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Electronic structures of U3P4 and U3As4 studied by synchrotron radiation photoemission spectroscopy
Journal of Magnetism and Magnetic Materials 52 (1985) 297-300 North-Holland, Amsterdam
297
E L E C T R O N I C S T R U C T U R E S O F U 3 P4 A N D U3As 4 S T U D I E D RADIATION PHOTOEMISSION SPECTROSCOPY S. S U G A , M. Y A M A M O T O ,
BY SYNCHROTRON
K. S O D A , T. M O R I
Synchrotron Radiation Laboratory, Institute for Solid State Physics, The University of Tokyo, Tanashi, Tokyo 188, Japan S. T A K A G I ,
N. NIITSUMA,
T. S U Z U K I
a n d T. K A S U Y A
Department of Physics, Tohoku University, Sendai 980, Japan
Photoemission spectra and their resonant behavior at the 5d core threshold were measured with synchrotron radiation. 5f derived features were observed close to the Fermi level as well as at 2.0(1.89) eV for U3P4(U3As4). An additional peak was observed around 5.5 eV for U3As 4. This structure seems to correspond to the p - f bonding states in Ce monopnictides.
Because of their unusual electronic and magnetic properties, the uranium compounds are presently under considerable investigation [1]. We have here employed U3P4 and U3As 4 with the cubic Th3P 4 crystal structure [2] and studied their electronic structures by means of synchrotron radiation photoemission spectroscopy. A unit cell of these crystals contains four molecules, where the stable valence is thought to be U 4+, p3 and As3-. The U atoms occupy equivalent sites with the same local point symmetry(S4). In comparison with the case of Ce monopnictides, the U 5f states are more extended than the Ce 4f states. Therefore, the p - 5 f hybridization between the U 5f orbitals and the p orbitals of P or As is stronger than the p - 4 f hybridization and should be taken into account to understand the electronic properties of U3P4 and U3As 4 [2,3]. We have measured the photoemission spectra in a wide excitation photon energy ( h v ) region between 32 and 140 eV, in order to reveal these hybridization effects near the Fermi level (EF). These results are compared with those of Ce pnictides to see the difference between the 4f and 5f states. The measurements were performed on clean surfaces of single crystals cleaved in situ under an ultrahigh vacuum better than 1 × 10 -1° Torr. The energy scale for the photoemission spectra was calibrated by the Fermi edge of gold with an accuracy better than +0.1 eV. The total resolution of the measurements was set to 0.25 eV at hv = 32 eV and better than 0.8 eV at hv = 120 eV. The spectra were normalized by the photon flux evaluated by the photoelectron yield of freshly 0304-8853/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
evaporated gold. The 5d5/2 a n d 5d3/2 core states of uranium were observed at E B (binding energy from EF) = 94.8 and 106.4 eV, respectively. Fig. 1 shows the ~.. ~° 2
/
UsP4 .... U3As4m
,-°~ (D E_-_
u 5f [ 1~=7o ev /
,/I4
U 6%~=
~l .... tt) tt) -~ o®
.f~
As 4s i
.C
i 20
0. 0
/]
P 5S
i
i
10
0
binding e n e r g y ( e V )
~ 2 ", >, -~ v
AS 5d I
d II ,~
~,
.¢_
-= ;, 1 ¢~
~
"--,j g
"-"-<-zJ\ll D
o I ~ 0 80
I~--120 eV
C
B
Av ~
vt
°6.
s/t
Auger |
I
|
|
I
1
60 40 ' 20 binding e n e r g y ( e V )
I
It
0
Fig. 1. Photoemission spectra of U3P4 and U3As 4 measured at room temperature on clean surfaces cleaved in ultrahigh vacuum.
298
S. Suga et al. /Resonant photoemission in U~P4 and U~As4
photoemission spectra of U 3P4 and U3As 4 measured on clean surfaces at hv = 70 a n d 120 eV. With the increase of E B, we have observed structures ascribable to U 5f, P 3s(As 4s), U 6P3/2, U 6Pa/2, (As 3d) a n d Auger structures. N o clear multiplet c o m p o n e n t is resolved for these structures [4] in U3P4 and U3As 4. The structures A, B, C, D are observed in b o t h materials at c o n s t a n t kinetic energies ( E k ) of 69.0, 59.3, 51.1 and 41.3 eV against the change of hu, a n d disappear below h~, = 92 eV. Consequently, these structures are assigned to the Auger electron emission associated with the U 5d core hole states. Judging from their energy positions, we ascribe the structures A, B, C, D to 5d5/2 5f 6P3/2, 5d5/2 5f 6pl/2, 5d5/2 6p3/2 6p3/2 and 5d5/2 6p3/2 6p1/2 Auger transitions. C o r r e s p o n d i n g structures associated with the 5d3/2 core hole states were not resolved in the present spectra. The upper panel of fig. 1 shows the detailed spectra in the low E B region. The effect of oxidization was checked on U3P4, where we found a strong peak a r o u n d E~ = 6 eV. T h e intensity of this peak is found to increase with the decrease of h u, which is a characteristic feature of the O 2p orbitals. In the case of clean U3P4 surfaces, no structure was observed in this region at any h v, which guaranteed the intrinsic nature of the observed spectra. As for U3As 4, some structures were observed in this region as shown in fig. 1. The intensity of the peak a r o u n d 5.5 eV, however, did not increase so m u c h in the low hu region in contrast to the case of the a d s o r b e d oxygen. Thus we discard the oxidization effect for the interpretation of this peak a n d rather assign this structure to the p - f hybridized states. Fig. 2 shows the typical spectra of U3P4 measured in the low E a region at various p h o t o n energies. One notices an oscillatory b e h a v i o r of the p h o t o e m i s s i o n intensity of the p r o m i n e n t peak at the lowest E B as h~, crosses the threshold energies of the U 5d ---, 5f excitation. The p h o t o n energy d e p e n d e n c e of the photoemission spectra in U3As 4 is found to be very similar to these results. Fig. 3 shows the hv d e p e n d e n c e of the photoemission intensity of U3As 4 at the c o n s t a n t binding energies of E B = 0.53 and 1.89 eV, c o r r e s p o n d i n g to the structures of U3P4 at E B = 0.4 a n d 2 eV in fig. 2. T h e c o n s t a n t initial state spectra (CIS) thus o b t a i n e d show a b r o a d m a x i m u m a r o u n d 40 eV followed by the gradual decrease toward the 5d threshold a r o u n d 91 eV. C o r r e s p o n d i n g m a x i m u m of the photoemission cross section is generally observed for the 4f states in various rare earths a n d in their c o m p o u n d s [4]. Passing through the m i n i m u m at h~,= 91 eV, the CIS spectra show a s h a r p peak with a m a x i m u m at hv = 97.5 eV. T h e n the CIS spectra show the second m i n i m u m at hv = 101 eV
U3P4
-
hO
101 eV
0 if)
~,
•
~o--~ 0
91 eV
o o¢ Q.
:52eV(x3) l
I
8
I
I
I
i
6 4 2 binding energy (eV)
I
0
Fig. 2. Photon energy dependence of the photoemission spectra o f U 3 P 4.
which is not so deep as the first dip. The third maxim u m of the CIS spectra shows a rather b r o a d and strong peak. Judging from the vanishing intensity of the low energy structure at E B = 0.5 eV for the excitation a r o u n d hu = 91 eV, we conclude that the U 5f states are prevailing in the region of E a = 0 . 5 eV. As for the
eJ ~2 ¢09 -
~
~
EB=1.89eV . . . .
(t) eq
r=
2 0¢ -
"0
40
60
80 100 photon e n e r g y (eV)
120
Fig. 3. Photon energy dependence of the photoemission intensity of the two structures of U3As 4 measured at E a = 0.53 and 1.89 eV (so called CIS spectra),
299
S. Suga et al. / Resonant photoemission in U~P4 and U~As4
structure at E B = 1.9 eV, the resonant behavior in the U 5d core excitation region and the broad CIS maximum around 40 eV suggests a strong contribution from the U 5f states. However, its intensity remains non zero even at hu = 91 eV. This result suggests that As 4p component is also considerably contributing to this structure. The contribution of the U 5f states can be evaluated from the difference spectrum. The difference spectrum between the two spectra measured at hu = 91 and 95 eV has demonstrated that U 5f states are rather concentrated between E F and E B --- 3 eV. The CIS spectra in the core excitation region are known to resemble the core absorption spectra. The one body excitation threshold energy from the U 5d5/2 and 5d3/2 core levels to the Fermi level is roughly evaluated as 94.8 and 106.4 eV from the 5d photoemission spectra, if one neglects the Coulomb energies between the core hole and excited electron. The latter threshold energy of 106.4 eV is in agreement with the peak energy of the CIS. On the other hand, the former energy (94.8 eV) is slightly lower than the energy of the maximum (97.5 eV) of the CIS structure, which is, however, well understood by considering the Fano line shape. These results elucidate that the 5d core excitations take place to the empty 5f states just above E v. In order to discuss the U 5f states in more detail, we have measured t h e photoemission spectra in low E B region with higher resolution. Fig. 4 shows the results of U3 P4. The broad structure around 2 eV is already interpreted as due to the P 3p states hybridized with U 5f states. In the spectra measured at h~, = 36 and 38 eV, a sharp peak is observed at E B = 0.3-0.4 eV. The low E B shoulder around 0.1 eV is due to the E v cut off broadened by the instrumental resolution. In the case of the spectra measured at h~, = 95 and 101 eV, a clear peak is observed at E B = 0.71 eV. It seems that the peak observed at low photon energies reflects rather the 6d conduction band, which is expected to begin from 1 eV below the Fermi energy. The results in U3As 4 are quite similar to these results. We consider that the 5f character is also mixing with this 6d band even though it is difficult to separate the p band contribution at E v. It should be noted that the 6d states also show the resonant behavior similar to the 5f states. Anyway, in a strong contrast to the case of insulating U O 2 [5,6], we observed high density of states with 5f character at E v. N o w we discuss these results in comparison with Ce monopnictides. Because of the stronger p f, and d - f mixing effects in U3P4 and U3As 4, these compounds are thought to be in the valence fluctuation regime such as in CeN and a-Ce. Even in such a case, we expect a p - f bonding peak at or beyond the valence band edge. A
peak around 5.5 eV observed in U3As 4 may correspond to this peak. The depression of the corresponding peak in U3P4 is due to the stronger mixing matrix(mostly d - f mixing) effect in U 3P4 similar to the case of Ce monopnictides series, which reduces the life time of this bonding orbit due to the Auger decay processes. In the present case, we expect a real bound state well separated from the bottom of the valence band because of the strong p - f mixing effect. From the band calculation [2], the valence bands in Th3P 4 are expected to have a width of about 5 eV, with a narrow indirect gap from the conduction band minimum. In U 3 P4, the top of the valence bands is shifted upwards by more than 1.5 eV due to the p - f mixing effect and overlaps with the conduction band. Therefore, the 6d conduction band is expected to start from the energy about 1 eV below E F. The 5f photoemission peak close to E v is thought to be partly mixed with this 6d band, although it is mostly due to the strong valence fluctuation effect similar to but stronger than a-Ce. Finally, we discuss the Auger structures in more detail. One can evaluate the effective hole-hole correlation energies from the difference between the observed E k and the calculated kinetic energies E~ (algebraic sum of one body energies estimated from the photoemission spectra). These correlation energies are evaluated as 2.7, 3.8, 3.5 and 4.7 eV for the structures A, B, C and D. One can also observe another Auger structure 5d5/2 5f 5f at E k = 87.7 eV in the spectra
U3P4
- - 5 o l ,v
o
~95
eV
~0
j
-
-
E 0 o IE
36 eV
0 4
3 2 1 binding energy (eV)
0
Fig. 4. High resolution photoemission spectra of U3P4 near the Fermi level.
300
S. Suga et al. / Resonant photoemission in U~P4 and U3As 4
measured at hp = 115 and 110 eV(not shown here). The effective correlation energy U(5f, 5f) is evaluated as small as 1.2 eV in this case. The small magnitude of this energy a n d the coincidence of the one body threshold energy with the energy of the 5d core a b s o r p t i o n structures is consistent with the itinerant character of the U 5f states in semimetallic U3P4 and U3As 4. However, the latter fact m a y be also explained by a fairly strongly correlated 5f electron model as follows. The final states for the 5d core UPS, corresponding to the observed E B ( U 5d), may be a p p r o x i m a t e d as 5d 5f 3 due to a screening in the case of strong correlation. On the other hand, the intermediate state for the resonant photoemission is given by 5d 5f 3, which is responsible for the threshold of the CIS spectra. Thus the coincidence of these two threshold energies can be interpreted in a correlated model. Therefore, further studies are required to fully u n d e r s t a n d the hybridization a n d correlation effects in these c o m p o u n d s .
References [1] A.J. Arko, C.G. Olson, D.M. Wieliczka, Z. Fisk and J i . Smith, Phys. Rev. Lett. 53 (1984) 2050. [2] T. Suzuki, S. Takagi, N. Niitsuma, K. Takegahara, 1. Kasuya, A. Yanase, T. Sakakibara, M. Date, P.J. Markowski and Z. Henkie, High Field Magnetism, ed. M. Date (North-Holland, Amsterdam, 1983) p. 183. [3] J. Schoenes, M. Kung, R. Hauert and Z. Henkie, Solid State Commun. 47 (1983) 23. [4] S. Suga, M. Taniguchi, M. Seki, H. Sakamoto, H. Kanzaki, M. Yamamoto, A. Kurita, Y. Kaneko and T. Koda, Solid State Commun. 49 (1984) 1005. [5] Y. Baer and J. Schoenes, Solid State Commun. 33 (1980) 885. [6] B. Reilh, N. M~trtensson, D.E. Eastman, A.J. Arko and O. Vogt, Phys. Rev. B26 (1982) 1842.
297
E L E C T R O N I C S T R U C T U R E S O F U 3 P4 A N D U3As 4 S T U D I E D RADIATION PHOTOEMISSION SPECTROSCOPY S. S U G A , M. Y A M A M O T O ,
BY SYNCHROTRON
K. S O D A , T. M O R I
Synchrotron Radiation Laboratory, Institute for Solid State Physics, The University of Tokyo, Tanashi, Tokyo 188, Japan S. T A K A G I ,
N. NIITSUMA,
T. S U Z U K I
a n d T. K A S U Y A
Department of Physics, Tohoku University, Sendai 980, Japan
Photoemission spectra and their resonant behavior at the 5d core threshold were measured with synchrotron radiation. 5f derived features were observed close to the Fermi level as well as at 2.0(1.89) eV for U3P4(U3As4). An additional peak was observed around 5.5 eV for U3As 4. This structure seems to correspond to the p - f bonding states in Ce monopnictides.
Because of their unusual electronic and magnetic properties, the uranium compounds are presently under considerable investigation [1]. We have here employed U3P4 and U3As 4 with the cubic Th3P 4 crystal structure [2] and studied their electronic structures by means of synchrotron radiation photoemission spectroscopy. A unit cell of these crystals contains four molecules, where the stable valence is thought to be U 4+, p3 and As3-. The U atoms occupy equivalent sites with the same local point symmetry(S4). In comparison with the case of Ce monopnictides, the U 5f states are more extended than the Ce 4f states. Therefore, the p - 5 f hybridization between the U 5f orbitals and the p orbitals of P or As is stronger than the p - 4 f hybridization and should be taken into account to understand the electronic properties of U3P4 and U3As 4 [2,3]. We have measured the photoemission spectra in a wide excitation photon energy ( h v ) region between 32 and 140 eV, in order to reveal these hybridization effects near the Fermi level (EF). These results are compared with those of Ce pnictides to see the difference between the 4f and 5f states. The measurements were performed on clean surfaces of single crystals cleaved in situ under an ultrahigh vacuum better than 1 × 10 -1° Torr. The energy scale for the photoemission spectra was calibrated by the Fermi edge of gold with an accuracy better than +0.1 eV. The total resolution of the measurements was set to 0.25 eV at hv = 32 eV and better than 0.8 eV at hv = 120 eV. The spectra were normalized by the photon flux evaluated by the photoelectron yield of freshly 0304-8853/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
evaporated gold. The 5d5/2 a n d 5d3/2 core states of uranium were observed at E B (binding energy from EF) = 94.8 and 106.4 eV, respectively. Fig. 1 shows the ~.. ~° 2
/
UsP4 .... U3As4m
,-°~ (D E_-_
u 5f [ 1~=7o ev /
,/I4
U 6%~=
~l .... tt) tt) -~ o®
.f~
As 4s i
.C
i 20
0. 0
/]
P 5S
i
i
10
0
binding e n e r g y ( e V )
~ 2 ", >, -~ v
AS 5d I
d II ,~
~,
.¢_
-= ;, 1 ¢~
~
"--,j g
"-"-<-zJ\ll D
o I ~ 0 80
I~--120 eV
C
B
Av ~
vt
°6.
s/t
Auger |
I
|
|
I
1
60 40 ' 20 binding e n e r g y ( e V )
I
It
0
Fig. 1. Photoemission spectra of U3P4 and U3As 4 measured at room temperature on clean surfaces cleaved in ultrahigh vacuum.
298
S. Suga et al. /Resonant photoemission in U~P4 and U~As4
photoemission spectra of U 3P4 and U3As 4 measured on clean surfaces at hv = 70 a n d 120 eV. With the increase of E B, we have observed structures ascribable to U 5f, P 3s(As 4s), U 6P3/2, U 6Pa/2, (As 3d) a n d Auger structures. N o clear multiplet c o m p o n e n t is resolved for these structures [4] in U3P4 and U3As 4. The structures A, B, C, D are observed in b o t h materials at c o n s t a n t kinetic energies ( E k ) of 69.0, 59.3, 51.1 and 41.3 eV against the change of hu, a n d disappear below h~, = 92 eV. Consequently, these structures are assigned to the Auger electron emission associated with the U 5d core hole states. Judging from their energy positions, we ascribe the structures A, B, C, D to 5d5/2 5f 6P3/2, 5d5/2 5f 6pl/2, 5d5/2 6p3/2 6p3/2 and 5d5/2 6p3/2 6p1/2 Auger transitions. C o r r e s p o n d i n g structures associated with the 5d3/2 core hole states were not resolved in the present spectra. The upper panel of fig. 1 shows the detailed spectra in the low E B region. The effect of oxidization was checked on U3P4, where we found a strong peak a r o u n d E~ = 6 eV. T h e intensity of this peak is found to increase with the decrease of h u, which is a characteristic feature of the O 2p orbitals. In the case of clean U3P4 surfaces, no structure was observed in this region at any h v, which guaranteed the intrinsic nature of the observed spectra. As for U3As 4, some structures were observed in this region as shown in fig. 1. The intensity of the peak a r o u n d 5.5 eV, however, did not increase so m u c h in the low hu region in contrast to the case of the a d s o r b e d oxygen. Thus we discard the oxidization effect for the interpretation of this peak a n d rather assign this structure to the p - f hybridized states. Fig. 2 shows the typical spectra of U3P4 measured in the low E a region at various p h o t o n energies. One notices an oscillatory b e h a v i o r of the p h o t o e m i s s i o n intensity of the p r o m i n e n t peak at the lowest E B as h~, crosses the threshold energies of the U 5d ---, 5f excitation. The p h o t o n energy d e p e n d e n c e of the photoemission spectra in U3As 4 is found to be very similar to these results. Fig. 3 shows the hv d e p e n d e n c e of the photoemission intensity of U3As 4 at the c o n s t a n t binding energies of E B = 0.53 and 1.89 eV, c o r r e s p o n d i n g to the structures of U3P4 at E B = 0.4 a n d 2 eV in fig. 2. T h e c o n s t a n t initial state spectra (CIS) thus o b t a i n e d show a b r o a d m a x i m u m a r o u n d 40 eV followed by the gradual decrease toward the 5d threshold a r o u n d 91 eV. C o r r e s p o n d i n g m a x i m u m of the photoemission cross section is generally observed for the 4f states in various rare earths a n d in their c o m p o u n d s [4]. Passing through the m i n i m u m at h~,= 91 eV, the CIS spectra show a s h a r p peak with a m a x i m u m at hv = 97.5 eV. T h e n the CIS spectra show the second m i n i m u m at hv = 101 eV
U3P4
-
hO
101 eV
0 if)
~,
•
~o--~ 0
91 eV
o o¢ Q.
:52eV(x3) l
I
8
I
I
I
i
6 4 2 binding energy (eV)
I
0
Fig. 2. Photon energy dependence of the photoemission spectra o f U 3 P 4.
which is not so deep as the first dip. The third maxim u m of the CIS spectra shows a rather b r o a d and strong peak. Judging from the vanishing intensity of the low energy structure at E B = 0.5 eV for the excitation a r o u n d hu = 91 eV, we conclude that the U 5f states are prevailing in the region of E a = 0 . 5 eV. As for the
eJ ~2 ¢09 -
~
~
EB=1.89eV . . . .
(t) eq
r=
2 0¢ -
"0
40
60
80 100 photon e n e r g y (eV)
120
Fig. 3. Photon energy dependence of the photoemission intensity of the two structures of U3As 4 measured at E a = 0.53 and 1.89 eV (so called CIS spectra),
299
S. Suga et al. / Resonant photoemission in U~P4 and U~As4
structure at E B = 1.9 eV, the resonant behavior in the U 5d core excitation region and the broad CIS maximum around 40 eV suggests a strong contribution from the U 5f states. However, its intensity remains non zero even at hu = 91 eV. This result suggests that As 4p component is also considerably contributing to this structure. The contribution of the U 5f states can be evaluated from the difference spectrum. The difference spectrum between the two spectra measured at hu = 91 and 95 eV has demonstrated that U 5f states are rather concentrated between E F and E B --- 3 eV. The CIS spectra in the core excitation region are known to resemble the core absorption spectra. The one body excitation threshold energy from the U 5d5/2 and 5d3/2 core levels to the Fermi level is roughly evaluated as 94.8 and 106.4 eV from the 5d photoemission spectra, if one neglects the Coulomb energies between the core hole and excited electron. The latter threshold energy of 106.4 eV is in agreement with the peak energy of the CIS. On the other hand, the former energy (94.8 eV) is slightly lower than the energy of the maximum (97.5 eV) of the CIS structure, which is, however, well understood by considering the Fano line shape. These results elucidate that the 5d core excitations take place to the empty 5f states just above E v. In order to discuss the U 5f states in more detail, we have measured t h e photoemission spectra in low E B region with higher resolution. Fig. 4 shows the results of U3 P4. The broad structure around 2 eV is already interpreted as due to the P 3p states hybridized with U 5f states. In the spectra measured at h~, = 36 and 38 eV, a sharp peak is observed at E B = 0.3-0.4 eV. The low E B shoulder around 0.1 eV is due to the E v cut off broadened by the instrumental resolution. In the case of the spectra measured at h~, = 95 and 101 eV, a clear peak is observed at E B = 0.71 eV. It seems that the peak observed at low photon energies reflects rather the 6d conduction band, which is expected to begin from 1 eV below the Fermi energy. The results in U3As 4 are quite similar to these results. We consider that the 5f character is also mixing with this 6d band even though it is difficult to separate the p band contribution at E v. It should be noted that the 6d states also show the resonant behavior similar to the 5f states. Anyway, in a strong contrast to the case of insulating U O 2 [5,6], we observed high density of states with 5f character at E v. N o w we discuss these results in comparison with Ce monopnictides. Because of the stronger p f, and d - f mixing effects in U3P4 and U3As 4, these compounds are thought to be in the valence fluctuation regime such as in CeN and a-Ce. Even in such a case, we expect a p - f bonding peak at or beyond the valence band edge. A
peak around 5.5 eV observed in U3As 4 may correspond to this peak. The depression of the corresponding peak in U3P4 is due to the stronger mixing matrix(mostly d - f mixing) effect in U 3P4 similar to the case of Ce monopnictides series, which reduces the life time of this bonding orbit due to the Auger decay processes. In the present case, we expect a real bound state well separated from the bottom of the valence band because of the strong p - f mixing effect. From the band calculation [2], the valence bands in Th3P 4 are expected to have a width of about 5 eV, with a narrow indirect gap from the conduction band minimum. In U 3 P4, the top of the valence bands is shifted upwards by more than 1.5 eV due to the p - f mixing effect and overlaps with the conduction band. Therefore, the 6d conduction band is expected to start from the energy about 1 eV below E F. The 5f photoemission peak close to E v is thought to be partly mixed with this 6d band, although it is mostly due to the strong valence fluctuation effect similar to but stronger than a-Ce. Finally, we discuss the Auger structures in more detail. One can evaluate the effective hole-hole correlation energies from the difference between the observed E k and the calculated kinetic energies E~ (algebraic sum of one body energies estimated from the photoemission spectra). These correlation energies are evaluated as 2.7, 3.8, 3.5 and 4.7 eV for the structures A, B, C and D. One can also observe another Auger structure 5d5/2 5f 5f at E k = 87.7 eV in the spectra
U3P4
- - 5 o l ,v
o
~95
eV
~0
j
-
-
E 0 o IE
36 eV
0 4
3 2 1 binding energy (eV)
0
Fig. 4. High resolution photoemission spectra of U3P4 near the Fermi level.
300
S. Suga et al. / Resonant photoemission in U~P4 and U3As 4
measured at hp = 115 and 110 eV(not shown here). The effective correlation energy U(5f, 5f) is evaluated as small as 1.2 eV in this case. The small magnitude of this energy a n d the coincidence of the one body threshold energy with the energy of the 5d core a b s o r p t i o n structures is consistent with the itinerant character of the U 5f states in semimetallic U3P4 and U3As 4. However, the latter fact m a y be also explained by a fairly strongly correlated 5f electron model as follows. The final states for the 5d core UPS, corresponding to the observed E B ( U 5d), may be a p p r o x i m a t e d as 5d 5f 3 due to a screening in the case of strong correlation. On the other hand, the intermediate state for the resonant photoemission is given by 5d 5f 3, which is responsible for the threshold of the CIS spectra. Thus the coincidence of these two threshold energies can be interpreted in a correlated model. Therefore, further studies are required to fully u n d e r s t a n d the hybridization a n d correlation effects in these c o m p o u n d s .
References [1] A.J. Arko, C.G. Olson, D.M. Wieliczka, Z. Fisk and J i . Smith, Phys. Rev. Lett. 53 (1984) 2050. [2] T. Suzuki, S. Takagi, N. Niitsuma, K. Takegahara, 1. Kasuya, A. Yanase, T. Sakakibara, M. Date, P.J. Markowski and Z. Henkie, High Field Magnetism, ed. M. Date (North-Holland, Amsterdam, 1983) p. 183. [3] J. Schoenes, M. Kung, R. Hauert and Z. Henkie, Solid State Commun. 47 (1983) 23. [4] S. Suga, M. Taniguchi, M. Seki, H. Sakamoto, H. Kanzaki, M. Yamamoto, A. Kurita, Y. Kaneko and T. Koda, Solid State Commun. 49 (1984) 1005. [5] Y. Baer and J. Schoenes, Solid State Commun. 33 (1980) 885. [6] B. Reilh, N. M~trtensson, D.E. Eastman, A.J. Arko and O. Vogt, Phys. Rev. B26 (1982) 1842.