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Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 472–475

Observation of bulk electronic states of Kondo semiconductor YbB12

by high-resolution soft X-ray photoemission spectroscopy

A. Shigemoto a, J. Yamaguchi a,∗, A. Sekiyama a, S. Imada a, A. Yamasaki a, A. Irizawa a,T. Ukawa a, T. Muro b, F. Iga c, T. Takabatake c, S. Suga a

a Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japanb Japan Synchrotron Research Institute, Mikazuki, Sayo, Hyogo 679-5198, Japan

c Graduate School of Advanced Sciences of Matter, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8530, Japan

Available online 3 January 2007

bstract

Bulk valence band photoemission spectra of the Kondo semiconductor YbB12 were measured with use of soft X-ray (hν = 700 eV) at 20 and00 K and the Kondo resonance behavior was confirmed. Sharp bulk Yb2+ 4f peaks are observed in the vicinity of Fermi level on fractured singlerystals. From the spectra is estimated the Yb mean valence as 2.86 ± 0.01 at 20 K and 2.91 ± 0.01 at 200 K. The whole peaks of the Yb3+ 4fultiplets are found to shift by ∼ 30 meV toward higher binding energies with decreasing the temperature from 200 to 20 K. On the other hand,

he shift of the peak energy position of the Yb2+ 4f spectra toward lower binding energies with decreasing the temperature is found to be less than0 meV. The temperature dependence of the Yb 4f photoemission spectra is well explained by the non-crossing approximation (NCA) calculationased on the single impurity Anderson model (SIAM).

2007 Elsevier B.V. All rights reserved.

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eywords: Kondo semiconductor; YbB12; SIAM; 4f peak shift

. Introduction

Among rare-earth compounds with strongly correlated f elec-rons, Kondo semiconductors have attracted much attention. Atigh temperatures, they show local magnetic moments and areetallic. In contrast, they have non-magnetic insulating ground

tate at low temperatures and behave as semiconductors with amall energy gap at the Fermi level (EF) [1]. The mechanism ofhe gap formation has, however, not been fully clarified yet.

YbB12 is one of a few Yb-based Kondo semiconductors [1,2].n Yb compounds, the Kondo resonance peak is observed inhotoemission (PES) spectra on the occupied side of the EF. Toate, many high-resolution PES studies have been performed oningle crystal samples of Yb compounds as YbAl3[3,4], YbCu2i2[5], and YbInCu4[6,7]. Although, the single impurity Ander-on model (SIAM) has been applied to interpret these PES

esults, controversial conclusions are derived about the appli-ability of the SIAM to the temperature dependence of peaknergy positions and valences.

∗ Corresponding author. Tel.: +81 6 6850 6422; fax: +81 6 6845 4632.E-mail address: yamaguchi@decima.mp.es.osaka-u.ac.jp (J. Yamaguchi).

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368-2048/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.elspec.2006.12.052

In the present letter, we report on a high-resolution PES studyf Kondo semiconductor YbB12. We have analyzed the tem-erature dependence of the bulk Yb 4f PES spectra by usinghe non-crossing approximation (NCA) calculation within theramework of SIAM. We have confirmed the relevance of SIAMredictions in the case of YbB12.

. Experimental

Single-crystalline YbB12 samples (UB12-type crystal struc-ure) were grown by the floating-zone method using an imageurnace [1]. The magnetic susceptibility follows a Curie–Weissehavior above 170 K. It shows a broad maximum aroundmax ∼ 80 K and then decreases rapidly to reach a constantalue characteristic of the non-magnetic insulating groundtate [2]. The Kondo temperature TK estimated as 3 × Tmaxs, therefore, TK ∼ 240 K (∼ 21 meV). The small energy gapbtained by means of transport measurements [1], opticalonductivity [8], and low- hν PES measurements [9] is about

–15 meV at low temperatures.

The high-resolution soft X-ray PES measurements were per-ormed with synchrotron radiation (hν = 700 eV) at beamlineL25SU of SPring-8, by using the GAMMADATA-SCIENTA

opy and Related Phenomena 156–158 (2007) 472–475 473

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Fig. 2. Temperature dependence of the Yb 4f PES spectra of YbB12 at 20 and200 K. The energy resolution was set to 60 meV. (a) The first peak of the Yb3+ 4fmip

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A. Shigemoto et al. / Journal of Electron Spectrosc

ES200 hemispherical photoelectron analyzer. Energy calibra-ion was performed for Pd Fermi edge at each measuringemperature. The energy resolution estimated by the Au Fermidge was 60 meV. The base pressure was 4 × 10−8 Pa. Cleanurfaces were obtained by fracturing in situ at 200 K.

. Results and discussion

Fig. 1 shows the valence band soft X-ray PES spectrumf YbB12 measured at 200 K. The 4f12 final state multiplettructures from the Yb3+ initial state are observed between 6nd 12 eV. Two sharp peaks are observed just below EF and at.3 eV, which are derived from the 4f13J = 7/2 (abbreviateds 4f7/2) and its spin-orbit partner J = 5/2 (4f5/2) final statesrom the Yb2+ initial state in the bulk, respectively. In addition,road peaks are observed in the Yb2+ 4f region at ∼1 and

2 eV. Although these broad peaks are from the surface layer9,10], their spectral intensity is much weaker in the presentesults compared with the previous low-hν PES experiments9,10]. Therefore, we can evaluate the intrinsic peak energyositions of the bulk Yb 4f PES spectra and the Yb valenceith much higher accuracy.Fig. 2(a and b) shows temperature dependence of the first

omponent (3H6) of the 4f12 multiplets and the 4f137/2 compo-

ent, respectively. Here, the spectra have been normalized byhe integrated intensity of the 3H6 intensity between 5.3 and.3 eV. As shown in Fig. 2(a), the peak of the first component3H6) of the 4f12 multiplets is found to shift toward higher bind-ng energies (EB) with decreasing the temperature. As shown inig. 2(b), on the other hand, the shift of the peak energy posi-

ion of the 4f7/2 component toward lower EB with decreasing

he temperature is at most 10 meV. Simultaneously the intensityncreases. The 4f5/2 component exhibits a similar behavior.

In order to estimate the temperature dependent Yb valencend the accurate peak energy positions of Yb 4f PES spectra,

ig. 1. Valence band PES spectrum of YbB12 measured athν = 700 eV at 200 K.he sharp Yb2+ 4f7/2 and 4f5/2 peaks are observed near EF and at 1.3 eV, respec-

ively. The broad structures at ∼ 1 and ∼ 2 eV are derived from the surefaceomponents. The Yb3+ 4f multiplets are observed between 6 and 12 eV.

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ultiples (3H6). The peak energy position shifts toward higher EB with decreas-ng the temperature from 200 to 20 K. (b) The Yb2+ 4f7/2 peak near EF. Theeak shift toward lower EB with decreasing the temperature is at most 10 meV.

e have carried out numerical fitting of these 4f spectra. Forimplicity, line spectra are convoluted with the Gaussian func-ion for the energy resolution and the Lorentzian function forhe life time broadening. The Yb valence can be estimated fromhe intensity ratio of the Yb2+ and Yb3+ 4f components. How-ver, the surface components observed in the Yb2+ 4f regionhould be excluded. In order to extract the intrinsic contribu-ions of Yb 4f bulk components, two structures (split by thepin-orbit interaction) are also assumed for surface and subsur-ace [4]. The Yb3+ 4f multiplet structures are fitted by the atomicultiplet calculation results [11]. The temperature independentahan’s asymmetry parameter α = 0.17 is assumed for Yb2+

nd Yb3+ 4f components [12]. The fitting is, however, not satis-actory to reproduce the experimental PES spectra, because finitepectral weights remain in the non-4f density of state (DOS)egion between 2 and 10 eV. Therefore, we added the broadtructure, which is assumed for the contribution of the B 2sptates [13]. As an example, the fitting result of the experimentalES spectrum at 200 K is shown Fig. 3. The spectrum is welleproduced by this numerical fitting. The Yb3+ 4f multiplets arehown by vertical lines. From the fitting results of the valenceand spectra, we have estimated the Yb valence (z) from theelation z = 2 + nf, where nf is the 4f hole occupation numberf = 1/[1 + (13/14)(I2+/I3+)], I2+ and I3+ are the integrated

ntensities of the bulk Yb2+ and Yb3+ 4f components, respec-ively. The Yb valence is estimated as z = 2.86 ± 0.01 at 20 Knd 2.91 ± 0.01 at 200 K, respectively (i.e. nf = 0.86 and 0.91ith the ambiguity of ±0.01). These values are in good agree-ent with those inferred from analyses of the high temperatureagnetic susceptibility (estimated valence is z = 2.85) [14], the-ray PES spectra at room temperature (z = 2.9) [15], and the

ow-hν PES spectra at 30 K (z = 2.86 ± 0.06) [14]. The wholeeaks of the Yb3+ 4f multiplets are found to shift by ∼ 30 meV

oward higher binding energies with decreasing the temperaturerom 200 to 20 K. The energy positions of the center of gravityCOG) of Yb3+ 4f multiplets are 7.73 eV at 20 K and 7.70 eVt 200 K. On the other hand, the peak energy positions of the

474 A. Shigemoto et al. / Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 472–475

Fig. 3. Numerical fitting for valence band spectrum of YbB12 measured at 200 K.Surface and subsurface components as well as the bulk Yb2+ 4f components areaob

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Fig. 4. NCA calculation at 20 and 200 K. (a) The trapezoidal conduction bandw2(

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ssumed in the Yb2+ 4f region. The Yb3+ 4f multiplets are fitted by the resultsf atomic multiplet calculation. The broad structure between 2 and 10 eV in theottom panel is assumed as the contribution of the B 2sp states.

ulk Yb2+ 4f7/2 component are ∼35 meV at 20 K and ∼43 meVt 200 K. The peak shift toward lower EB with decreasing theemperature is at most 10 meV.

In order to interpret the above temperature dependence of theulk Yb 4f PES spectra in the valence band, we have carriedut the NCA calculation based on SIAM. In this NCA cal-ulation, the 4f14, 4f13 (J = 7/2, 5/2), and 4f12 initial statesre considered, and the degeneracy Nf of 4f13

7/2, 4f135/2, and 4f12

tates are considered as 8, 6, and 91, respectively. The spin-rbit splitting of the 4f13 states (the energy difference betweenf137/2 and 4f13

5/2 states) is adjusted to 1.26 eV. The hybridiza-ion of the 4f state with a conduction band extending within

6 eV from EF with trapezoidal shape is assumed. In addition,narrow pseudogap is assumed at 20 K near EF [see Fig. 4(a)].he Fermi–Dirac distribution function and Gaussian function for

he energy resolution (60 meV FWHM) are taken into account.ther optimized parameters are the bare 4f energy level εf, the

veraged hybridization strength � = (π/2B)∫ B

−BρV 2(E) dE

where ρV 2(E) represents the energy dependent hybridizationtrength and B is the half of the conduction band width), andhe Coulomb repulsion Uff between two 4f electrons localizedn the same lattice site. The calculated PES spectra at 20 and00 K are shown in Fig. 4(b and c), where, 4f12 multiplet struc-ures are represented by a single component, whose intensity andeak energy position correspond to the integrated intensity of

he whole multiplets and the COG, respectively. The calculatedpectra are obtained for the parameter set as εf = −0.858 eV,

= 53.6 meV, and Uff = 8.295 eV. As shown in Fig. 4(b), theOGs of f12 spectra are 7.725 eV at 20 K and 7.690 eV at 200 K.

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as assumed. The inset shows that a narrow pseudogap is assumed near EF at0 K. Temperature dependence of the 4f12 spectra (b) and 4f13

7/2 spectra near EF

c).

herefore, the peak shift is predicted to be 35 meV. On the otherand, in Fig. 4(c), the peak energy positions of the 4f13

7/2 spectrare 35 meV at 20 K and 45 meV at 200 K. This peak shift has thepposite sign relative to that of the 4f12 multiplets and its magni-ude is much smaller than the peak shift of the 4f12 spectra. Theseemperature dependences of the 4f spectra according to the NCAalculation are in reasonable agreement with the observed Yb 4fES spectra. The 4f hole occupation number nf obtained from

he present NCA calculation is nf = 0.86 at 20 K and nf = 0.91t 200 K, which are in good agreement with the present exper-mental results. In this NCA calculation, TK is estimated fromhe peak energy position of the predicted magnetic excitationpectrum at low temperatures. From the magnetic excitationpectrum calculated by using the above parameter set, TK isstimated as ∼ 395 K (∼ 34 meV) at 20 K. This is slightly largerhan the value of TK ∼ 240 K estimated by the magnetic suscep-ibility. The temperature dependence of the Yb 4f PES spectran YbB12, namely the Yb valence and the Kondo peak posi-ion or TK as well as the positions of 4f12 multiplets are almostnderstood in the framework of SIAM or one can say that theeviation from the SIAM is not prominent in the case of Kondoemiconductor YbB12 in contrast to such a case as YbAl3[4].

. Summary

We have preformed high-resolution soft X-ray PES measure-ents in Kondo semiconductor YbB12 and observed sharp bulkb2+ 4f spectra in the vicinity of EF, where the contributions of

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[Tsunekawa, T. Muro, T. Matsushita, S. Suga, H. Ishii, T. Hanyu, A. Kimura,H. Namatame, M. Taniguchi, T. Miyahara, F. Iga, M. Kasaya, H. Harima,

A. Shigemoto et al. / Journal of Electron Spectrosc

he surface components are suppressed. From the numerical fit-ing of valence band spectra including the Yb3+ 4f multiplets, theb valence is estimated as 2.86 ± 0.01 at 20 K and 2.91 ± 0.01

t 200 K. The temperature dependence of the Yb 4f PES spectras found to be almost explained by the NCA calculation withinhe framework of SIAM.

cknowledgement

This work was supported by a Grant-in-Aid for Creative Sci-ntific Research (15GS0213) from the Ministry of Education,ulture, Sports, Science, and Technology (MEXT), Japan.

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