Photon Factory Activity Report 2004 #22 Part B (2005)
Electronic Structure of Condensed Matter

19B/2003G008

Ir 5d state in CuIr2S4 studied by photoemission and x-ray emission spectroscopies
Katsuyuki KITAMOTO, Yukihiro TAGUCHI*, Tatsunori NODA, Yukihiro FUJITA,
Kojiro MIMURA, Kouichi ICHIKAWA, Hiroki ISHIBASHI, Osamu AITA
Osaka Prefecture University, Gakuencho, Sakai, Osaka 599-8531, Japan
Introduction
CuIr2S4 exhibits metal-insulator and paramagneticdiamagnetic transitions around 226 K [1]. It is consideredthat the transitions are associated with simultaneous
charge-ordering and spin-dimerization of Ir ions [2].
While band-structure calculations show a sharp peak of Ir
5d partial density of states at 1 eV below the Fermi levelfor CuIr2S4 with the crystal structure corresponding to thehigh-temperature metallic phase [3], there has not existed
a corresponding sharp structure in the photoemission
spectrum [4]. In this work we have measured thephotoemission spectrum of CuIr2S4 at various excitation
photon energies in order to identify Ir 5d in the valenceband by the use of energy dependence of photoionization
cross-section. A sulfur L2,3 x-ray emission spectrum has
also been measured to confirm the hybridization of S 3pwith Ir 5d states.

Normalized Intensity

Intensity (arb. units)

Results and Discussion
Fig. 1 shows the valence-band photoemission spectraof CuIr2S4 taken with hv=180, 480, and 630 eV at room
temperature. The sample used was a sintered polycrystal.
The sample surface was cleaned in situ by scraping. Thespectra are normalized at 14 eV, a peak due to S 3s. The
intensity of peak at 2.8 eV and that of shoulder around 6eV show photon-energy dependence similar to ionization
cross-sections of Cu 3d and S 3p, respectively. The insetof Fig. 1 shows photon-energy dependence of intensity at

31 eV
Cu 3dS 3p
Ir 5d2

CuIr2S4PES

10200

400600

Photon energy (eV)hv (eV)
180480630

151050Binding energy (eV)Fig. 1. Valence-band photoemission spectra of CuIr2S4
taken with hv=180, 480 and 630 eV at room temperature.
The spectra are normalized at 14 eV. Solid circles in theinset denote the intensity at 1 eV relative to that at 14 eV.

108

1 eV (solid circles) relative to that at 14 eV along with theatomic photoionization cross-sections of Cu 3d (blue line),
Ir 5d (red) and S 3p (solid) relative to that of S 3s. All theratios in the inset of Fig. 1 are normalized at 630 eV. With
increasing excitation-photon energy the relative intensity
at 1 eV decreases up to ~480 eV and then increases. Only
the ratio of (Ir 5d)/(S 3s) increases monotonically in theenergy region from 180 to 630 eV. This indicates that theshoulder structure at 1 eV has contribution from Ir 5d,
although not so large as is predicted by the calculations.
The degeneracy of Ir 5d t2g is the cause of the sharp peakat 1 eV in the calculations. The IrS6 octahedron suffers
trigonal field even in the high-temperature cubic phase [2].
This lifts the degeneracy of t2g and may be partly
responsible for the discrepancy between experiments andcalculations. Alternatively the correlation effect on Ir 5delectrons may result in the discrepancy. We note that thecontribution from Ir 5d to the 1-eV shoulder in thephotoemission spectrum was neglected in our previous
analysis with cluster-model calculations [5] and that weoverestimated the on-site Ir 5d Coulomb energy forCuIr2S4.
It is considered that the decrease of relative intensity at
1 eV in the photon-energy region below 480 eV is caused
mainly by S 3p hybridized with Ir 5d. A shoulder
structure at the energy corresponding to 1 eV below theFermi level is also seen in the S L2,3 x-ray emission
spectrum of CuIr2S4 taken with excitation-photon energyof 188 eV at room temperature (not shown). According to
the E1 selection rule the x-ray emission spectrum
corresponds to the radiative transition from the sulfurderived states with d- and/or s-symmetry in the valenceband to the S 2p levels.

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References[1] T. Furubayashi et al., J. Phys. Soc. Jpn. 63, 3333
(1994).
[2] P.G. Radaelli et al., Nature 416, 155 (2002).
[3] T. Sasaki et al., J. Phys. Soc. Jpn. 73, 1875 (2004).
[4] T. Okane et al., J. Magn. Magn. Mater., 272-276, e297
(2004).
[5] K. Kitamoto et al., Phys. Rev. B 68, 195124 (2003).
* taguchi@ms.osakafu-u.ac.jp