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Electronic structure of UC studied by X-ray photoemission and bremsstrahlung isochromat spectroscopy
PHYSICA
Physica B 186-188 (1993) 77-79 North-Holland
Electronic structure of UC studied by X-ray photoemission and bremsstrahlung isochromat spectroscopy Takeo Ejima a, Katsutoshi Murata al, Shoji Suzuki a, Takashi Takahashi a, Shigeru Sato a, Tadao Kasuya a, Yoshichika 6nukibl Hiroshi Yamagami c, Akira Hasegawa d and Takehiko
Ishii e
aDepartment of Physics, Faculty of Sc&nce, Tohoku University, Sendai, Japan blnstitute of Materials Science, University of Tsukuba, Japan CCollege of General Education, Tohoku University, Sendai, Japan OCollege of General Education, Niigata University, Japan elnstitute for Solid State Physics, University of 7bkyo, Japan X-ray photoemission and bremsstrahlung isochromat spectra are measured for UC. Valence state spectra are compared with the relativistic APW band calculation. Shape analysis of the U 4f core spectrum is made by Doniach-Sunjic lineshape with broadening. The discrepancies between valence spectra and the APW calculations, and presence of U 4f satellites show a correlation effect between U 5f electrons.
1. Introduction Hill plots predict that the itinerant behavior of U 5f states in uranium compounds depends on the separation between U atoms. Thereby most magnetic compounds have a uranium separation greater than 3.6 A., but several materials with greater separations are exceptionally nonmagnetic. This feature stresses the importance of interactions through the hybridization of U 5f states with the remaining valence electrons [1]. There have been several reports on XPS-BIS studies of uranium compounds [2,8], such as the alloy system Yl_xUxPd3. So far, however, there is no established description of uranium compounds in terms of the impurity Anderson model for the 5f electron except for the case of UO 2 [3]. This situation can be contrasted with the case of Ce compounds [1l]. NaCl-type UC is a suitable material for the fundamental study of the nature of 5f electrons [4]. In this study, we have measured the XPS and BIS spectra of metallic UC and compared with self-consistent relativistic APW band calculation for examining the validity of the itinerant feature for this material. In addition, U 4 f inner core spectrum is measured to
Correspondence to: T. Ejima, Department of Physics, Faculty of Science, Tohoku University, Sendai 980, Japan. 1Present address: Siemens-Asahi Medical Technology Ltd., Atsugi, Kanagawa 243-02, Japan.
obtain states.
complementary information on
electronic
2. Experiments The XPS and BIS measurements were performed on ESCA-LAB MKII(VG). The total resolution was 0.73eV for XPS and 0.69eV for BIS. Because uranium compounds are chemically active, oxidization of the sample was checked before and after measurements by observation of the O Is core spectrum. We repeated filing to obtain fresh sample surfaces. The pressure during the measurements was less than 3.0 × 10-1°mbar for XPS and 1 . 0 x l 0 - g m b a r for BIS. Mg Kcx radiation served as an exciting light for XPS measurements. The photon energy detected in BIS measurements was 1486.5 eV. The UC sample was a single crystal; sample properties are described elsewhere [4].
3. Results and discussion Figure l(a) shows the valence band XPS and unoccupied BIS spectrum for UC. Both spectra are normalized by assuming that the density-of-states (DOS) for BIS spectrum coincide with that for the XPS spectrum at the Fermi energy. The valence band
0921-4526/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
78
T. Ejirna et al. / Electronic structure of UC I
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BINDING ENERGY (eV) Fig. 1. (a) Occupied and unoccupied valence state spectra of UC obtained by XPS and BIS measurements. (b) Comparison of valence state XPS-BIS spectra with a DOS curve calculated by the relativistic APW method. XPS spectrum exhibits a single sharp peak A at the binding energy of 0.6 eV. In the binding energy region from - 2 to - 4 eV, there is a shoulder-like structure B that is expected as a tail from the peak. In addition, a broad hump C appears at around -10eV. The BIS spectrum shows also a single peak a at 2.4 eV with a tail c extending to +10eV. Shoulders a' and b are clearly seen at 1 and 3 eV above E v, respectively. The average reproducibility of the present data is within 2%. The DOS are calculated using a self-consistent relativistic APW method by the authors (HY and HA) as an extension of the theoretical investigation of the electronic structures of UC and ThC. The results explain well the recent de Haas-van Alphen effect for UC. For this reason UC is believed to be typical of uranium compounds in which the U 5f states have itinerant character [5]. Figure l(b) compares the present data and the calculated DOS for UC. From the calculation, it is obvious that the C 2p states form the valence band and the U 5f states hybridize mainly with the C 2p states. U 5d bands superpose over the valence band region. In addition, the C 2s band lies around - 9 eV. From the
atomic data [9], the photoionization cross-section of U 5f states in the energy range of Mg Ka radiation is approximately 20 times greater than that of U 6d states and 40 times greater than those of U 7s and C 2s states, respectively. Note that the cross-section of C 2p states is negligibly small. As a result, we can interpret the valance XPS spectra as follows. The sharp peak A is ascribed to the U 5f states. The shoulder B originates predominantly from U 5f states. The U 5d states of smaller magnitude overlap in the same energy region. The hump C is assigned as the C 2s band. The present DOS calculation agrees qualitatively with the measured XPS valence band. In the measured BIS spectrum, the energy positions of the shoulder structure a' and the sharp main peak a are in good agreement with those of the calculated two-peak U 5f DOS. In the calculation, these two peaks are identified as spin-orbit-split pairs of the U 5f level. For this reason we can assume that the shoulder a' and the peak a originate from U 5f states. On the higher-energy side from the main BIS peak a, we can observe the long tailing c extending to + 12 eV. The U 5f DOS are not so high in the corresponding energy region, but we can assign the tailing as the U 5f states because of the large cross-section of the states. As is seen from fig. l(b), the nearly constant DOS, which is assumed to be from U 5d states, overlaps the same binding energy region. Comparing the experimental results and the DOS by APW calculations, we have found several discrepancies, as follows: (1) Peak A in the XPS spectrum shows a higher intensity and an asymmetric shape. We confirmed that the asymmetric shape was not generated by the instrumental and lifetime broadening. (2) Peak a in the BIS spectrum exhibits a strong intensity enhancement and narrowing. Shoulder a' is clearly observed. (3) The long tailing c shows a higher intensity and a broader extension up to + 12 eV. Consequently it seems that a certain amount of the 5f intensity around E v is transferred to the satellite bands B and b located at both sides of E v as in the case of 4f materials [11]. Although the present XPSBIS spectra were qualitatively explained in terms of the 5f band model, the above discrepancies tell us that the renormalized band calculation by introducing correlation effect between U 5f electrons is necessary even for metallic UC. Figure 2 shows the U 4f core level spectrum and the results of the lineshape analysis for UC. Two asymmetric pairs, c~ and 13, are spin-orbit-split pairs; a is assigned as 4f7/2 component and 13 is as 4fsj e. A so-called Ka3. 4 X-ray satellite is observed on the low
T. Efima et al. / Electronic structure of UC i
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These satellites could be interpreted in terms of the impurity Anderson model [3]. The presence of the satellite indicates the more localized nature of the U 5f states in UC. As for the asymmetric core lineshape, the DS mechanism may be very effective if the DOS at E F is very large [10]. However, in addition to this, other contribution of an unresolved multiplet splitting, final state mixing to the lineshape could account for the observed results. This work was supported by the Grant-in-Aid for Scientific Research on Priority Areas 'Physics of Actinide Compounds', No. 02216108, from the Ministry of Education, Science and Culture, Japan•
--~-A
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Fig. 2. Uranium 4f core level spectrum and fitting result with a DS lineshape for UC. a and 13 are spin-orbit-split pairs, corresponding to 4f7/2 and 4f5/2. Solid line, convoluted resuits; broken lines A and B, deconvoluted curves, a is the singularity index; L, the Lorentzian width; G, the Gaussian width. The integrated intensity ratio of the satellite to the main core line is 8.14 (±0.4)%, and ratio of 4f5/2 to 4f7/2 is 72 (-41)%. binding energy side of the a peak. In order to clear the information of the satellite, we convoluted the spectral shape of the U 4f core level using the analytical expression of Doniach-Sunjic (DS), where the asymmetry of the core line is described by the singularity index a [7]. In the figure, the solid line represents the results of the convolution. Agreement with the experimental curve is relatively good. The broken line A corresponds to the deconvoluted main peaks with inclusion of Kot3, 4 satellites. Line B represents deconvoluted satellites of the U 4f core spectrum, where their profiles are assumed to have the same lineshapes as the main peak. The relevant parameter for the DS lineshape is a = 0.51, which is close to = 0.56 in UPt 5 [6].
References
[1] J.M. Fournier, in: Structure and Bonding, 59/60 (Springer-Verlag, Berlin, 1985) p. 1. [2] L.Z. Liu, J.W. Allen, C.L. Seaman, M.B. Maple, Y. Dalichaouch, J.-S. Kang, M.S. Torikachvili and M.A. Lopez de la Torte, Phys. Rev. Lett. 68 (1992) 1034. [3] O. Gunnarsson, D.D. Sarma, F.U. HiUebrecht and K. Sch6nhammer, J. Appl. Phys. 63 (1988) 3676. [4] Y. ()nuki, I. Umehara, Y. Kurosawa, K. Satoh and H. Matsui, J. Phys. Soc. Jpn. 59 (1990) 229. [5] A. Hasegawa and H. Yamagami, J. Phys. Soc. Jpn. 59 (1990) 218. [6] W.-D. Schneider and C. Laubschat, Phys. Rev. B 23 (1981) 997. [7] G.K. Wertheim and P.H. Citrin, in: Photoemission in Solids, Vol. I, eds. M. Cardona and L. Ley (SpringerVerlag, Berlin, 1978) p. 197. [8] Y. Baer, in: Handbook on the Physics and Chemistry of the Actinides, eds. A.J. Freeman and G.H. Lander (Elsevier, Amsterdam, 1984) p. 271. [9] J.J. Yeh and I. Lindau, Atomic Data Nucl. Data Tables, 32 (1985) 1. [10] F. Greuter, E. Hauser, P. Oelhafen, H.-J. G~intherodt, B. Reihl and O. Vogt, Physica B 102 (1980) 117. [11] J.W. Allen, S-J. Oh, O. Gunnarsson, K. Sch6nhammer, M.B. Maple, M.S. Torikachvili and I. Lindau, Adv. Phys. 35 (1986) 275.
Physica B 186-188 (1993) 77-79 North-Holland
Electronic structure of UC studied by X-ray photoemission and bremsstrahlung isochromat spectroscopy Takeo Ejima a, Katsutoshi Murata al, Shoji Suzuki a, Takashi Takahashi a, Shigeru Sato a, Tadao Kasuya a, Yoshichika 6nukibl Hiroshi Yamagami c, Akira Hasegawa d and Takehiko
Ishii e
aDepartment of Physics, Faculty of Sc&nce, Tohoku University, Sendai, Japan blnstitute of Materials Science, University of Tsukuba, Japan CCollege of General Education, Tohoku University, Sendai, Japan OCollege of General Education, Niigata University, Japan elnstitute for Solid State Physics, University of 7bkyo, Japan X-ray photoemission and bremsstrahlung isochromat spectra are measured for UC. Valence state spectra are compared with the relativistic APW band calculation. Shape analysis of the U 4f core spectrum is made by Doniach-Sunjic lineshape with broadening. The discrepancies between valence spectra and the APW calculations, and presence of U 4f satellites show a correlation effect between U 5f electrons.
1. Introduction Hill plots predict that the itinerant behavior of U 5f states in uranium compounds depends on the separation between U atoms. Thereby most magnetic compounds have a uranium separation greater than 3.6 A., but several materials with greater separations are exceptionally nonmagnetic. This feature stresses the importance of interactions through the hybridization of U 5f states with the remaining valence electrons [1]. There have been several reports on XPS-BIS studies of uranium compounds [2,8], such as the alloy system Yl_xUxPd3. So far, however, there is no established description of uranium compounds in terms of the impurity Anderson model for the 5f electron except for the case of UO 2 [3]. This situation can be contrasted with the case of Ce compounds [1l]. NaCl-type UC is a suitable material for the fundamental study of the nature of 5f electrons [4]. In this study, we have measured the XPS and BIS spectra of metallic UC and compared with self-consistent relativistic APW band calculation for examining the validity of the itinerant feature for this material. In addition, U 4 f inner core spectrum is measured to
Correspondence to: T. Ejima, Department of Physics, Faculty of Science, Tohoku University, Sendai 980, Japan. 1Present address: Siemens-Asahi Medical Technology Ltd., Atsugi, Kanagawa 243-02, Japan.
obtain states.
complementary information on
electronic
2. Experiments The XPS and BIS measurements were performed on ESCA-LAB MKII(VG). The total resolution was 0.73eV for XPS and 0.69eV for BIS. Because uranium compounds are chemically active, oxidization of the sample was checked before and after measurements by observation of the O Is core spectrum. We repeated filing to obtain fresh sample surfaces. The pressure during the measurements was less than 3.0 × 10-1°mbar for XPS and 1 . 0 x l 0 - g m b a r for BIS. Mg Kcx radiation served as an exciting light for XPS measurements. The photon energy detected in BIS measurements was 1486.5 eV. The UC sample was a single crystal; sample properties are described elsewhere [4].
3. Results and discussion Figure l(a) shows the valence band XPS and unoccupied BIS spectrum for UC. Both spectra are normalized by assuming that the density-of-states (DOS) for BIS spectrum coincide with that for the XPS spectrum at the Fermi energy. The valence band
0921-4526/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
78
T. Ejirna et al. / Electronic structure of UC I
I
I
I
(a)
I
I
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BINDING ENERGY (eV) Fig. 1. (a) Occupied and unoccupied valence state spectra of UC obtained by XPS and BIS measurements. (b) Comparison of valence state XPS-BIS spectra with a DOS curve calculated by the relativistic APW method. XPS spectrum exhibits a single sharp peak A at the binding energy of 0.6 eV. In the binding energy region from - 2 to - 4 eV, there is a shoulder-like structure B that is expected as a tail from the peak. In addition, a broad hump C appears at around -10eV. The BIS spectrum shows also a single peak a at 2.4 eV with a tail c extending to +10eV. Shoulders a' and b are clearly seen at 1 and 3 eV above E v, respectively. The average reproducibility of the present data is within 2%. The DOS are calculated using a self-consistent relativistic APW method by the authors (HY and HA) as an extension of the theoretical investigation of the electronic structures of UC and ThC. The results explain well the recent de Haas-van Alphen effect for UC. For this reason UC is believed to be typical of uranium compounds in which the U 5f states have itinerant character [5]. Figure l(b) compares the present data and the calculated DOS for UC. From the calculation, it is obvious that the C 2p states form the valence band and the U 5f states hybridize mainly with the C 2p states. U 5d bands superpose over the valence band region. In addition, the C 2s band lies around - 9 eV. From the
atomic data [9], the photoionization cross-section of U 5f states in the energy range of Mg Ka radiation is approximately 20 times greater than that of U 6d states and 40 times greater than those of U 7s and C 2s states, respectively. Note that the cross-section of C 2p states is negligibly small. As a result, we can interpret the valance XPS spectra as follows. The sharp peak A is ascribed to the U 5f states. The shoulder B originates predominantly from U 5f states. The U 5d states of smaller magnitude overlap in the same energy region. The hump C is assigned as the C 2s band. The present DOS calculation agrees qualitatively with the measured XPS valence band. In the measured BIS spectrum, the energy positions of the shoulder structure a' and the sharp main peak a are in good agreement with those of the calculated two-peak U 5f DOS. In the calculation, these two peaks are identified as spin-orbit-split pairs of the U 5f level. For this reason we can assume that the shoulder a' and the peak a originate from U 5f states. On the higher-energy side from the main BIS peak a, we can observe the long tailing c extending to + 12 eV. The U 5f DOS are not so high in the corresponding energy region, but we can assign the tailing as the U 5f states because of the large cross-section of the states. As is seen from fig. l(b), the nearly constant DOS, which is assumed to be from U 5d states, overlaps the same binding energy region. Comparing the experimental results and the DOS by APW calculations, we have found several discrepancies, as follows: (1) Peak A in the XPS spectrum shows a higher intensity and an asymmetric shape. We confirmed that the asymmetric shape was not generated by the instrumental and lifetime broadening. (2) Peak a in the BIS spectrum exhibits a strong intensity enhancement and narrowing. Shoulder a' is clearly observed. (3) The long tailing c shows a higher intensity and a broader extension up to + 12 eV. Consequently it seems that a certain amount of the 5f intensity around E v is transferred to the satellite bands B and b located at both sides of E v as in the case of 4f materials [11]. Although the present XPSBIS spectra were qualitatively explained in terms of the 5f band model, the above discrepancies tell us that the renormalized band calculation by introducing correlation effect between U 5f electrons is necessary even for metallic UC. Figure 2 shows the U 4f core level spectrum and the results of the lineshape analysis for UC. Two asymmetric pairs, c~ and 13, are spin-orbit-split pairs; a is assigned as 4f7/2 component and 13 is as 4fsj e. A so-called Ka3. 4 X-ray satellite is observed on the low
T. Efima et al. / Electronic structure of UC i
I
i
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,
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These satellites could be interpreted in terms of the impurity Anderson model [3]. The presence of the satellite indicates the more localized nature of the U 5f states in UC. As for the asymmetric core lineshape, the DS mechanism may be very effective if the DOS at E F is very large [10]. However, in addition to this, other contribution of an unresolved multiplet splitting, final state mixing to the lineshape could account for the observed results. This work was supported by the Grant-in-Aid for Scientific Research on Priority Areas 'Physics of Actinide Compounds', No. 02216108, from the Ministry of Education, Science and Culture, Japan•
--~-A
I
79
~
-400 -380 Binding Energy (eV)
I
Fig. 2. Uranium 4f core level spectrum and fitting result with a DS lineshape for UC. a and 13 are spin-orbit-split pairs, corresponding to 4f7/2 and 4f5/2. Solid line, convoluted resuits; broken lines A and B, deconvoluted curves, a is the singularity index; L, the Lorentzian width; G, the Gaussian width. The integrated intensity ratio of the satellite to the main core line is 8.14 (±0.4)%, and ratio of 4f5/2 to 4f7/2 is 72 (-41)%. binding energy side of the a peak. In order to clear the information of the satellite, we convoluted the spectral shape of the U 4f core level using the analytical expression of Doniach-Sunjic (DS), where the asymmetry of the core line is described by the singularity index a [7]. In the figure, the solid line represents the results of the convolution. Agreement with the experimental curve is relatively good. The broken line A corresponds to the deconvoluted main peaks with inclusion of Kot3, 4 satellites. Line B represents deconvoluted satellites of the U 4f core spectrum, where their profiles are assumed to have the same lineshapes as the main peak. The relevant parameter for the DS lineshape is a = 0.51, which is close to = 0.56 in UPt 5 [6].
References
[1] J.M. Fournier, in: Structure and Bonding, 59/60 (Springer-Verlag, Berlin, 1985) p. 1. [2] L.Z. Liu, J.W. Allen, C.L. Seaman, M.B. Maple, Y. Dalichaouch, J.-S. Kang, M.S. Torikachvili and M.A. Lopez de la Torte, Phys. Rev. Lett. 68 (1992) 1034. [3] O. Gunnarsson, D.D. Sarma, F.U. HiUebrecht and K. Sch6nhammer, J. Appl. Phys. 63 (1988) 3676. [4] Y. ()nuki, I. Umehara, Y. Kurosawa, K. Satoh and H. Matsui, J. Phys. Soc. Jpn. 59 (1990) 229. [5] A. Hasegawa and H. Yamagami, J. Phys. Soc. Jpn. 59 (1990) 218. [6] W.-D. Schneider and C. Laubschat, Phys. Rev. B 23 (1981) 997. [7] G.K. Wertheim and P.H. Citrin, in: Photoemission in Solids, Vol. I, eds. M. Cardona and L. Ley (SpringerVerlag, Berlin, 1978) p. 197. [8] Y. Baer, in: Handbook on the Physics and Chemistry of the Actinides, eds. A.J. Freeman and G.H. Lander (Elsevier, Amsterdam, 1984) p. 271. [9] J.J. Yeh and I. Lindau, Atomic Data Nucl. Data Tables, 32 (1985) 1. [10] F. Greuter, E. Hauser, P. Oelhafen, H.-J. G~intherodt, B. Reihl and O. Vogt, Physica B 102 (1980) 117. [11] J.W. Allen, S-J. Oh, O. Gunnarsson, K. Sch6nhammer, M.B. Maple, M.S. Torikachvili and I. Lindau, Adv. Phys. 35 (1986) 275.