.

The signals from the capping AlOx layer were separately
measured using the sample with a thick (5 nm) capping
layer as shown at the bottom of Fig.1(a). The featurelessspectrum of AlOx guarantees that the valence-band
photoemission spectra reflect the electronic structure ofSmO.
The on-resonance spectrum, in which the Sm 4f
contribution is enhanced, looks significantly different fromthe off-resonance spectrum. This indicates that the offresonance spectrum predominantly consists of O 2p bands
strongly hybridized with Sm 5d orbitals. The off-resonancespectrum shows a clear Fermi edge as shown in Fig. 1(b).
This Fermi edge most likely originates from the Sm 5dbands, indicating that Sm has a mixed valence with finite
4f55d1 (Sm3+) contribution.
References[1] F. Lu, J. Zhao, H. Weng, Z. Fang, and X. Dai, Phys.
Rev. Lett. 110, 096401 (2013).

Photon Factory Activity Report 2019 #37 (2020)[2] C.-J. Kang, D.-C. Ryu, J. Kim, K. Kim, J.-S. Kang, J.
D. Denlinger, G. Kotliar, and B. I. Min, Phys. Rev.
Mater. 3, 081201 (2019)[3] Y. Uchida, K. Kaminaga, T. Fukumura, and T.
Hasegawa, Phys. Rev. B 95, 125111 (2017)* shoya.sakamoto@issp.u-tokyo.ac.jp