Synchrotron SOLEIL CASSIOPEE

Valence-band Electronic Structure of FeSi studied by high-resolution
Angle-Resolved Photoelectron Spectroscopy (ARPES)
Y. Takeichi1, A. Nishide1, K. Nakatsuji1, T. Okuda1, F. Komori1, J. Yoshinobu1, H. Kondoh2,
I. Matsuda1, F. Bertran3, P. Le Fevre3, A. Taleb-Ibrahimi3, H. Yamagami4, A. Kakizaki1
1
ISSP, the Univ. of Tokyo, Kashiwanoha, Kashiwa 277-8581, Japan2
Department of Chemistry, Keio University, Yokohama 223-8522, Japan
3
SOLEIL / LURE, Saint-Aubin, BP 48, 91192 Gif-sur-Yvette Cedex, France
4
Department of Physics, Kyoto Sangyo Univ., Kamigamo, Kita-ku, Kyoto 603-8555, Japan
Among transition metal mono-silicides: MSi(M=Cr, Mn, Fe, Co, Ni), FeSi has been intensively
studied as one of the possible candidates for thed-electron Kondo insulators. While FeSi has an
insulating ground state, electric conductivity shows asemiconductor to insulator crossover at 300K.
Another interesting feature is the temperaturedependence of its magnetic susceptibility, which rises
nearly exponentially up to 500K making a broad
maximum followed by a Curie-Weiss law at evenhigh temperatures. To explain these unusual electricand magnetic properties, many theoretical models
have been discussed so far. They are for example,
thermally
excitedspin-fluctuation
theory,phenomenological models assuming two narrow dbands in the vicinity of the band gap. Such a model
density of states is quite similar to those for Kondo
insulator description, and due to this similarity, it wasoften claimed that FeSi is the first Kondo insulator
containing no f-electrons.
A number of valence band photoemission
spectroscopy studies have been performed toelucidate the electronic structures near the Fermi level(EF) [2]. In spite of careful measurement with
sufficiently high-energy resolution smaller than theexpected semiconductor band gap, results are notconclusive about the existence of the band gap [3].
Recently, Klein et al. made an effort to cleave a single
crystal at (001) plane and observed ARPES spectra[4]. Though the sample surface consists of several
faces, they observed that the spectral features arealmost consistent with calculated band structure alongthe XM plane in the Brillouin zone showing asharp peak feature at 300meV below EF. In thefollowing, we present the valence band structures ofFeSi(001) single crystal surface observed by ARPESand discuss on the electronic structures of thismaterial.
Experiments were carried out at CASSIOPEE
beamline of SOLEIL. The ARPES apparatus consistsof a sample preparation chamber equipped with anAuger electron spectrometer (AES) and a low energy
electron diffraction (LEED) apparatus and an analysis
chamber with a hemispherical electron energyanalyzer (SCIENTA R4000). Clean FeSi(001) single

crystal surface was obtained by the proceduredescribed elsewhere [5].

Figure 1 sows the valence band structure ofFeSi(001) single crystal surface observed along the-X direction in the Brillouin zone, where the Fe 3d
originated valence band shows a very small energy
dispersion. The results are basically consistent withthe calculated valence band structure with an energy
gap very close to the EF. The obtained band
dispersion well reproduces the theoretical calculation,
except the reduction of the bandwidth of about 30%
due to the renormalization effect.
This work resolves the controversial evidences,which have been provided for the arguments of theKondo-like interaction in this material.

Fig. 1.

ARPES spectra of FeSi(100).

[1] C. Fu and S. Doniach, Phys. Rev. B 51, 17439(1995).
[2] A. Kakizaki, et al., J. Phys. Soc. Jpn., 51, 2597
(1982).
[3] K. Breuer, et al., Phys. Rev. B 56, R7061 (1995).
[4] M. Klein, et al., Phys. Rev. Lett., 101, 046406(2008).
[5] K. Kura, et al. J. Phys. Soc. Jpn., 77, 024709(2008).