Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 671–673

High resolution soft X-ray photoemission of Kondo insulator YbB12

A. Shigemotoa,∗, S. Imadaa, A. Sekiyamaa, A. Yamasakia, A. Irizawaa,T. Murob, Y. Saitohc, F. Igad, T. Takabataked, S. Sugaa

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

c Japan Atomic Energy Research Institute, SPring-8, Mikazuki, Sayo, Hyogo 679-5148, Japand Graduate School of Advanced Sciences of Matter, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8530, Japan

Available online 25 February 2005

Abstract

We have measured photoemission spectra of a single crystal YbB12, which is known to be a Kondo insulator, with high energy resolutionat 700 eV in the temperature range between 200 and 20 K to study the Kondo resonance behavior in the bulk. The sharp bulk Yb2+ 4f13 peaksobserved on fractured YbB12, which is only weakly influenced by the surface electronic structures, facilitate high accuracy evaluation of thet©

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emperature dependences of the Kondo peak energy and the bulk valence.2004 Elsevier B.V. All rights reserved.

ACS:71.20.Eh; 79.60.−i

eywords:Kondo insulator; Photoemission

YbB12 is known to be a Kondo insulator with a tem-erature dependent energy gap induced by strong electronorrelation. Below the Kondo temperature (TK), the electricesistivity increases in accordance with the increase of theybridization strength between the local 4f electron and theonduction electrons. Meanwhile, the magnitude of the hy-ridization is characterized by the Kondo temperatureTK.heTK of YbB12 is around 220 K (∼=19 meV), which is esti-ated as 3Tmax, whereTmax = 75 K corresponds to the max-

mum of the magnetic susceptibility[1,2].In YbB12, the Kondo peak has been observed by the low

xcitation energy photoemission spectroscopy[3,4]. Besideshe contributions of the spectral weights of the boron 2p andtterbium 5d states, the spectral weights of the surface Yb2+f components overlap strongly with those of the bulk Yb2+f components near the Fermi level (EF). This makes it dif-cult to accurately estimate the valence in the bulk by lowxcitation energy photoemission spectroscopy. Therefore weave performed high-energy photoemission spectroscopy of

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

A. Shigemoto).

YbB12 by using synchrotron radiation at the excitationergy hν of 700 eV with high energy resolution. At this eergy, the ratio of the photoionization cross sections Yb2p is 104 times larger than athν = 21.2 eV and the meanfree path of the photoelectrons reaches several Yb labeneath the sample surface. Therefore the results athν =700 eV are much more sensitive to the bulk Yb 4f electrstates.

We measured photoemission spectra of a single crYbB12 with use of the synchrotron radiation at BL25SUSPring-8. Emitted photoelectrons were analyzed by theenta SES-200 analyzer. The base pressure in the anchamber was∼4.0 × 10−8 Pa. We fractured samples at 20and took spectra in the sequence ofT = 200, 150, 75, 40 an20 K. The spectrum of Pd, which was electrically conneto the sample, was measured at each temperature torately determine theEF position. In addition, Au is used foenergy calibration. The energy resolution estimated byFermi edge was 60 meV.

Fig. 1(a) shows the valence-band spectrum at 20 K onfractured surface. The spectral features in the range from12 eV are assigned to the 4f12 final state multiplet structureThe atomic multiplet calculation can well reproduce the12

368-2048/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.elspec.2005.01.144

672 A. Shigemoto et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 671–673

Fig. 1. Photoemission spectra of YbB12 at hν = 700 eV. (a) Experimen-tal spectrum obtained at 20 K. (b) The total fitting result including Yb2+bulk, subsurface, and surface components. For fitting of Yb3+ multiplets,the atomic calculation by Gerken is employed. (c) Yb2+ bulk peaks with theaymmetric parameterα = 0.15. (d) Yb2+ subsurface and surface peaks. Theasymmetric parameterα for the subsurface peaks is taken to be the same asfor the Yb2+ bulk peaks.

multiplet structures[5]. The 4f13 final state multiplets are ob-served betweenEF and 3 eV, where the spectrum is governedby the 4f13 spin-orbit doublet and the binding energy of the4f7/2 peak is estimated to be 34 meV at 20 K. Although theYb2+ surface peaks (4f5/2, 4f7/2) have been drastically sup-pressed, they are still recognizable as broad humps near 2 and1 eV (Fig. 1a).

Then, we have performed numerical fitting to the exper-imental spectra. The reproduced spectrum is shown inFig.1(b). The Yb2+ bulk components are calculated with the Ma-

han’s asymmetric parameterα = 0.15 [6]. We have also as-sumed the third spin-orbit doublet representing the subsur-face components. The mean valence of the Yb ion is thenestimated to be 2.94 at 20 K and 2.96± 0.01 at 200 K. Thesevalues were not so accurately estimated by the low energyexcitation photoemission spectroscopy[3].

Fig. 2 shows the detailed temperature dependence of theYb2+ (4f7/2) and Yb3+ ( 3H6) 4f peaks. The spectrum is nor-malized by the intensity of the Yb3+ ( 3H6) 4f peak. TheYb2+ 4f13 peaks shift slightly toward lower binding energieswith decreasing the temperature. The amounts of the energyshifts of both 4f5/2 and 4f7/2 peaks are estimated as 6 meVat most. On the other hand, the spectrum shows a notice-able peak shift of the Yb3+ multiplet structures. The energyshift toward higher binding energies is about 30 meV withdecreasing the temperature. The energy separation betweenthe center of gravity of the Yb2+ and Yb3+ 4f peaks at 0 Kis approximated in the single impurity Anderson model byεf + Uff in the lowest order[7], whereεf andUff represent theenergy level of an f electron and on-site Coulomb repulsiveenergy between the Yb 4f electrons. However, the spectra atfinite temperatures should be calculated by non crossing ap-proximation (NCA) calculation in the single impurity model.The peak shifts of both 4f13 and 4f12 peaks with temperatureare qualitative consistent with the prediction of NCA withvertex correction.

tra ofY nics nt en-ew rdh . Thisr hem 94 at2

F 3H6 co sity of

ig. 2. The temperature dependence of the Yb2+ 4f7/2 peak and the Yb3+3H6 peak.

In summary, we have measured photoemission specbB12 athν = 700 eV and obtained the bulk Yb 4f electrotructures. We have observed the temperature dependergy shifts of the Yb2+ 4f components are at most−6 meV,hereas the Yb3+ multiplet peaks shift up to 30 meV towaigher binding energies as the temperature decreasesesult is qualitatively explicable by an NCA calculation. Tean valence of Yb ions is accurately estimated as 2.0 K and 2.96 at 200 K.

mponent. The spectral intensities have been normalized by the inten

A. Shigemoto et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 671–673 673

This work was supported by a Grant-in-Aid for CreativeScientific Reasearch (15GS0213) from the Ministry of Edu-cation, Science, Sports and Culture, Japan.

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