- Email: [email protected]
Temperature dependence study of inverse photoemission spectra at Ce 4d absorption edge of CePd3
ELSPEC 3398
Journal of Electron Spectroscopy and Related Phenomena 92 (1998) 81–85
Temperature dependence study of inverse photoemission spectra at Ce 4d absorption edge of CePd 31 K. Kanai a,*, Y. Tezuka a, H. Ishii a, S. Nozawa a, S. Shin a, A. Kotani b, G. Schmerber c, J.P. Kappler c, J.C. Parlebas c a
Synchrotoron Radiation Laboratory, Institute for Solid State Physics, University of Tokyo, 3-2-1 Midoricho, Tanashi, Tokyo 188, Japan b Institute for Solid State Physics, University of Tokyo, 7-22-1 Roppongi, Minatoku, Tokyo 106, Japan c IPCMS-GEMME (UMR 46 CNR), Universite´ Louis Pasteur, 23, rue du Loess, 67037 Strasbourg, France
Abstract A resonant inverse photoemission study (RIPES) at Ce 4d → 4f absorption edge of CePd 3 has been carried out. The excitation energy dependence of 4f spectral resonance indicates 4d–4f multiplet splitting in the intermediate state of the RIPES process. Asymmetric constant final state spectra (CFS) show Fano-type line shape in contrast to the 3d–4f RIPES. A clear temperature dependence of 4f structures near Fermi level is observed in the RIPES spectra. This fact indicates the process of the low lying excited state starting to be occupied and the 4f electron localizing as temperature increases, which is consistent with the NCA calculation. It is found, however, that the 4f electron still shows strong itinerant character at a temperature around Kondo temperature. q 1998 Elsevier Science B.V. All rights reserved Keywords: RIPES; CePd 3; Inverse photoemission; Cd 4d absorption; Resonance effect; Temperature dependence
1. Introduction Resonant inverse photoemission spectroscopy (RIPES) is a new technique to investigate the unoccupied electronic state of solids. Recently Weibel et al. [1] have performed the measurements of RIPES of several Ce compounds near the Ce M 5 edge. Kanai et al. [2] have also measured the RIPES of CePd 7 near the Ce N 4,5 edge. The great ability to research the 4f electronic structure by resonant enhancement of the 4f signal was testified. In this research, the measurements have been performed on the Ce 3d → 4f and 4d → 4f absorption edge. The 4f cross section * Corresponding author. 1 Presented at the Todai Symposium 1997 and the 6th ISSP International Symposium on Frontiers in Synchrotron Radiation Spectroscopy, Tokyo, Japan, 27–30 October 1997.
increases when the excitation energy tuned to the Ce N 4,5 and M 5 absorption edge and the Ce-4f contribution can be extracted. The normal IPES process is represented for the transition to Ce 4f states as follows: ld 10 4f n i + e − → ld 10 4f n + 1 i + hn
(1)
Here, n is the configuration number of the 4f electrons in the ground state and the d means 3d or 4d core which is involved in the measurements. For the incident-energy range of the present experiment the contribution from non-f conduction bands also coexists with the f contribution in the normal IPES. In the RIPES experiment, on the other hand, the resonant processes are expressed by the following processes: ld 10 4f n i + e − → ld 9 4f n + 2 i → ld 10 4f n + 1 i + hn
0368-2048/98/$19.00 q 1998 Elsevier Science B.V. All rights reserved PII S 0 36 8- 2 04 8 (9 8 )0 0 10 5 -4
(2)
82
K. Kanai et al./Journal of Electron Spectroscopy and Related Phenomena 92 (1998) 81–85
Since the initial and final states are the same in Eqs. (1) and (2), they interfere with each other. Therefore, the 4f cross section increases when the excitation energy is tuned to Ce d → 4f absorption edge and we can extract the Ce 4f contribution. There are two RIPES according to the 3d or 4d absorption edges in Ce compounds. It is thought that the incoherent processes such as the normal fluorescence are not very strong in on-resonant spectra near the Ce 4d → 4f threshold as compared with the 3d → 4f threshold. A clear resonant effect was observed at Ce N 4,5 absorption edge and several features different from the M 5 edge were pointed out [2]. Many Ce intermetallic compounds belong to the dense Kondo system, which has a shallow and narrow 4f band. In this system, the strong hybridization between the Ce 4f electron and conduction electrons plays an important role in its electronic structure. At low temperature the 4f electrons wander through the crystal by Kondo effect on each Ce site. In this picture, the localized magnetic moments on Ce 3+ ion at a temperature above Kondo temperature, T K, are screened by the spin of conduction electrons and so vanish below T K. The large susceptibility, characteristic of Fermi liquid at low temperature, changes following a Curie–Weiss law above T K. For example, it is known that CePd 3 is the typical valence fluctuation system, where the 4f electron has an itinerant character. It shows no magnetic order and has temperature-independent magnetic susceptibility at low temperature. Descriptions of Ce-4f electron as the local moment which follows a Curie–Weiss law apply only where the temperature is above T K. This transformation of the property of the 4f electron is reflected in the photoemission spectroscopy (PES) and Bremstrahlung isochromat spectroscopy (BIS) spectra. It was reported that high resolution PES data reflect the process of destroying the singlet ground state of the CeSi 2 and Yb intermetallic compounds [3,4]. In the case of CeSi 2, as temperature increases, the broadening of the structure near the Fermi level consists of the tail of the Kondo peak, and a crystal field splitting side-band was also observed [3]. On the other hand, in the X-BIS region, the high sensitivity of this measurement to the change of Ce-4f state has been demonstrated and a great reduction of Kondo peak was observed for CePd3 [5]. In the general case, the BIS spectrum shows the
two-peak structure near the Fermi level. The sharp one just above the Fermi level called the ‘f 1 peak’ corresponds to the 4f 1 configuration in the final state. The other one at higher energy side, called the ‘f 2 peak’, corresponds to 4f 2 configuration in the final state. Roughly speaking, the intensities of the f 1 and f 2 peaks reflect the weight of f 0 and f 1 configurations in the initial state, respectively. The I( f 2)/[I( f 1) + I(f 2)] peak intensity ratio gives direct information about the initial state configuration [6], where I(f 1) and I(f 2) represent the integrated intensities of f 0 and f 1 peaks. When temperature is increased to T K the singlet ground state is destroyed and the system starts to occupy the lower doublet state, i.e. the transfer of the weight of f 0 configuration to that of f 1 configuration starts. This dramatic change of 4f state should bring a strong temperature dependence of IPES spectra. That is, the I(f 1)/[I(f 1) + I(f 2)] peak intensity ratio decreases proportionally to the reduction of the f 0 configuration weight in the initial state. In this paper, we present the temperature dependence of the RIPES spectra of CePd 3. The RIPES measurement is performed at the Ce4d → 4f threshold. The aim of this work is to observe the variation of the ‘f peak’ intensities in RIPES spectra in the temperature region from 20 to 290 K. It is expected that we can obtain the information about the changes of pure 4f components from clear resonant enhancement and larger cross section of 4f components. For BIS and 3d RIPES measurement in the soft X-ray region, it is cumbersome to separate the f components from the others (e.g. Ce-5d and normal fluorescence), so there remains an ambiguity in subtracting the background.
2. Experiment The crystal structure of the polycrystalline sample has been confirmed to be in the AuCu 3 structure by X-ray diffraction patterns. A Kondo temperature of CePd 3 is reported to be about 240 K [7, 8]. Measurements were performed in an ultra-high vacuum chamber where the base pressure is about 5 × 10 −11 Torr. Clean samples were obtained by scraping the surface with a diamond file in a vacuum every 40 min. A filament-cathode type electron gun was used for the excitation source. The kinetic energy,
K. Kanai et al./Journal of Electron Spectroscopy and Related Phenomena 92 (1998) 81–85
83
E k, of the electron was calibrated by the electron energy analyzer. The IPES was measured by the soft X-ray emission spectrometer of the Institute for Solid State Physics [9], which is Rowland mount type. The Fermi level position and a resolution of the system was determined by referring to the Fermi edge in the IPES spectra of Au which was evaporated on the sample holder. Energy-resolution of this system is 0.8 eV at E k = 120 eV.
3. Results and discussion The RIPES spectra measured at 30 K are shown in Fig. 1 with respect to the kinetic energy of the incident electron around the resonant point. The spectra exhibit the 4f structures resonantly enhanced near Fermi level. The broad band indicated by vertical bar at the high energy side of 4f structures is normal fluorescence created by 5p → 4d emission on the Ce
Fig. 1. The IPES spectra of CePd 3 near the Ce 4d → 4f edge measured at 30 K. The vertical bar indicates the normal fluorescence. The abscissa is the energy above Fermi level (E F = 0 eV). The numbers written on the side of the right axis represent the kinetic energy, E k of the electron.
Fig. 2. The intensities of f 1 and f 2 peaks plotted against the kinetic energy of the electron, E k. Dotted lines represent the maximum points of the CFS curves for f 1 and f 2 peaks.
site. The broad structure coming out from the 4f components near the Fermi level around 130 eV of excitation energy is normal fluorescence created by 4f → 4d emission off the resonance region. Fig. 2 shows the excitation energy dependence of the f peak intensities for the resonant spectra at 30 K. These CFS spectra are obtained by plotting the f 1, f 2 peaks intensities against the excitation energy E k. Around 130 eV of the normal fluorescence created by 4f → 4d emission on the Ce site emerges from behind the resonant 4f structure as E k steps up to the offresonance region at the higher energy side. The intensities of this fluorescence were taken away from the spectrum to make an estimate of f peak intensities. The broad and asymmetric line shapes of these spectra indicate the large 4d–4f multiplet splitting and Fanotype line shape, respectively. The peak of f 2 spectrum is higher than that of f 1 by about 3 eV. This indicates that the average energy difference between the intermediate states of 4f 1 and 4f 2 final state can be estimated at ,3 eV. With a simple model this energy difference can be represented as the separation DE = E(4d 9 , f 3)-E(4d 9, f 2) < e f + 2U ff + U fc < 3 eV. On the other hand, this energy difference is expected to give information about the hopping integral V between l4d 9, f 2i and l4d 9, f 3i in the intermediate state. This might suggest how the strong 4f–4d interaction influences the hybridization between 4f electron and conduction electron [10]. Fig. 3(a) shows the comparison between the spectrum measured at 30 K (open diamond) and the one at
84
K. Kanai et al./Journal of Electron Spectroscopy and Related Phenomena 92 (1998) 81–85
Fig. 3. (a) Comparison between the IPES spectra on-resonance (E k = 120 eV) measured at 30 and 290 K. The spectra at 30 and 290 K are presented by open and filled diamonds, respectively. The intensities are normalized by f 2 peaks. (b) Schematic energy diagram of IPES process for valence fluctuation systems [5].
290 K (closing diamond). Each spectrum was measured at E k = 120 eV, where the f 1 peak is the most enhanced. The abscissa is the energy above the Fermi level. The intensities are normalized by those of f 2 peaks. It is clear that the intensity of the f 1 peak is dramatically reduced at 290 K compared with at 30 K. (Note that the intensities of f 2 peaks increase.) This reflects the fact that the system starts to thermally occupy the lowest magnetic state. The 4f electrons which had been itinerant by the Kondo effect at 30 K become partially localized at 290 K. A little broadening to the low-energy side of f 2 peak at 30 K is due to a larger intensity of the f 1 peak broadened by the resolution. Meanwhile, the intensity of the f 1 peak remains enhanced. That shows that the 4f electron remains itinerant at 290 K. In Fig. 3(b), the schematic total energy diagram of the valence fluctuation system is quoted from Ref. [5]. The solid arrow shows the transition from the initial state of singlet ground state which was composed of mixed f 0 and f 1 configurations. In this case, the final states of IPES process are the f 1 or f 2 configuration. On the other hand, in the case of T $ T K, the transition from f 1 to f 2 configuration, indicated by the dotted arrow, also takes place, and the weight of the f 1 final state is reduced. Therefore, the intensity of the f 1 peak
decreases. In Fig. 3(a), the surface effect needs to be kept in mind in the case where the excitation energy is around 120 eV. It is said that the surface valence of rare earth compounds, the 4f electron of which has strong hybridization character, is often lower than that of bulk, and so the surface contribution to the spectrum differs from that of bulk. However, in this case, the 4f electron at surface localizes for all the temperature ranges, so that the reduction of the intensity of the f 1 peak corresponds to the change of the bulk feature. In Fig. 4 the temperature dependence of the ratio of f peak intensities measured at E k = 120 eV is presented.
Fig. 4. Ratios of f-peak intensities, I( f 2)/[I( f 1) + I( f 2)] measured at 120 eV are plotted against temperature. Temperature on abscissa is normalized with T K assumed to be 240 K.
K. Kanai et al./Journal of Electron Spectroscopy and Related Phenomena 92 (1998) 81–85
This curve is obtained by plotting the f 2 peak intensity, I(f 2), normalized by the total peak intensity, I(f 1) + I(f 2), against temperature. Assuming that the hybridization at the intermediate state of the resonant process connecting the 4d 9 4f 2 and 4d 9 4f 3 configuration is small, the ratio of I(f 2)/[I(f 1) + I(f 2)] provides direct information about the initial state occupancy, n f. For low temperature in comparison with T K, there is not a prominent temperature dependence; however for T,T K it is strongly pushed up through the T K. The temperature dependence of n f was calculated by noncrossing approximation (NCA) computational method for dilute magnetic alloys by using the ‘Kondo model’ [11]. The inclination of the experimental result is consistent with that of this calculation. For very low temperature, in comparison with T K, the 4f occupancy, n f, is constant and the increase of the ratio, I(f 2)/[I(f 1) + I(f 2)] near T K corresponds to the increase of the 4f occupancy, n f, near T K. This reflects the process of a singlet ground state being broken up by thermal excitation, and 4f electrons becoming localized near T K. The characteristic ‘Kondo scaling’ behavior of the Kondo effect in the impurity systems is clearly observed. At this point, the Kondo impurity model seems to give a good description of the temperature variation of the low temperature 4f valence, n f, in a qualitative aspect. On the other hand, the fact that the f 1 peak shows still stronger resonance enhancement in the on-resonant spectrum at 290 K indicates that 4f electron still remains itinerant to some extent up to the temperature above T K. This is consistent with the magnetic susceptibility data which is Curie–Weisslike above room temperature.
4. Conclusion We measured the clear variation of 4f structure in the RIPES spectrum at different temperatures, which is resonantly enhanced near the Ce-N 4,5 absorption edge. There is an apparent inclination in the temperature
85
dependence of the f peaks ratio. The peak intensity ratio, I(f 2)/[I( f 1) + I( f 2)] is strongly pushed up around the T K. This reflects the process of destroying the singlet ground state by thermal excitation. The dramatic transformation of the system is brought about with slight changes of 4f occupancy. Also, we would like to emphasize the great ability of this new technique, IPES and RIPES measurement, to observe the small changes of the electronic structure of strongly correlated systems.
Acknowledgements We would like to thank Prof. T. Jo, Dr. A. Tanaka, Dr T. Uozumi and Dr R. Takayama for their helpful disscussions.
References [1] P. Weibel, M. Grioni, D. Malterre, B. Dardel, Y. Baer, Phys. Rev. Lett. 72 (1994) 1252. [2] K. Kanai, Y. Tezuka, M. Fujisawa, Y. Harada, S. Shin, G. Schmerber, J.P. Kappler, J.P. Parlebas, A. Kotani, Phys. Rev. 55 (1997) 2623. [3] F. Patthey, W.D. Schneider, Y. Baer, B. Delley, Phys. Rev. Lett. 58 (1987) 2810. [4] L.H. Tjeng et al., Phys. Rev. Lett. 71 (1993) 1419. [5] D. Malterre, M. Grioni, P. Weibel, Y. Baer, Phys. Rev. Lett. 68 (1992) 2656; M. Grioni, D. Malterre, P. Weibel, B. Dardel, Y. Baer, Physica B 186–188 (1993) 38. [6] O. Gunnarsson, K. Scho¨nhammer, Z. Phys. B30 (1978) 297; Phys. Rev. B28 (1983) 4315. [7] J.M. Lawrence, J.D. Thompson, Y.Y. Chen, Phys. Rev. Lett. 54 (1985) 2537. [8] J. Arts, F.R. deBoer, P.F. de Chaˆtel, A. Menovsky, Solid State Commun. 56 (1985) 623. [9] S. Shin, A. Agui, M. Fujisawa, Y. Tezuka, T. Ishii, N. Hirai, Rev. Sci. Instrum. 66 (1995) 1584. [10] A. Tanaka, T. Jo, Physica B206-207 (1995) 74; J. Phys. Soc. Jpn 65 (1996) 615. [11] N.E. Bickers, D.L. Cox, J.W. Wilkins, Phys. Rev. Lett. 54 (1985) 230; Phys. Rev. B36 (1987) 2036.
Journal of Electron Spectroscopy and Related Phenomena 92 (1998) 81–85
Temperature dependence study of inverse photoemission spectra at Ce 4d absorption edge of CePd 31 K. Kanai a,*, Y. Tezuka a, H. Ishii a, S. Nozawa a, S. Shin a, A. Kotani b, G. Schmerber c, J.P. Kappler c, J.C. Parlebas c a
Synchrotoron Radiation Laboratory, Institute for Solid State Physics, University of Tokyo, 3-2-1 Midoricho, Tanashi, Tokyo 188, Japan b Institute for Solid State Physics, University of Tokyo, 7-22-1 Roppongi, Minatoku, Tokyo 106, Japan c IPCMS-GEMME (UMR 46 CNR), Universite´ Louis Pasteur, 23, rue du Loess, 67037 Strasbourg, France
Abstract A resonant inverse photoemission study (RIPES) at Ce 4d → 4f absorption edge of CePd 3 has been carried out. The excitation energy dependence of 4f spectral resonance indicates 4d–4f multiplet splitting in the intermediate state of the RIPES process. Asymmetric constant final state spectra (CFS) show Fano-type line shape in contrast to the 3d–4f RIPES. A clear temperature dependence of 4f structures near Fermi level is observed in the RIPES spectra. This fact indicates the process of the low lying excited state starting to be occupied and the 4f electron localizing as temperature increases, which is consistent with the NCA calculation. It is found, however, that the 4f electron still shows strong itinerant character at a temperature around Kondo temperature. q 1998 Elsevier Science B.V. All rights reserved Keywords: RIPES; CePd 3; Inverse photoemission; Cd 4d absorption; Resonance effect; Temperature dependence
1. Introduction Resonant inverse photoemission spectroscopy (RIPES) is a new technique to investigate the unoccupied electronic state of solids. Recently Weibel et al. [1] have performed the measurements of RIPES of several Ce compounds near the Ce M 5 edge. Kanai et al. [2] have also measured the RIPES of CePd 7 near the Ce N 4,5 edge. The great ability to research the 4f electronic structure by resonant enhancement of the 4f signal was testified. In this research, the measurements have been performed on the Ce 3d → 4f and 4d → 4f absorption edge. The 4f cross section * Corresponding author. 1 Presented at the Todai Symposium 1997 and the 6th ISSP International Symposium on Frontiers in Synchrotron Radiation Spectroscopy, Tokyo, Japan, 27–30 October 1997.
increases when the excitation energy tuned to the Ce N 4,5 and M 5 absorption edge and the Ce-4f contribution can be extracted. The normal IPES process is represented for the transition to Ce 4f states as follows: ld 10 4f n i + e − → ld 10 4f n + 1 i + hn
(1)
Here, n is the configuration number of the 4f electrons in the ground state and the d means 3d or 4d core which is involved in the measurements. For the incident-energy range of the present experiment the contribution from non-f conduction bands also coexists with the f contribution in the normal IPES. In the RIPES experiment, on the other hand, the resonant processes are expressed by the following processes: ld 10 4f n i + e − → ld 9 4f n + 2 i → ld 10 4f n + 1 i + hn
0368-2048/98/$19.00 q 1998 Elsevier Science B.V. All rights reserved PII S 0 36 8- 2 04 8 (9 8 )0 0 10 5 -4
(2)
82
K. Kanai et al./Journal of Electron Spectroscopy and Related Phenomena 92 (1998) 81–85
Since the initial and final states are the same in Eqs. (1) and (2), they interfere with each other. Therefore, the 4f cross section increases when the excitation energy is tuned to Ce d → 4f absorption edge and we can extract the Ce 4f contribution. There are two RIPES according to the 3d or 4d absorption edges in Ce compounds. It is thought that the incoherent processes such as the normal fluorescence are not very strong in on-resonant spectra near the Ce 4d → 4f threshold as compared with the 3d → 4f threshold. A clear resonant effect was observed at Ce N 4,5 absorption edge and several features different from the M 5 edge were pointed out [2]. Many Ce intermetallic compounds belong to the dense Kondo system, which has a shallow and narrow 4f band. In this system, the strong hybridization between the Ce 4f electron and conduction electrons plays an important role in its electronic structure. At low temperature the 4f electrons wander through the crystal by Kondo effect on each Ce site. In this picture, the localized magnetic moments on Ce 3+ ion at a temperature above Kondo temperature, T K, are screened by the spin of conduction electrons and so vanish below T K. The large susceptibility, characteristic of Fermi liquid at low temperature, changes following a Curie–Weiss law above T K. For example, it is known that CePd 3 is the typical valence fluctuation system, where the 4f electron has an itinerant character. It shows no magnetic order and has temperature-independent magnetic susceptibility at low temperature. Descriptions of Ce-4f electron as the local moment which follows a Curie–Weiss law apply only where the temperature is above T K. This transformation of the property of the 4f electron is reflected in the photoemission spectroscopy (PES) and Bremstrahlung isochromat spectroscopy (BIS) spectra. It was reported that high resolution PES data reflect the process of destroying the singlet ground state of the CeSi 2 and Yb intermetallic compounds [3,4]. In the case of CeSi 2, as temperature increases, the broadening of the structure near the Fermi level consists of the tail of the Kondo peak, and a crystal field splitting side-band was also observed [3]. On the other hand, in the X-BIS region, the high sensitivity of this measurement to the change of Ce-4f state has been demonstrated and a great reduction of Kondo peak was observed for CePd3 [5]. In the general case, the BIS spectrum shows the
two-peak structure near the Fermi level. The sharp one just above the Fermi level called the ‘f 1 peak’ corresponds to the 4f 1 configuration in the final state. The other one at higher energy side, called the ‘f 2 peak’, corresponds to 4f 2 configuration in the final state. Roughly speaking, the intensities of the f 1 and f 2 peaks reflect the weight of f 0 and f 1 configurations in the initial state, respectively. The I( f 2)/[I( f 1) + I(f 2)] peak intensity ratio gives direct information about the initial state configuration [6], where I(f 1) and I(f 2) represent the integrated intensities of f 0 and f 1 peaks. When temperature is increased to T K the singlet ground state is destroyed and the system starts to occupy the lower doublet state, i.e. the transfer of the weight of f 0 configuration to that of f 1 configuration starts. This dramatic change of 4f state should bring a strong temperature dependence of IPES spectra. That is, the I(f 1)/[I(f 1) + I(f 2)] peak intensity ratio decreases proportionally to the reduction of the f 0 configuration weight in the initial state. In this paper, we present the temperature dependence of the RIPES spectra of CePd 3. The RIPES measurement is performed at the Ce4d → 4f threshold. The aim of this work is to observe the variation of the ‘f peak’ intensities in RIPES spectra in the temperature region from 20 to 290 K. It is expected that we can obtain the information about the changes of pure 4f components from clear resonant enhancement and larger cross section of 4f components. For BIS and 3d RIPES measurement in the soft X-ray region, it is cumbersome to separate the f components from the others (e.g. Ce-5d and normal fluorescence), so there remains an ambiguity in subtracting the background.
2. Experiment The crystal structure of the polycrystalline sample has been confirmed to be in the AuCu 3 structure by X-ray diffraction patterns. A Kondo temperature of CePd 3 is reported to be about 240 K [7, 8]. Measurements were performed in an ultra-high vacuum chamber where the base pressure is about 5 × 10 −11 Torr. Clean samples were obtained by scraping the surface with a diamond file in a vacuum every 40 min. A filament-cathode type electron gun was used for the excitation source. The kinetic energy,
K. Kanai et al./Journal of Electron Spectroscopy and Related Phenomena 92 (1998) 81–85
83
E k, of the electron was calibrated by the electron energy analyzer. The IPES was measured by the soft X-ray emission spectrometer of the Institute for Solid State Physics [9], which is Rowland mount type. The Fermi level position and a resolution of the system was determined by referring to the Fermi edge in the IPES spectra of Au which was evaporated on the sample holder. Energy-resolution of this system is 0.8 eV at E k = 120 eV.
3. Results and discussion The RIPES spectra measured at 30 K are shown in Fig. 1 with respect to the kinetic energy of the incident electron around the resonant point. The spectra exhibit the 4f structures resonantly enhanced near Fermi level. The broad band indicated by vertical bar at the high energy side of 4f structures is normal fluorescence created by 5p → 4d emission on the Ce
Fig. 1. The IPES spectra of CePd 3 near the Ce 4d → 4f edge measured at 30 K. The vertical bar indicates the normal fluorescence. The abscissa is the energy above Fermi level (E F = 0 eV). The numbers written on the side of the right axis represent the kinetic energy, E k of the electron.
Fig. 2. The intensities of f 1 and f 2 peaks plotted against the kinetic energy of the electron, E k. Dotted lines represent the maximum points of the CFS curves for f 1 and f 2 peaks.
site. The broad structure coming out from the 4f components near the Fermi level around 130 eV of excitation energy is normal fluorescence created by 4f → 4d emission off the resonance region. Fig. 2 shows the excitation energy dependence of the f peak intensities for the resonant spectra at 30 K. These CFS spectra are obtained by plotting the f 1, f 2 peaks intensities against the excitation energy E k. Around 130 eV of the normal fluorescence created by 4f → 4d emission on the Ce site emerges from behind the resonant 4f structure as E k steps up to the offresonance region at the higher energy side. The intensities of this fluorescence were taken away from the spectrum to make an estimate of f peak intensities. The broad and asymmetric line shapes of these spectra indicate the large 4d–4f multiplet splitting and Fanotype line shape, respectively. The peak of f 2 spectrum is higher than that of f 1 by about 3 eV. This indicates that the average energy difference between the intermediate states of 4f 1 and 4f 2 final state can be estimated at ,3 eV. With a simple model this energy difference can be represented as the separation DE = E(4d 9 , f 3)-E(4d 9, f 2) < e f + 2U ff + U fc < 3 eV. On the other hand, this energy difference is expected to give information about the hopping integral V between l4d 9, f 2i and l4d 9, f 3i in the intermediate state. This might suggest how the strong 4f–4d interaction influences the hybridization between 4f electron and conduction electron [10]. Fig. 3(a) shows the comparison between the spectrum measured at 30 K (open diamond) and the one at
84
K. Kanai et al./Journal of Electron Spectroscopy and Related Phenomena 92 (1998) 81–85
Fig. 3. (a) Comparison between the IPES spectra on-resonance (E k = 120 eV) measured at 30 and 290 K. The spectra at 30 and 290 K are presented by open and filled diamonds, respectively. The intensities are normalized by f 2 peaks. (b) Schematic energy diagram of IPES process for valence fluctuation systems [5].
290 K (closing diamond). Each spectrum was measured at E k = 120 eV, where the f 1 peak is the most enhanced. The abscissa is the energy above the Fermi level. The intensities are normalized by those of f 2 peaks. It is clear that the intensity of the f 1 peak is dramatically reduced at 290 K compared with at 30 K. (Note that the intensities of f 2 peaks increase.) This reflects the fact that the system starts to thermally occupy the lowest magnetic state. The 4f electrons which had been itinerant by the Kondo effect at 30 K become partially localized at 290 K. A little broadening to the low-energy side of f 2 peak at 30 K is due to a larger intensity of the f 1 peak broadened by the resolution. Meanwhile, the intensity of the f 1 peak remains enhanced. That shows that the 4f electron remains itinerant at 290 K. In Fig. 3(b), the schematic total energy diagram of the valence fluctuation system is quoted from Ref. [5]. The solid arrow shows the transition from the initial state of singlet ground state which was composed of mixed f 0 and f 1 configurations. In this case, the final states of IPES process are the f 1 or f 2 configuration. On the other hand, in the case of T $ T K, the transition from f 1 to f 2 configuration, indicated by the dotted arrow, also takes place, and the weight of the f 1 final state is reduced. Therefore, the intensity of the f 1 peak
decreases. In Fig. 3(a), the surface effect needs to be kept in mind in the case where the excitation energy is around 120 eV. It is said that the surface valence of rare earth compounds, the 4f electron of which has strong hybridization character, is often lower than that of bulk, and so the surface contribution to the spectrum differs from that of bulk. However, in this case, the 4f electron at surface localizes for all the temperature ranges, so that the reduction of the intensity of the f 1 peak corresponds to the change of the bulk feature. In Fig. 4 the temperature dependence of the ratio of f peak intensities measured at E k = 120 eV is presented.
Fig. 4. Ratios of f-peak intensities, I( f 2)/[I( f 1) + I( f 2)] measured at 120 eV are plotted against temperature. Temperature on abscissa is normalized with T K assumed to be 240 K.
K. Kanai et al./Journal of Electron Spectroscopy and Related Phenomena 92 (1998) 81–85
This curve is obtained by plotting the f 2 peak intensity, I(f 2), normalized by the total peak intensity, I(f 1) + I(f 2), against temperature. Assuming that the hybridization at the intermediate state of the resonant process connecting the 4d 9 4f 2 and 4d 9 4f 3 configuration is small, the ratio of I(f 2)/[I(f 1) + I(f 2)] provides direct information about the initial state occupancy, n f. For low temperature in comparison with T K, there is not a prominent temperature dependence; however for T,T K it is strongly pushed up through the T K. The temperature dependence of n f was calculated by noncrossing approximation (NCA) computational method for dilute magnetic alloys by using the ‘Kondo model’ [11]. The inclination of the experimental result is consistent with that of this calculation. For very low temperature, in comparison with T K, the 4f occupancy, n f, is constant and the increase of the ratio, I(f 2)/[I(f 1) + I(f 2)] near T K corresponds to the increase of the 4f occupancy, n f, near T K. This reflects the process of a singlet ground state being broken up by thermal excitation, and 4f electrons becoming localized near T K. The characteristic ‘Kondo scaling’ behavior of the Kondo effect in the impurity systems is clearly observed. At this point, the Kondo impurity model seems to give a good description of the temperature variation of the low temperature 4f valence, n f, in a qualitative aspect. On the other hand, the fact that the f 1 peak shows still stronger resonance enhancement in the on-resonant spectrum at 290 K indicates that 4f electron still remains itinerant to some extent up to the temperature above T K. This is consistent with the magnetic susceptibility data which is Curie–Weisslike above room temperature.
4. Conclusion We measured the clear variation of 4f structure in the RIPES spectrum at different temperatures, which is resonantly enhanced near the Ce-N 4,5 absorption edge. There is an apparent inclination in the temperature
85
dependence of the f peaks ratio. The peak intensity ratio, I(f 2)/[I( f 1) + I( f 2)] is strongly pushed up around the T K. This reflects the process of destroying the singlet ground state by thermal excitation. The dramatic transformation of the system is brought about with slight changes of 4f occupancy. Also, we would like to emphasize the great ability of this new technique, IPES and RIPES measurement, to observe the small changes of the electronic structure of strongly correlated systems.
Acknowledgements We would like to thank Prof. T. Jo, Dr. A. Tanaka, Dr T. Uozumi and Dr R. Takayama for their helpful disscussions.
References [1] P. Weibel, M. Grioni, D. Malterre, B. Dardel, Y. Baer, Phys. Rev. Lett. 72 (1994) 1252. [2] K. Kanai, Y. Tezuka, M. Fujisawa, Y. Harada, S. Shin, G. Schmerber, J.P. Kappler, J.P. Parlebas, A. Kotani, Phys. Rev. 55 (1997) 2623. [3] F. Patthey, W.D. Schneider, Y. Baer, B. Delley, Phys. Rev. Lett. 58 (1987) 2810. [4] L.H. Tjeng et al., Phys. Rev. Lett. 71 (1993) 1419. [5] D. Malterre, M. Grioni, P. Weibel, Y. Baer, Phys. Rev. Lett. 68 (1992) 2656; M. Grioni, D. Malterre, P. Weibel, B. Dardel, Y. Baer, Physica B 186–188 (1993) 38. [6] O. Gunnarsson, K. Scho¨nhammer, Z. Phys. B30 (1978) 297; Phys. Rev. B28 (1983) 4315. [7] J.M. Lawrence, J.D. Thompson, Y.Y. Chen, Phys. Rev. Lett. 54 (1985) 2537. [8] J. Arts, F.R. deBoer, P.F. de Chaˆtel, A. Menovsky, Solid State Commun. 56 (1985) 623. [9] S. Shin, A. Agui, M. Fujisawa, Y. Tezuka, T. Ishii, N. Hirai, Rev. Sci. Instrum. 66 (1995) 1584. [10] A. Tanaka, T. Jo, Physica B206-207 (1995) 74; J. Phys. Soc. Jpn 65 (1996) 615. [11] N.E. Bickers, D.L. Cox, J.W. Wilkins, Phys. Rev. Lett. 54 (1985) 230; Phys. Rev. B36 (1987) 2036.