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Studies of uranium compounds by photoemission and bremsstrahlung isochromat spectroscopy
PHYSICA
Physica B 186-188 (1993) 21-25 North-Holland
Studies of uranium compounds by photoemission and bremsstrahlung isochromat spectroscopy Takehiko Ishii Institute for Solid State Physics, University of Tokyo, Japan The valence state spectra of UPS, XPS and BIS of uranium compounds are discussed using the spectra of UGe 2 and UC as examples of the optically traced 5f partial densities of states and related information about the electronic structures. The 5f spectra in different compounds are similar in that they have sharp Fermi edges and intensities that decrease monotonically toward high binding energy. The BIS spectra have profile intensities that decrease toward high energy. A discussion is made on the localized-electron model versus the itinerant-electron model in the representative case of UC. The spectral profile on the 5d-5f resonance excitation is also discussed.
1. Introduction
Several years ago, a lot of experimental work on ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and Bremsstrahlung isochromat spectroscopy (BIS) of U compounds were reported [1-3]. The materials attracted many investigators since they show quite distinctive solid state properties. Some of them show anomalous temperature dependences of electrical conductivity and magnetic susceptibility, and anomalously large specific heat constants, as in the case of Ce compounds forming heavy-fermion systems, whereas others do not show such anomalous properties and are ordinary semiconductors or metals. Since it is well known that the anomalous solid state properties of Ce compounds as heavy-fermion materials are caused by localized 4f electrons with strong correlation interactions, hybridization with other itinerant electrons, and the configuration interaction, it is assumed that the localization of 5f electrons responsible for the characteristic aspects of U compounds is not strong enough. The situation can be contrasted with the case of 3d electrons in transition metal compounds, which are practically itinerant and the correlation interaction modifies their one-electron nature. The energy states of Ce compounds are well described by the impurity Anderson model, but those of transition metal compounds are described by the energy band model with corrections
Correspondence to: T. Ishii, Institute for Solid State Physics, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106, Japan.
for the correlation effect. The investigation of U compounds appears fascinating because no unique model to explain their properties has yet been found. Among the basic information that is required for understanding the nature of matter is the electronic structure. This information can be obtained from optical measurements such as UPS, XPS and BIS. The 4f spectra of Ce compounds derived from UPS energy distribution curves (EDCs), particularly from resonance o n - o f f difference spectra, have two-peak profiles and they are well reproduced by the impurity Anderson model with the configuration interaction. The appearance of satellites in XPS core-level lines can also be explained by this model [4]. On the other hand, the angle-resolved UPS spectra of transition metals are well explained by the energy band model and the calculated dispersion of energy bands as well as the calculated density of states (DOS) curves are well reproduced by experimentally observed EDCs. In the case of materials with localized 3d orbitals such as transition metal insulator compounds, UPS and XPS spectra can be explained in terms of the cluster model, in which convoluted multiplet lines arising from a few 3d electron configurations in the field of ligand potentials in clusters reproduce observed EDCs. In the case of the BIS spectra of Ce compounds, transition metals, and transition metal compounds, the situations are more or less similar to the case of UPS and XPS spectra and the interpretation of spectra proceeds along similar lines. The situation is quite different in U compounds. Even in those that show solid state properties similar to those of Ce compounds, their optical spectra are
0921-4526/93/$06.00 © 1993- Elsevier Science Publishers B.V. All rights reserved
22
T. Ishii / PES and B1S of U compounds
not the same. A conspicuous aspect of the valence band photoelectron EDCs of U compounds is that the ionization cross section of the U 5f electron is quite large [5]. As a consequence, if the valence states of a U compound other than the U 5f states are composed of states originating in electrons with ionization cross sections much smaller than that of a U 5f electron, the observed valence state spectra reflect the partial DOS curves of U 5f electrons. If constituent electrons other than U 5f electrons have ionization cross sections comparable with those of U 5f electrons, as in the case of compounds involving transition metals, the observed spectra exhibit features arising from the constituent electrons. Even in compounds such as these, it is possible to draw out the partial DOS curves of U 5f electrons by the 5d-5f resonant photoemission. The partial DOS curves are simulated by the difference curves between the on-resonance and off-resonance EDCs. The purpose of this paper is to discuss the U 5f partial DOS curve in U compounds. Among various U compounds we have dealt with, U G e 2 and UC are selected as representative examples. The DOS curves for occupied states are derived from the UPS and XPS spectra and those for unoccupied states are simulated by the BIS spectra. The solid state properties of U G e 2 are not well known. It shows ferrimagnetic ordering below 52 K. The magnetic susceptibility follows the Curie-Weiss law about 200K. The effective Bohr magneton per U atom and the paramagnetic Curie temperature are estimated to be 2.73p. a and 89 K, respectively. This material crystallizes in the monoclinic ZrSi2-type structure. The temperature dependence of magnetoresistance suggests that it is a compensated metal. Since the cyclotron mass is considerably large, U 5f electrons appear to be localized and heavy• UC is a semimetal with the crystal structure of the NaCl type. The results of energy band calculations [6] agree well with the results of measurements of the de H a a s - v a n Alphen effects [7]. The U G e 2 specimens used in the experiments were in polycrystalline form [8]. Small specimens of UC were cut from a single crystal grown by annealing a polycrystalline ingot in an argon atmosphere [7,9]. The measurements of resonant photoemission and related UPS spectra were carried out using SORR I N G , a 0.38 GeV electron storage ring at the Synchrotron Radiation Laboratory of the Institute for Solid State Physics, the University of Tokyo. A modified Rowland mount monochromator and a cylindrical mirror analyzer were used. XPS and UPS measurements were made using a commercially available instrument, VG E S C A - L A B MkII. Clean sample surfaces of U G e 2 were obtained by fracturing, and those
of UC by filing with a diamond file. The overall instrumental resolution for UPS varied from 0.350.7 eV at photon energies of 40-120 eV, respectively. The overall instrumental resolutions for XPS and BI were 0.8 and 0.7 eV, respectively. Other experimental details are described elsewhere [8-10].
2. Occupied valence state spectra
Figure 1 shows the EDCs of UGe2 measured by excitations at 40 and 1253.6 eV. For both spectra, no correction (including background subtraction) was made, and they are quite similar. In the spectral region observed here, the ionization cross sections of U 5f electrons are much larger than those of other valence electrons in this material. Thus, the spectra illustrated in fig. 1 reflect the 5f partial DOS curve mostly; this is verified by the 5d-5f resonant photoemission. In the UPS spectrum, a broad hump-like feature occurs around 2 eV. The comparison of the UPS spectrum with the o n - o f f difference spectrum of the 5d-Sf resonant photoemission shows that the 2 eV hump cannot be ascribed to the 5f partial states. The XPS spectrum is broader than the UPS spectrum. This is not ascribed merely to the instrumental resolution, which is lower in XPS EDCs. The main peak of the UPS spectrum is located at 0.15 eV, almost at the Fermi edge, whereas the corresponding highest peak is located at 0.75 eV in the XPS spectrum. The width of the peak of the UPS spectrum is about 1 eV,
°. e.
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BINDING ENERGY (eV) Fig. 1. Comparison of the UPS and XPS EDCs of UGe 2. The numbers on the curves represent the excitation energies.
T. lshii / PES and BIS of U compounds whereas the corresponding width of the XPS spectrum is larger by a factor of 2. The origin of this difference is not clear at present. One possibility is that the ionization cross sections of valence electrons other than 5f electrons are relatively larger in XPS emission. The difference of this type is manifest in a weak broad band with its center around 8.5 eV in the XPS spectrum. The corresponding feature in the UPS spectrum appears weak, although a weak oxygen band due to surface contamination occurring around 6 eV obscures the presence of the 8.5 eV band. The partial DOS curve of 5f electrons and that of other valence electrons in UGe 2 are shown in fig. 2. The 5f partial DOS curve was obtained as the resonance o n - o f f difference spectrum drawn up by subtracting the EDC measured by an excitation at 92 eV from that measured by an excitation at 97 eV. The curve is indicated as U 5f in fig. 2. The excitation threshold of the U 5d electron occurs around 95 eV, above which the ionization cross section of the 5f electron is enhanced. At 92 eV, on the contrary, the ionization cross sections of all valence electrons are very small, and are ascribed to the Fano minimum in
UGe2 ." U5f
/
z ~,
23
the 5d-5f resonance excitation where the ionization cross section of the 5f electron almost vanishes. This is found in EDC designated as '92 eV', and in this sense, it represents the partial DOS curve for electrons other than 5f electrons. The ordinate scale of the EDC measured by the excitation at 92 eV has been expanded ten times so that the partial DOS curve of other electrons could be shown in greater detail. The spectrum obtained is shown by the curve designated as '92 eV, × 10' in fig. 2. The intensity of a broad feature around 8.5 eV is discernible in fig. 1 is very weak in the excitation energy region above 70 eV and is not found in the spectrum illustrated in fig. 2. Instead, a feature is found around 6 eV in the spectrum obtained by the 92 eV excitation and expanded 10 times. This feature is brought about by the contamination with oxygen. In the lowest part of the figure, the valence band XPS spectrum of pure Ge obtained by Ley et al. [11] is shown. The XPS spectrum of pure Ge looks quite different from that of UGe2 measured at 92 eV. Pure Ge is a semiconductor and its spectrum does not show a well defined Fermi edge, whereas the sharp Fermi edge is found in the spectrum of UGe 2. Other parts of the spectral profile look totally different. To the author's knowledge, no theoretical calculation of the energy levels of UGe 2 has been reported. The author supposes that the 5f states in UGe2 are in a situation more or less similar to those of UC, for which an energy band calculation has been reported [6]. Therefore, the discussion of the valence state energy levels is made here using UC as a representative example. Figure 3 shows the XPS valence state EDC and the 5f partial DOS curve calculated by
92 eV
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Fig. 2. Top curve: the 5f partial DOS curve of UGe 2 obtained as the resonance on-off difference spectrum, where the on-resonance EDC was obtained by an excitation at 97 eV. Two middle curves, EDCs obtained by excitations at 92 eV, assumed to represent the partial DOS curves of electrons other than 5f electrons. In one of the spectra the ordinate scale is expanded ten times. Bottom curve: the valence band XPS spectrum of pure Ge obtained from ref. [11].
5f I
DOS Jr'-
, I
6 4 2 0 BINDING ENERGY (eV) Fig. 3. XPS valence band EDC of UGe 2 and the 5f partial DOS curve calculated by the self-consistent relativistic APW method [6]. The DOS curve is cut at the Fermi level with the Fermi-Dirac distribution function for 300 K.
24
T. lshii / PES and BIS of U compounds
Hasegawa and Yamagami [6] using the self-consistent relativistic APW method. Since the results of this energy-band calculation explain the results of de Haas-van Alphen experiments carried out by Onuki et al. [7], we are tempted to compare the observed valence state EDC with the DOS curve calculated in this energy band calculation [9]. In fig. 3, the 5f partial DOS curve illustrated was obtained by cutting off the calculated 5f partial DOS curve with the Fermi-Dirac distribution function at 300 K at which the experiments were carried out. At first glance, a fair qualitative agreement between the experimental and theoretical curves is found, but for a more complete agreement, we meet a few difficulties. First, a hump at around 2 eV may not be ascribed to the 5f state. The total DOS curve appears to suggest that a considerable amount of state density exists in this area. Second, the main peak of the experimental curve does not show a composite structure. If we convolute the partial DOS curve shown in fig. 3 with a Gaussian function expressing the instrumental and phonon broadening, we obtain a broadened peak that still has a doublet profile. This a kind of qualitative disagreement. In the calculated DOS curve, a sharp and intense peak exists just above the Fermi level. If the location of the Fermi level moves upward, a peak with an appreciable magnitude occurs below it. In such a case the spectra obtained by convoluting the DOS curves with a broadening function still have sharp peaks near the Fermi edge and no long tailings, as found in the experimentally observed EDC. Again, some disagreement is found. It is not known at present whether a localized electron model based on the impurity Anderson Hamiltonian and the configuration interaction reproduces a single peak at the Fermi edge and a long tailing on the high binding energy side. In the case of rare earth compounds with two or more 4f electrons, such calculations lead to spectra quite different from those found experimentally in U compounds. It appears that the assumption of an unresolved satellite [12] is not realistic in the U compounds measured here, since the UPS spectra show sharp main peaks. Thus, more elaborate investigations are required to clarify the valence state spectra.
3. R e s o n a n c e
processes
Resonant photoemission has been discussed in detail by many authors. Here, only one aspect that is distinctive of U - G e compounds is pointed out. Figure 4 shows the total yield spectrum of UGe~ and the constant initial state (CIS) spectrum measured for the
UGe2
/
/
Z TOTAL YIELD
z
Z © ,..3 © [.., © 1
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80 100 120 PHOTON ENERGY(eV) Fig. 4. Total yield spectrum of UGe2 and the CIS spectrum for the binding energy of 0.3 eV. intensity at 0.3 eV. Generally, total yield spectra are very similar to absorption spectra. This is obvious, since the most of the emitted electrons are from the 5f states, as mentioned above. The distinctive aspect is that the excitation curves consist of two peaks, at 97 and 112 eV, similar to those found in other U - G e compounds such as U3Ge 4 [8]. In rare earth compounds, the 4d-4f resonance is dominant. The excitation spectra are composed of two parts: one is a giant broad band with high intensity, and the other is a group of very weak lines occurring in the region below the onset of the giant band. The spectral area of the weak lines is often referred to as the prethreshold region. The weak lines originate in the multiplet levels caused by the electrostatic interactions between 4f electrons and a photoproduced 4d hole. Weak lines are originally forbidden owing to the spin selection rule, which is violated by the weak spin-orbit interaction. In U - G e compounds, such weak lines in the prethreshold region are not found, but a neatly shaped band with an intermediate intensity occurs at 97 eV. Yamazaki and Kotani have pointed out the importance of the spin-orbit coupling for explaining the inner-shell excitation spectra of actinide compounds [13]. Using their idea, the spectra shown in fig. 4 can be interpreted as follows. The strong spin-orbit interactions modify the multiplet coupling considerably and spin-forbidden lines are replaced with some allowed lines. Instead, a new line emerges, probably consisting of many unresolved component lines. The cause of the giant band is similar to the case of the 4d-4f resonance in rare earths. A practical calculation of the 5d-5f resonance along this direction has not yet been made.
T. Ishii / PES and B1S of U compounds
4. BIS spectra It is widely believed that a spectrum composed of an XPS and a BIS spectrum connected together at the F e r m i edge represents the D O S curve over a wide energy range. The author's colleagues have obtained such curves for various U compounds [9,10,14]. The results show that the BIS spectra of U compounds look m o r e or less similar, but this was already widely known• H e r e , the example of the spectrum of U C is discussed. Figure 5 shows the XPS and BIS spectra of U C connected together at the Fermi edge. The two spectra are connected in such a manner that the intensities at the Fermi edge are equal. The total D O S curve as well as the 5f partial D O S curve calculated by the self-consistent A P W m e t h o d is shown for comparison. The 5f partial D O S curve is the same as that shown in fig. 3. The BIS spectrum has a strong peak at 1.4 eV and a shoulder-like feature around 0.7eV. The calculated D O S curve suggests that two peaks occur in a region within 2.4 eV from the Fermi edge. If the instrumental resolution is not high enough to resolve the expected two features, the highest peak at 1.4 eV and a shoulder
c-,
-~
UC s. s" i.
z ~
• "
XPS
Z
././"
DOS
1
TOTAL ....
5f P A R T I A L ,4
--8
-4 0 4 8 ELECTRON ENERGY (eV)
Fig. 5. XPS and BIS spectra of UC connected at the Fermi level. Both spectra are connected so that the intensities at the Fermi edge are equal. Total and 5f partial DOS curves calculated by the self-consistent relativistic APW method [6] are also shown.
25
around 0 . 7 e V are possible candidates for the two peaks in the calculated D O S curves. If we ignore the disagreement between the 5f partial D O S curve and the observed XPS spectrum as described in fig. 3, and assume that the E D C and the D O S curve are in fair agreement, the whole spectra could be explained in terms of the energy band picture. What we find about the X P S - B I S spectrum here is that the one-electron picture explains a part of the observed aspect of the spectra• It will therefore be interesting to see whether any refinement of the one-electron calculation along this direction is possible.
References [1] A.J. Arko, D.D. Koelling, B.D. Dunlap, C. Capasso and M. del Giudice, J. Less-Common Metals 48 (1989) 133, and references therein. [2] O. Gunnarsson, D.D. Sarma, F.U. Hillebrecht and K. Sch6nhammer, J. Appl. Phys. 63 (1988) 3676, and references therein. [3] J.M. Imer, D. Malterre, M. Gioni, P. Weibel, B. Bardel and Y. Baer, Phys. Rev. B 44 (1991) 10455. [4] O. Gunnarsson and K. Sch6nhammer, Phys. Rev. B 28 (1983) 4815. [5] J.J. Yeh and I. Lindau, Atom. Data Nucl. Data Tables 32 (1985) 1. [6] A. Hasegawa and H. Yamagami, J. Phys. Soc. Jpn. 59 (1990) 218. [7] Y. Onuki, I. Umehara, Y. Kurosawa, K. Satoh and H. Matsui, J. Phys. Soc. Jpn. 59 (1990) 229. [8] K. Soda, T. Mori, Y. Onuki, T. Komatsubara, S. Suga, A. Kakizaki and T. Ishii, J. Phys. Soc. Jpn. 60 (1991) 3059, and references therein. [9] T. Ejima, K. Murata, S. Suzuki, T. Takahashi, S. Sato, T. Kasuya, Y. Onuki, H. Yamagami, A. Hasegawa and T. Ishii, Physica B 186-188 (1993) 77. [10] S. Suzuki, S. Sato, T. Ejima, K. Murata, Y. Kubo, T. Takahashi, S. Suga, T. Komatsubara, N. Sato, M. Kasaya, Y. Onuki, T. Takabatake, T. Kasuya, K. Soda, T. Mori, A. Kakizaki and T. Ishii, Jpn. J. Appl. Phys. Series 8, to be published. [11] L. Ley, S. Kowalczyk, R. PoUak and D. Shirly, Phys. Rev. Lett. 29 (1972) 1088. [12] A.J. Arko, D.D. Koelling, B.D. Dunlap, A.W. Mitchell, C. Capasso and M. del Giudice, J. Appl. Phys. 63 (1988) 3680. [13] T. Yamazaki and A. Kotani, J. Phys. Soc. Jpn. 60 (1991) 49. [14] T. Ejima, Y. Kudo, S. Suzuki, T. Takahashi, S. Sato, T. Kasuya, T. Takabatake and T. Ishii, Physica B 186-188 (1993) 86.
Physica B 186-188 (1993) 21-25 North-Holland
Studies of uranium compounds by photoemission and bremsstrahlung isochromat spectroscopy Takehiko Ishii Institute for Solid State Physics, University of Tokyo, Japan The valence state spectra of UPS, XPS and BIS of uranium compounds are discussed using the spectra of UGe 2 and UC as examples of the optically traced 5f partial densities of states and related information about the electronic structures. The 5f spectra in different compounds are similar in that they have sharp Fermi edges and intensities that decrease monotonically toward high binding energy. The BIS spectra have profile intensities that decrease toward high energy. A discussion is made on the localized-electron model versus the itinerant-electron model in the representative case of UC. The spectral profile on the 5d-5f resonance excitation is also discussed.
1. Introduction
Several years ago, a lot of experimental work on ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and Bremsstrahlung isochromat spectroscopy (BIS) of U compounds were reported [1-3]. The materials attracted many investigators since they show quite distinctive solid state properties. Some of them show anomalous temperature dependences of electrical conductivity and magnetic susceptibility, and anomalously large specific heat constants, as in the case of Ce compounds forming heavy-fermion systems, whereas others do not show such anomalous properties and are ordinary semiconductors or metals. Since it is well known that the anomalous solid state properties of Ce compounds as heavy-fermion materials are caused by localized 4f electrons with strong correlation interactions, hybridization with other itinerant electrons, and the configuration interaction, it is assumed that the localization of 5f electrons responsible for the characteristic aspects of U compounds is not strong enough. The situation can be contrasted with the case of 3d electrons in transition metal compounds, which are practically itinerant and the correlation interaction modifies their one-electron nature. The energy states of Ce compounds are well described by the impurity Anderson model, but those of transition metal compounds are described by the energy band model with corrections
Correspondence to: T. Ishii, Institute for Solid State Physics, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106, Japan.
for the correlation effect. The investigation of U compounds appears fascinating because no unique model to explain their properties has yet been found. Among the basic information that is required for understanding the nature of matter is the electronic structure. This information can be obtained from optical measurements such as UPS, XPS and BIS. The 4f spectra of Ce compounds derived from UPS energy distribution curves (EDCs), particularly from resonance o n - o f f difference spectra, have two-peak profiles and they are well reproduced by the impurity Anderson model with the configuration interaction. The appearance of satellites in XPS core-level lines can also be explained by this model [4]. On the other hand, the angle-resolved UPS spectra of transition metals are well explained by the energy band model and the calculated dispersion of energy bands as well as the calculated density of states (DOS) curves are well reproduced by experimentally observed EDCs. In the case of materials with localized 3d orbitals such as transition metal insulator compounds, UPS and XPS spectra can be explained in terms of the cluster model, in which convoluted multiplet lines arising from a few 3d electron configurations in the field of ligand potentials in clusters reproduce observed EDCs. In the case of the BIS spectra of Ce compounds, transition metals, and transition metal compounds, the situations are more or less similar to the case of UPS and XPS spectra and the interpretation of spectra proceeds along similar lines. The situation is quite different in U compounds. Even in those that show solid state properties similar to those of Ce compounds, their optical spectra are
0921-4526/93/$06.00 © 1993- Elsevier Science Publishers B.V. All rights reserved
22
T. Ishii / PES and B1S of U compounds
not the same. A conspicuous aspect of the valence band photoelectron EDCs of U compounds is that the ionization cross section of the U 5f electron is quite large [5]. As a consequence, if the valence states of a U compound other than the U 5f states are composed of states originating in electrons with ionization cross sections much smaller than that of a U 5f electron, the observed valence state spectra reflect the partial DOS curves of U 5f electrons. If constituent electrons other than U 5f electrons have ionization cross sections comparable with those of U 5f electrons, as in the case of compounds involving transition metals, the observed spectra exhibit features arising from the constituent electrons. Even in compounds such as these, it is possible to draw out the partial DOS curves of U 5f electrons by the 5d-5f resonant photoemission. The partial DOS curves are simulated by the difference curves between the on-resonance and off-resonance EDCs. The purpose of this paper is to discuss the U 5f partial DOS curve in U compounds. Among various U compounds we have dealt with, U G e 2 and UC are selected as representative examples. The DOS curves for occupied states are derived from the UPS and XPS spectra and those for unoccupied states are simulated by the BIS spectra. The solid state properties of U G e 2 are not well known. It shows ferrimagnetic ordering below 52 K. The magnetic susceptibility follows the Curie-Weiss law about 200K. The effective Bohr magneton per U atom and the paramagnetic Curie temperature are estimated to be 2.73p. a and 89 K, respectively. This material crystallizes in the monoclinic ZrSi2-type structure. The temperature dependence of magnetoresistance suggests that it is a compensated metal. Since the cyclotron mass is considerably large, U 5f electrons appear to be localized and heavy• UC is a semimetal with the crystal structure of the NaCl type. The results of energy band calculations [6] agree well with the results of measurements of the de H a a s - v a n Alphen effects [7]. The U G e 2 specimens used in the experiments were in polycrystalline form [8]. Small specimens of UC were cut from a single crystal grown by annealing a polycrystalline ingot in an argon atmosphere [7,9]. The measurements of resonant photoemission and related UPS spectra were carried out using SORR I N G , a 0.38 GeV electron storage ring at the Synchrotron Radiation Laboratory of the Institute for Solid State Physics, the University of Tokyo. A modified Rowland mount monochromator and a cylindrical mirror analyzer were used. XPS and UPS measurements were made using a commercially available instrument, VG E S C A - L A B MkII. Clean sample surfaces of U G e 2 were obtained by fracturing, and those
of UC by filing with a diamond file. The overall instrumental resolution for UPS varied from 0.350.7 eV at photon energies of 40-120 eV, respectively. The overall instrumental resolutions for XPS and BI were 0.8 and 0.7 eV, respectively. Other experimental details are described elsewhere [8-10].
2. Occupied valence state spectra
Figure 1 shows the EDCs of UGe2 measured by excitations at 40 and 1253.6 eV. For both spectra, no correction (including background subtraction) was made, and they are quite similar. In the spectral region observed here, the ionization cross sections of U 5f electrons are much larger than those of other valence electrons in this material. Thus, the spectra illustrated in fig. 1 reflect the 5f partial DOS curve mostly; this is verified by the 5d-5f resonant photoemission. In the UPS spectrum, a broad hump-like feature occurs around 2 eV. The comparison of the UPS spectrum with the o n - o f f difference spectrum of the 5d-Sf resonant photoemission shows that the 2 eV hump cannot be ascribed to the 5f partial states. The XPS spectrum is broader than the UPS spectrum. This is not ascribed merely to the instrumental resolution, which is lower in XPS EDCs. The main peak of the UPS spectrum is located at 0.15 eV, almost at the Fermi edge, whereas the corresponding highest peak is located at 0.75 eV in the XPS spectrum. The width of the peak of the UPS spectrum is about 1 eV,
°. e.
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BINDING ENERGY (eV) Fig. 1. Comparison of the UPS and XPS EDCs of UGe 2. The numbers on the curves represent the excitation energies.
T. lshii / PES and BIS of U compounds whereas the corresponding width of the XPS spectrum is larger by a factor of 2. The origin of this difference is not clear at present. One possibility is that the ionization cross sections of valence electrons other than 5f electrons are relatively larger in XPS emission. The difference of this type is manifest in a weak broad band with its center around 8.5 eV in the XPS spectrum. The corresponding feature in the UPS spectrum appears weak, although a weak oxygen band due to surface contamination occurring around 6 eV obscures the presence of the 8.5 eV band. The partial DOS curve of 5f electrons and that of other valence electrons in UGe 2 are shown in fig. 2. The 5f partial DOS curve was obtained as the resonance o n - o f f difference spectrum drawn up by subtracting the EDC measured by an excitation at 92 eV from that measured by an excitation at 97 eV. The curve is indicated as U 5f in fig. 2. The excitation threshold of the U 5d electron occurs around 95 eV, above which the ionization cross section of the 5f electron is enhanced. At 92 eV, on the contrary, the ionization cross sections of all valence electrons are very small, and are ascribed to the Fano minimum in
UGe2 ." U5f
/
z ~,
23
the 5d-5f resonance excitation where the ionization cross section of the 5f electron almost vanishes. This is found in EDC designated as '92 eV', and in this sense, it represents the partial DOS curve for electrons other than 5f electrons. The ordinate scale of the EDC measured by the excitation at 92 eV has been expanded ten times so that the partial DOS curve of other electrons could be shown in greater detail. The spectrum obtained is shown by the curve designated as '92 eV, × 10' in fig. 2. The intensity of a broad feature around 8.5 eV is discernible in fig. 1 is very weak in the excitation energy region above 70 eV and is not found in the spectrum illustrated in fig. 2. Instead, a feature is found around 6 eV in the spectrum obtained by the 92 eV excitation and expanded 10 times. This feature is brought about by the contamination with oxygen. In the lowest part of the figure, the valence band XPS spectrum of pure Ge obtained by Ley et al. [11] is shown. The XPS spectrum of pure Ge looks quite different from that of UGe2 measured at 92 eV. Pure Ge is a semiconductor and its spectrum does not show a well defined Fermi edge, whereas the sharp Fermi edge is found in the spectrum of UGe 2. Other parts of the spectral profile look totally different. To the author's knowledge, no theoretical calculation of the energy levels of UGe 2 has been reported. The author supposes that the 5f states in UGe2 are in a situation more or less similar to those of UC, for which an energy band calculation has been reported [6]. Therefore, the discussion of the valence state energy levels is made here using UC as a representative example. Figure 3 shows the XPS valence state EDC and the 5f partial DOS curve calculated by
92 eV
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Fig. 2. Top curve: the 5f partial DOS curve of UGe 2 obtained as the resonance on-off difference spectrum, where the on-resonance EDC was obtained by an excitation at 97 eV. Two middle curves, EDCs obtained by excitations at 92 eV, assumed to represent the partial DOS curves of electrons other than 5f electrons. In one of the spectra the ordinate scale is expanded ten times. Bottom curve: the valence band XPS spectrum of pure Ge obtained from ref. [11].
5f I
DOS Jr'-
, I
6 4 2 0 BINDING ENERGY (eV) Fig. 3. XPS valence band EDC of UGe 2 and the 5f partial DOS curve calculated by the self-consistent relativistic APW method [6]. The DOS curve is cut at the Fermi level with the Fermi-Dirac distribution function for 300 K.
24
T. lshii / PES and BIS of U compounds
Hasegawa and Yamagami [6] using the self-consistent relativistic APW method. Since the results of this energy-band calculation explain the results of de Haas-van Alphen experiments carried out by Onuki et al. [7], we are tempted to compare the observed valence state EDC with the DOS curve calculated in this energy band calculation [9]. In fig. 3, the 5f partial DOS curve illustrated was obtained by cutting off the calculated 5f partial DOS curve with the Fermi-Dirac distribution function at 300 K at which the experiments were carried out. At first glance, a fair qualitative agreement between the experimental and theoretical curves is found, but for a more complete agreement, we meet a few difficulties. First, a hump at around 2 eV may not be ascribed to the 5f state. The total DOS curve appears to suggest that a considerable amount of state density exists in this area. Second, the main peak of the experimental curve does not show a composite structure. If we convolute the partial DOS curve shown in fig. 3 with a Gaussian function expressing the instrumental and phonon broadening, we obtain a broadened peak that still has a doublet profile. This a kind of qualitative disagreement. In the calculated DOS curve, a sharp and intense peak exists just above the Fermi level. If the location of the Fermi level moves upward, a peak with an appreciable magnitude occurs below it. In such a case the spectra obtained by convoluting the DOS curves with a broadening function still have sharp peaks near the Fermi edge and no long tailings, as found in the experimentally observed EDC. Again, some disagreement is found. It is not known at present whether a localized electron model based on the impurity Anderson Hamiltonian and the configuration interaction reproduces a single peak at the Fermi edge and a long tailing on the high binding energy side. In the case of rare earth compounds with two or more 4f electrons, such calculations lead to spectra quite different from those found experimentally in U compounds. It appears that the assumption of an unresolved satellite [12] is not realistic in the U compounds measured here, since the UPS spectra show sharp main peaks. Thus, more elaborate investigations are required to clarify the valence state spectra.
3. R e s o n a n c e
processes
Resonant photoemission has been discussed in detail by many authors. Here, only one aspect that is distinctive of U - G e compounds is pointed out. Figure 4 shows the total yield spectrum of UGe~ and the constant initial state (CIS) spectrum measured for the
UGe2
/
/
Z TOTAL YIELD
z
Z © ,..3 © [.., © 1
I
80 100 120 PHOTON ENERGY(eV) Fig. 4. Total yield spectrum of UGe2 and the CIS spectrum for the binding energy of 0.3 eV. intensity at 0.3 eV. Generally, total yield spectra are very similar to absorption spectra. This is obvious, since the most of the emitted electrons are from the 5f states, as mentioned above. The distinctive aspect is that the excitation curves consist of two peaks, at 97 and 112 eV, similar to those found in other U - G e compounds such as U3Ge 4 [8]. In rare earth compounds, the 4d-4f resonance is dominant. The excitation spectra are composed of two parts: one is a giant broad band with high intensity, and the other is a group of very weak lines occurring in the region below the onset of the giant band. The spectral area of the weak lines is often referred to as the prethreshold region. The weak lines originate in the multiplet levels caused by the electrostatic interactions between 4f electrons and a photoproduced 4d hole. Weak lines are originally forbidden owing to the spin selection rule, which is violated by the weak spin-orbit interaction. In U - G e compounds, such weak lines in the prethreshold region are not found, but a neatly shaped band with an intermediate intensity occurs at 97 eV. Yamazaki and Kotani have pointed out the importance of the spin-orbit coupling for explaining the inner-shell excitation spectra of actinide compounds [13]. Using their idea, the spectra shown in fig. 4 can be interpreted as follows. The strong spin-orbit interactions modify the multiplet coupling considerably and spin-forbidden lines are replaced with some allowed lines. Instead, a new line emerges, probably consisting of many unresolved component lines. The cause of the giant band is similar to the case of the 4d-4f resonance in rare earths. A practical calculation of the 5d-5f resonance along this direction has not yet been made.
T. Ishii / PES and B1S of U compounds
4. BIS spectra It is widely believed that a spectrum composed of an XPS and a BIS spectrum connected together at the F e r m i edge represents the D O S curve over a wide energy range. The author's colleagues have obtained such curves for various U compounds [9,10,14]. The results show that the BIS spectra of U compounds look m o r e or less similar, but this was already widely known• H e r e , the example of the spectrum of U C is discussed. Figure 5 shows the XPS and BIS spectra of U C connected together at the Fermi edge. The two spectra are connected in such a manner that the intensities at the Fermi edge are equal. The total D O S curve as well as the 5f partial D O S curve calculated by the self-consistent A P W m e t h o d is shown for comparison. The 5f partial D O S curve is the same as that shown in fig. 3. The BIS spectrum has a strong peak at 1.4 eV and a shoulder-like feature around 0.7eV. The calculated D O S curve suggests that two peaks occur in a region within 2.4 eV from the Fermi edge. If the instrumental resolution is not high enough to resolve the expected two features, the highest peak at 1.4 eV and a shoulder
c-,
-~
UC s. s" i.
z ~
• "
XPS
Z
././"
DOS
1
TOTAL ....
5f P A R T I A L ,4
--8
-4 0 4 8 ELECTRON ENERGY (eV)
Fig. 5. XPS and BIS spectra of UC connected at the Fermi level. Both spectra are connected so that the intensities at the Fermi edge are equal. Total and 5f partial DOS curves calculated by the self-consistent relativistic APW method [6] are also shown.
25
around 0 . 7 e V are possible candidates for the two peaks in the calculated D O S curves. If we ignore the disagreement between the 5f partial D O S curve and the observed XPS spectrum as described in fig. 3, and assume that the E D C and the D O S curve are in fair agreement, the whole spectra could be explained in terms of the energy band picture. What we find about the X P S - B I S spectrum here is that the one-electron picture explains a part of the observed aspect of the spectra• It will therefore be interesting to see whether any refinement of the one-electron calculation along this direction is possible.
References [1] A.J. Arko, D.D. Koelling, B.D. Dunlap, C. Capasso and M. del Giudice, J. Less-Common Metals 48 (1989) 133, and references therein. [2] O. Gunnarsson, D.D. Sarma, F.U. Hillebrecht and K. Sch6nhammer, J. Appl. Phys. 63 (1988) 3676, and references therein. [3] J.M. Imer, D. Malterre, M. Gioni, P. Weibel, B. Bardel and Y. Baer, Phys. Rev. B 44 (1991) 10455. [4] O. Gunnarsson and K. Sch6nhammer, Phys. Rev. B 28 (1983) 4815. [5] J.J. Yeh and I. Lindau, Atom. Data Nucl. Data Tables 32 (1985) 1. [6] A. Hasegawa and H. Yamagami, J. Phys. Soc. Jpn. 59 (1990) 218. [7] Y. Onuki, I. Umehara, Y. Kurosawa, K. Satoh and H. Matsui, J. Phys. Soc. Jpn. 59 (1990) 229. [8] K. Soda, T. Mori, Y. Onuki, T. Komatsubara, S. Suga, A. Kakizaki and T. Ishii, J. Phys. Soc. Jpn. 60 (1991) 3059, and references therein. [9] T. Ejima, K. Murata, S. Suzuki, T. Takahashi, S. Sato, T. Kasuya, Y. Onuki, H. Yamagami, A. Hasegawa and T. Ishii, Physica B 186-188 (1993) 77. [10] S. Suzuki, S. Sato, T. Ejima, K. Murata, Y. Kubo, T. Takahashi, S. Suga, T. Komatsubara, N. Sato, M. Kasaya, Y. Onuki, T. Takabatake, T. Kasuya, K. Soda, T. Mori, A. Kakizaki and T. Ishii, Jpn. J. Appl. Phys. Series 8, to be published. [11] L. Ley, S. Kowalczyk, R. PoUak and D. Shirly, Phys. Rev. Lett. 29 (1972) 1088. [12] A.J. Arko, D.D. Koelling, B.D. Dunlap, A.W. Mitchell, C. Capasso and M. del Giudice, J. Appl. Phys. 63 (1988) 3680. [13] T. Yamazaki and A. Kotani, J. Phys. Soc. Jpn. 60 (1991) 49. [14] T. Ejima, Y. Kudo, S. Suzuki, T. Takahashi, S. Sato, T. Kasuya, T. Takabatake and T. Ishii, Physica B 186-188 (1993) 86.