UVSOR-II, BL7U

Temperature and substitution dependence of extremely low-energy
photoemission spectra on Sm1-xEuxB6
J. Yamaguchi1, A. Sekiyama1, M. Y. Kimura1, H. Sugiyama1, S. Komori1, Y. Tomida1,G. Kuwahara,1 T. Ito2, S. Kimura2, S. Yeo3, S.-I. Lee4, H.-D. Kim5, and S. Suga1
1
Graduate school of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531,
Japan
2
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
3
Korea Atomic Energy Research Institute, Daejeon 305-6000, Korea
4
Department of Physics, Sogang University, Seoul 121-742, Korea
5Pohang Accelerator Laboratory, Pohang 790-784, Korea

Sm1-xEuxB6
Intensity (arb. units)

SmB6 and YbB12 have been well known as valencefluctuating (VF) Kondo semiconductors (insulators)
and intensively studied because of their physicalproperties. The Kondo semiconductors behave as
metals with localized f magnetic moments at hightemperatures (T), whereas they develop a narrowhybridization gap (~10 meV) at the Fermi level (EF) at
low T. It is thought that the gap formation originatesfrom the hybridization of the narrow f band with broaditinerant valence-bands, but the mechanism of the gap
is still controversial.
In our previous hard x-ray photoemission(HAXPES) studies for the Lu substitution Kondo
semiconductor alloys Yb1-xLuxB12 [1], we have foundfrom the T dependence of the Yb 4f spectral analyses
that the 4f lattice coherence plays important roles fordeveloping the gap. The gap for YbB12 is suggested to
be rapidly closed by the Lu substitution of x = 0.125
due to the collapse of the 4f lattice coherence. On theother hand, our HAXPES study for the Eu
substitution alloys Sm1-xEuxB6 shows that the T
dependence of the Sm2+ 4f spectra for x = 0.15 isqualitatively similar to that for pure YbB12. Thus, it isexpected from our HAXPES results that a finite gap
is still open for Sm0.85Eu0.15B6, although the 4f latticecoherence could be broken by the Eu substitution. In
order to directly investigate the existence of the gap,
we have performed the extremely low-energy
photoemission (ELEPES) study on Sm1-xEuxB6 (x = 0,0.15, and 0.5). The ELEPES spectra for x = 0.15 and
0.5 were measured by use of synchrotron radiation(hν = 7 eV) at UVSOR-II BL7U and those for x = 0and 0.5 were measured with the Xe I (hν = 8.4 eV)
resonance line. The energy resolution was set to ~6meV in all measurements.
Figure 1 shows the T dependence of the ELEPES
spectra near EF for x = 0, 0.15, and 0.5. According to
the photoionization cross section [2], the observed
spectra are dominated by the non-4f (Sm and/or Eu 5dand B 2sp) states. For x = 0, we find that a so-calledleading-edge of the spectra is on the occupied side,which indicates the existence of the finite gap. Inaddition, the prominent peak is observed at ~15 meV,which is comparable to that due to the magnetic
excitation
observed
byneutron
scatteringx=0x = 0.15

x = 0.5

T (K) =
200
150
100
55155hν = 8.4 eVhν = 7 eVhν = 8.4 eV

100

50
0-50
Binding Energy (meV)Fig. 1. Temperature dependence of the ELEPES
spectra near EF for Sm1-xEuxB6.

measurements for SmB6 [3]. With increasing T from 5
to 200 K, The spectral weight on EF increases and thepeak shits toward the higher binding energy side. It
should be noted that the spectra for x = 0.15 show theessentially equivalent T dependence for x = 0, whichindicates that Sm0.85Eu0.15B6 is still a Kondo
semiconductor against the collapse of the 4f latticecoherence. In contrast, the spectra for x = 0.5 show noprominent peak and thus a typical metallic thermal
behavior. These ELEPES results indicate that SmB6 isa “robust” Kondo semiconductor against a rare-earthsubstitution, which is significantly different from
YbB12.
[1] J. Yamaguchi et al., Phys. Rev. B 79, 125121(2009).
[2] J. J. Yeh and I. Lindau, At. Data Nucl. Data Tables
32, 1 (1985).
[3] P. A. Alekseev et al., J. Solid State Chem. 133,
230-236 (1995).