Proc. Int. Conf. Strongly Correlated Electron Systems (SCES2013)
JPS Conf. Proc. 3, 011080 (2014)
http://dx.doi.org/10.7566/JPSCP.3.011080

Angle-Resolved
Photoemission
Study
Insulator-to-Metal Transition of Sm1-xYxSon

M. Kaneko1, 2, M. Saito3, T. Ito1, 4, K. Imura3, T. Hajiri1, 2, M. Matsunami2, 5,
S. Kimura2, 6, H. S. Suzuki7, N. K. Sato3
1

Graduate School of Engineering, Nagoya University, Nagoya 464-8603, JapanUVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
3
Graduate School of Science, Nagoya University, Nagoya 464-8603, Japan
4
Nagoya University Synchrotron Radiation Research Center, Nagoya University, Nagoya
464-8603, Japan
5
School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI),
Okazaki 444-8585, Japan
6
Graduate School of Frontier Biosciences and Physics Department, Osaka University, Osaka
565-0871, Japan
7
National Institute for Materials Science, Tsukuba 305-0047, Japan2

E-mail: t.ito@numse.nagoya-u.ac.jp(Received October 20, 2013)
Samarium monosulfide (SmS) is a prototypical compound that exhibits an insulator-to-metaltransition, namely black-golden phase transition, by applying external pressure or by achemical pressure owing to the substitution of Sm ions by yttrium (Y) ions. To directly
observe the change of the electronic structure of the black-golden phase transition, we have
performed angle-resolved photoemission spectroscopy on Sm1-xYxS. From x = 0 (insulator) to0.33 (metal), the Sm2+ 4f multiplet structure continuously shifts to the Fermi level (EF). On theother hand, the S 3p bands show the discontinuous shift with respect to x = 0.17. Furthermore,on metallic compound of x = 0.17, we find that an electron pocket appears at the X point,where the bottom of Sm 5d states is expected to be located just above EF in insulating SmS by
a band calculation. Thus, the observed electron pocket is considered to originate from Sm 4f –5d hybridization due to the chemical pressure and it is a possible origin of the insulator –metal transition of SmS.
KEYWORDS: SmS, ARPES, band structure, insulator – metal transition, heavy
fermion, valence fluctuation

1.

Introduction

Samarium monosulfide (SmS) is a prototypical valence fluctuating compound thatexhibits pressure-induced electronic transition, namely insulator (black) – metal (golden
yellow) phase transition by applying external pressure (~ 6.5 kbar at room temperature)[1, 2]. Across the insulator – metal transition, the valence of Sm changes from 2+ to 2+δ,
so-called valence fluctuating state. In spite of many efforts from experiment and theory[3], the origin of this anomalous phase transition especially the essential role of 4f
electrons has not been elucidated yet within 30 years. So far, there are two contrary■■■
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JPS Conf. Proc. 3, 011080 (2014)

models for understanding this state: “local” Sm2+ and Sm3+ coexist and configure amixed-valence state, and Sm 4f electrons and Sm 5d conduction electrons hybridize“itinerantly” and form a heavy fermion state [4, 5].
Angle-resolved photoemission spectroscopy (ARPES) is a powerful technique to
directly study the electronic structure of solids as a function of momentum. However,
the technique is hard to apply under pressure. Thus, to study the pressure-dependentchange of the electronic structure by ARPES, we need help of the chemical pressure
effect, i.e., substitution dependence. From this viewpoint, recently, it has been realized
on Sm1-xYxS. Actually, the lattice constant of Sm1-xYxS decreases like SmS as Y
substitution, and shows the slight jump around x = 0.17 [6] where the color of sample
surface changes from black to golden, and the resistivity changes from insulating tometallic one. To clarify the change of the electronic structure across the black-golden
phase transition, we have compared the normal emission spectra between x = 0 (black)and 0.33 (golden) in the previous study [6]. Then we found that the Sm2+ 4f multiplet
structures shift to lower binding energy side from x = 0 to 0.33 in consistent withincreasing metallic nature. However, both the origin of the change of the electronic
structure and its relation with the character of Sm 4f electrons have not been elucidated.
In the present work, we investigated the change of the electronic structure of
Sm1-xYxS across the insulator – metal transition using ARPES [6, 7]. The results indicatethat the Sm2+ 4f multiplet structure is gradually moved to the Fermi level (EF) from x =0 (black insulator) to 0.17 (black metal) and 0.33 (golden metal), while the S 3p bands
show the discontinuous energy shift. In addition, we have observed on x = 0.17 theelectron pocket appearing at the X point, where the Sm 5d states are expected just above
EF on SmS. Furthermore, the observed electron pocket shows typical hybridization with
the Sm2+ 4f multiplet state. These results suggest that the increase of the Sm 4f - 5d
hybridization intensity is a possible origin of the anomalous insulator – metal transition
under the pressure of SmS.
2. ExperimentSingle crystals of Sm1-xYxS (x = 0, 0.17 and 0.33) were grown by the Bridgman
technique using a high-frequency induction furnace installed at NIMS. ARPES
measurement was performed at BL5U of UVSOR-III, Institute for Molecular Science [8].
The total energy and angular resolutions were set at 60 – 180 meV and 0.16°, respectively.
A clean surface of Sm1-xYxS (001) plane was obtained by in-situ cleaving at 10 – 20 K
just before the measurement. The crystal orientation was determined by the Lauediffraction method.
3. Results and Discussion
Figure 1 show the photon-energy-dependent ARPES spectra at T = 10 K in thenormal emission geometry of Sm1-xYxS [x = 0 (a), 0.17 (b), and 0.33 (c)], respectively.
From the comparison with the ARPES on black SmS [6, 7], the highly-dispersive feature
at 4 eV is ascribed to the S 3p band, while nondispersive ones near the Fermi level (EF) to
the Sm2+ 4f multiplet structures. With increasing x, it is clear that the Sm2+■■■

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JPS Conf. Proc. 3, 011080 (2014)

(b)

(a)

(c)

Fig. 1. Photon-energy-dependent ARPES spectra at normal emission of (a) x = 0, (b) 0.17, and (c) 0.33
of Sm1-xYxS measured at T = 10 K.

Fig. 2. Schematic Brillouin zone in the k//-k⊥plane of black SmS (the lattice constant a =5.97 Å). The Γ and X points correspond to thephoton energies of h = 58 and 31 eV at EF,
respectively. The blue vertical line and theblue curved line indicate the ARPES tracelines in Fig. 1 (Fig. 3) and Fig. 5, respectively.

4f multiplet peaks shift to EF. On the other hand, the dispersive features show no distinct
change around 4 eV, though the photon energy of the S 3p folding point shifts to thehigher photon energy side.
In order to see the change of the electronic structure more clearly, we have mappedout the experimental band structures of Sm1-xYxS (x = 0, 0.17 and 0.33) in Fig. 3. The
experimental band structures correspond to the trace of the electronic structure along theΓX high-symmetry line of k⊥ axis (see Fig. 2) following





2m
(1)
h  EB   cos 2   V02

where h is photon energy, EB is binding energy,  is work function evaluated from the
EF edge of reference gold,  = 0°, and V0 is inner potential estimated from the symmetry
ki  

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JPS Conf. Proc. 3, 011080 (2014)

(a)

(b)

(c)

Fig. 3. Band structure along ΓX line (k⊥) of Sm1-xYxS [(a) x = 0, (b) 0.17 and (c) 0.33].

(a)

(b)

Fig. 4. Comparison of the EDCs of x = 0, 0.17 and 0.33 at the Γ point (a) and the X point. Solid anddotted lines indicate the Sm2+ 4f multiplet peaks of bulk and surface states, respectively [7]. Inset shows
the x-dependent S 3p peaks in enlarged scale.

of dispersive features. From our systematic APRES experiment, we have obtained theinner potentials V0 = 14.1 eV, 15.2 eV, and 17.5 eV for x = 0, 0.17, and 0.33,
respectively. With utilizing the clear dispersive features of S 3p band in Fig 3, we havechecked the consistency with the lattice constant and the size of the Brillouin zone along
ΓX line for each sample carefully, and successfully obtain the reasonable innerpotentials as shown above. In Fig. 4, the broad highly dispersive feature from 4 to 6 eVaround the Γ point also appears on each band structure, which is consistent with the inplane S 3p band dispersion of SmS [6]. Furthermore, even though the Sm2+ 4f multiplet
structure has no dispersion, the intensity is clearly modulated at the Γ point, which has
been ascribed to be the implication of the existence of the lattice effect even in the black
SmS [7].

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JPS Conf. Proc. 3, 011080 (2014)

(a)

(b)

Fig. 5. (a) The thorough ARPES image of x = 0.17 along the XWX line using h = 34 eV at T = 20 K. MDC
at EF is shown at the top of image. (b) ARPES spectra near EF around the X point. Dashed lines indicate theSm2+ 4f multiplet structures. Solid circles (crosses) and solid squares are ARPES peak positions of strong(weak) dispersive features of Sm 5d electron pocket and itinerant Sm 4f features, respectively.

To investigate the change of electronic structure with the Y substitution clearly, we
directly compare the energy distribution curves (EDCs) at the Γ and X points obtained
from the vertical cut (white dashed line) in Fig. 3. With choosing the electronic
structure at the exactly same Brillouin zone point, the direct comparison of electronic
structure among different Y substitution samples with different lattice constants
becomes possible. In Fig. 4, Sm2+ 4f multiplet structures of bulk (b) and surface (s)components systematically shift to EF with increasing x and touch to EF at x = 0.33 as
described above. With respect to S 3p features, however, we find very slight energy
shift at both Γ and X points as shown in the inset of Fig. 4. From black insulating (x = 0)
to black metallic (x = 0.17) phase, the S 3p band at the Γ (X) point shifts to about 20
meV (10 meV) to lower (higher) binding energy side. In addition, the S 3p peak at the X
point becomes broader than one at the Γ point. On the other hand, from black metallic (x= 0.17) to golden metallic (x = 0.33) phase, S 3p bands at both the Γ and X points showhigh binding energy shift of 90 meV and 130 meV, respectively. The observed
discontinuous change of the S 3p band at x = 0.17 might be the discontinuous change ofthe interactions between S 3p and other electrons, as the lattice constant jumps at x =0.17 [6]. To understand the origin of the discontinuous change of the electronic structure,
we have investigated the electronic structure near EF in the wide Brillouin zone.
Figure 5 shows an ARPES image and energy distribution curves of x = 0.17 near EF
around the X point (see Fig. 2). We successfully find an electron pocket along WXWhigh-symmetry line. Though the dispersive feature becomes much broader at the second
Brillouin zone than that at the first Brillouin zone possibly due to the matrix element
effect, we find a symmetric dispersive feature against to the strong dispersion at the X
point (see circles/crosses in Figs. 5(a) and 5(b)). According to the band calculation of
insulating SmS, Sm 5d states have been expected to be located just above EF at the X
point. Thus we conclude that the EF crossing band can be ascribed as the electron pocket
at the Brillouin zone boundary. In strong contrast to the nondispersive Sm2+ 4f multiplet■■■

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JPS Conf. Proc. 3, 011080 (2014)

structure (6H (s) and 6F (b)) around 1.2 eV, 6H (b) structure shows anomalous dispersive
feature around the X point, where the bottom of the electron pocket merges (see squares
in Figs. 5 (a) and 5(b)). From the analogue of the occupied c-f hybridized band reported
on heavy fermion compounds [9], the observed anomaly seems to suggest the itinerant
Sm 4f character hybridized with the Sm 5d band. From the observed electron pocket
formation at x = 0.17, we expect that the increase of the hybridization intensity between
Sm 5d and S 3p electrons in the metallic phase is one possible origin of the larger
energy shift as well as the broadening of S 3p band around the X point. To further
insight into the anomalous phase transition in SmS, more systematic ARPES studies on
Sm1-xYxS are intended.
3. Conclusion
We performed high-resolution angle-resolved photoemission spectroscopy on
Sm1-xYxS (x = 0, 0.17 and 0.33) to observe the change of the electronic structure acrossthe insulator – metal transition. With increasing x, the Sm2+ 4f multiplet structure shifts
to EF systematically, but the S 3p bands shift to the lower binding energy side from x = 0
to 0.17 and are pushed down to high binding energy side from x = 0.17 to 0.33. In themetallic phase of x = 0.17, we successfully observed the Sm 5d electron pocket thatpossibly hybridizes with Sm 4f state around the X point. The results suggest that theincrease of the Sm 4f – Sm 5d hybridization intensity is one possible origin of theinsulator – metal transition of SmS.
Acknowledgment
The authors gratefully acknowledge M. Sakai and T. Horigome for their technicalsupports for the experiments. This work was performed as a Use-of-UVSOR Facility
Program (BL5U, 2011, 2012).
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