High resolution photoemission study of CeRu2Si2

Solid State Communications.

103, No. 12, pp. 659462. 1997 0 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 003s1098/97 .$17.00+.00

Pergamon

Vol.

PII: soo38-1098(97)00266-4

HIGH RESOLUTION

PHOTOEMISSION

STUDY OF CeRuzSi2

M. Tsunekawa,’ S. Suga,‘** A. Kimura,” T. Matsushita,” T. Muro,’ S. Ueda,” H. Daimon,O S. Imada,” T. Nakatani,” Y. Saitoh,” T. Iwasaki,” A. Sekiyama,b A. Fujimori,b H. Ishii,” T. Miyahara,’ T. Hanyu,c H. Namatame,d M. Taniguchi,d E. Shigemasa,e 0. Sakai,f R. Takayama,’ R. Settai,g H. Azumag and Y. Onukig aDepartment of Material Physics, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan ‘Department of Physics, Faculty of Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan “Department of Physics, Faculty of Science, Tokyo Metropolitan University, Hachioji, Tokyo 192-03, Japan dDepartment of Physics, Faculty of Materials Science, Hiroshima University, Higashihiroshima 730, Japan ePhoton Factory, National Laboratory for High Energy Physics, Tsukuba, Ibaraki 305, Japan fDepartment of Physics, Faculty of Science, Tohoku University, Sendai, Miyagi 980-77, Japan gDepartment of Physics, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan (Received 7 February 1997; accepted 6 June 1997 by H. Kamimura) High resolution Ce 4d - 4f resonance photoemission study was done for CeRu2Si2. The Ce 4f spectrum has shown two prominent peaks around 0.3 and 2.5 eV. The 0.3 eV peak has clearly shown an additional structure closer to the Fermi level, whose intensity decreases at higher temperatures. This intensity reduction is ascribed to the tail of the Kondo resonance peak. 0 1997 Elsevier Science Ltd Keywords: D. heavy fermions, D. Kondo spectroscopy, E. synchrotron radiation.

1. INTRODUCTION The heavy fermion compound CeRu$iz has the tetragonal ThCr2Siz structure and has the electronic specific-heat coefficient of -350 mJ K-* mol-’ and the Kondo temperature T, of -20 K [l-3]. This material shows a metamagnetic transition at about 7.7 T, when a magnetic field is applied parallel to the [O 0 I] direction [4-81. A lot of studies have been performed to reveal the origin of its interesting properties in the past decade [9- 121. Neither superconductivity nor magnetic ordering has been observed down to 20 mK. The Fermi surface properties are intensively studied by the dHvA measurements [3, 13-171. In order to study its electronic structure, in particular, the Ce 4f-conduction electron hybridization effects and crystal field splitting effects on the 4f states, we have performed high resolution resonance photoemission spectroscopy (HRPES) and X-ray photoelectron spectroscopy (XPS). From the HRPES spectra we have obtained

effects,

E. photoelectron

detailed information on the Ce 4f states in the vicinity of the Fermi level (EF). In addition to the structures of the antibonding and bonding states due to the hybridization between the Ce 4f and conduction band electrons, the bonding (or well screened) structure has shown a doublet structure reflecting the Ce 4f spin-orbit splitting. The temperature dependence of this doublet is interpreted as due to the temperature dependence of the tail of the Kondo resonance peak above EF in comparison with the calculation by an Anderson impurity model using non crossing approximation (NCA). The Ce 3d XPS spectrum is analyzed by an Anderson impurity model.

* To whom correspondence should be addressed. e-mail: [email protected] 659

2. EXPERIMENTAL Single crystals of CeRu2Si2 were grown by the tri-arc Czochralski pulling methods in an argon gas atmosphere and were annealed at 1500°C under a high vacuum of 1 X lo-’ Torr for 5 h [3]. The crystal structure was checked by the X-ray diffraction experiment. Magnetic, thermal and optical measurements were done before the present study. The resonance photoemission study in the Ce 4d - 4f excitation region was done at the beam

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PHOTOEMISSION STUDY OF CeRu $Sir

line BL-3B of the Photon Factory, National Laboratory for High Energy Physics (KEK) in Tsukuba. The synchrotron radiation was monochromatized by a high resolution Dragon type monochromator. HRPES measurement was done with setting the monochromator to hv = 122 and 114 eV, corresponding to the resonancemaximum and -minimum, respectively. The energy resolution of the photon was set to 30 meV. The sample temperature was controlled between the room temperature and 30 K. For the XPS measurement, an X-ray tube equipped with Al and Mg twin anodes was used as an excitation photon source (hv = 1486.6 and 1253.6 eV). The photoelectron energy was analyzed by using a SCIENTA SES200 analyzer for HRPES with the total energy resolution of 40 meV and a double-pass cylindrical-mirror analyzer for XPS. Clean sample surfaces were obtained by scraping in situ with a diamond file in the ultrahigh vacuum chamber with the base pressure of -5 X lo-” Torr for HRPES and - 1 X lo-” Torr for XPS, respectively. During the HRPES and XPS measurements, the cleanliness of the surfaces was checked by the absence of 0 2p, 0 1s and C 1s photoemission signals. The surfaces were kept clean by occasionally repeated filing. The binding energy (Es) was calibrated for evaporated Au thin film on the surface of the sample. 3. RESULTS AND DISCUSSION Photoemission spectra measured for hv = 122 and 114 eV are shown in Fig. 1 by the open and filled marks after subtracting integral backgrounds. They correspond to the spectra at the resonance maximum and minimum. Their difference spectra shown in the lower panel represent the 4f predominated component. The prominent two peaks in the difference spectra are

10 5 0 binding energy (eV)

Fig. 1. Valence band photoemission spectra of CeRuzSiz measured with synchrotron radiation at 32 and 200 K. Difference spectra represent the contribution of the 4f component.

J=7l2

J 5/:

binding energy (eV)

Fig. 2. High resolution photoemission spectra of CeRuzSi2 in the vicinity of the Fermi level. called a “well screened” peak (smaller Es) and a “poorly screened” peak (larger E,), which correspond to the “bonding” and “antibonding” states between the Ce 4f and 4f’ final states. High resolution spectra obtained with better statistics for the “well screened” peak are shown in Fig. 2. The spectrum measured at 32 K reveals two prominent doublet peaks corresponding to the Ce 4f spin-orbit splitting of the states with the screening electron in the J = 7/2 and 5/2 states. No structure corresponding to the crystal field splitting was clarified. It is shown that the intensity of the peak just below EF is noticeably reduced at 200 K. We have normalized the intensity of both spectra at the “poorly screened” peak and compared the “well screened” peak at 32 and 200 K in Fig. 3. It is confirmed that the peak intensity is stronger at 32 K than at 200 K. The experimental spectrum at 32 K is thought to be composed of a temperature independent spectrum

1

bindi$&ergy

(eV7

Fig. 3. Upper panel: 4f spectra near the Fermi level normalized at the “poorly screened peak”. Lower panel: results of the NCA calculation. T stands for the temperature of the measurement and P stands for the instrumental resolution. pV2 is set to 0.0275 eV.

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PHOTOEMISSION STUDY OF CeRuZSi2

broadened with a Gaussian with the full width at half maximum (FWHM) of 3.8 kBT (T = 32 K). Then the spectrum at 200 K is broadened with a Gaussian with FWHM of 3.8 kBT (with T = 200 K). Thus obtained spectrum (not shown here) has still a prominent J = 512 peak in a strong contrast to the experimental spectrum, where the peak is completely suppressed. Thus the temperature dependence of this structure cannot be explained by thermal broadening effect alone and suggests the contribution from the “Kondo resonance peak” located just above EF. CeRu2Si2 is characterized by a Kondo temperature close to 20 K and the Kondo state will be realized below the comparable temperature. The Kondo peak itself is expected at the energy of about ksTK higher than EF and could be detected only by an extremely high resolution inverse photoemission spectroscopy. Its tail is, however, expected to be traced below EF by high resolution photoemission measurements, since the width is of the order of ksT,. The lower panel of Fig. 3 shows a set of NCA calculation done for the sample temperatures of 200 and 30 K by using the following parameters by assuming a trapezia1 conduction band density of states: instrumental resolution with Gaussian broadening I’ = 400 K, energy of the 4f state er = - 2 eV and the spin-orbit splitting energy of 0.32 eV. The crystal field splittings are assumed to be 13 and 26 meV for the first and second excited states. Calculations are done for different sets of hybridization strength pV2. The result for PV2 = 0.0275 eV is shown in the figure. The 4felectron number rz,at 30 K is estimated by the NCA calculation as 0.968 for this pV2. If the Kondo temperature TK is expressed as (1 - nf)NfA+ [18] (here A = ?rpV2) and Nr = 2 of the crystal field ground state is employed, one obtains T, = 22 K. The NCA calculations for larger pV* provide too much intensity of the peak just below EP compared with the experimental spectra. Even employing the above pV* value, still clear discrepancy is recognized between the calculated and experimental spectra. It has been reported that a y-Ce like surface state is present on the surface of various Ce compounds [ 19,201. Such a surface has the pV* smaller than the bulk value. Since the photoemission is a surface-sensitive measurement, there is a possibility that the smaller pV* of the surface is reflected in the experimental results. So we considered a line shape as a superposition of two line shapes with different values of pV2. We tentatively took pV2 = 0.0150 eV for the surface and pV2 = 0.0275 eV for the bulk and assumed equal weights for both components. The result is shown in Fig. 4. Even for different set of parameters (different pV2, different weights and so on), the superposed line shape cannot better reproduce

661

.$*NcA calculations 8 .9 rr _

t.

‘\ p”2&q

pv2=0.01 50 t. I. 0.5

1

.,

\ : ,:

binding energy (e$

Fig. 4. Experimental spectrum (dots) at T = 32 K is compared with an even sum of the two spectra of NCA calculations shown below. pV2 is in the unit of eV. the experimental results. In this sense, a resonance XPS measurement of the 4f state with the use of extremely high resolution soft X-ray monochromator (covering the region above 800 eV) and high brilliance synchrotron radiation is required. The experimental Ce 3d XPS spectrum measured with an Al-Kcu source is shown by the circles in the upper panel of Fig. 5. After subtracting an integral background given by the dashed curve, the spectrum is deconvoluted into three spin-orbit partners as often done for Ce compounds. The structure near EB = 933 eV not reproduced by this method is interpreted as an intrinsic energy loss peak of the 3d3,2 component. Both this structure and a corresponding structure accompanying the 3djR component near Es = 915 eV are subtracted from the experimental spectrum as shown by the dashed curve at the bottom. The spectrum corrected for the background and the energy loss peaks is subjected to deconvolution in order to evaluate the relative strength of the three spin-orbit doublets. The line spectrum shows

940

920 900 880 binding energy (cV)

860

Fig. 5. Ce 3d XPS spectrum measured at hv = 1486.6 eV and liquid nitrogen temperature. See text for detail.

PHOTOEMISSION STUDY OF CeRu $i 2

662

the result calculated on an Anderson impurity model. The Lorentzian line shape is assumed with a constant half width for all components. The resultant spectrum is added on the background in the upper panel as shown by a dotted curve. The experimental spectrum is semiquantitatively reproduced by the calculation. As a result of this analysis, the 4f electron number (nf) is evaluated as 0.96. Since the kinetic energy of the photoelectron in this XPS is much higher than that in the Ce 4d-4f resonance experiment, this result is thought to be less influenced by the surface effect. Further study is required to accurately evaluate the surface effect in future. 4. CONCLUSIONS High resolution 4d-4f resonance photoemission experiment is done for CeRuzSi2 at 200 and 32 K. The Ce 4f spectrum has revealed remarkable temperature dependence of the fine structures. The clear increase of the photoemission intensity near Er at 32 K is ascribed to the temperature dependent behavior of the tail of the Kondo peak. The 4f electron number evaluated from the Ce 3d XPS is about 0.96. Further studies such as high resolution Ce 3d-4f resonance photoemission measurements of the valence band and Ce 4d state will be performed in near future.

4. 5. 6. 7.

8. 9. IO.

11.

12. 13. 14.

Acknowledgements-The

authors are much obliged to Professor A. Yagishita and Mr T. Susaki for their technical support. This work was approved by the Photon Factory Program Advisory Committee (923002) and supported by the Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan.

15. 16. 17.

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