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Electronic structure of doped C60: strong correlation or lattice distortion?
PHYSICA
Physica B 186-188 (1993) 1068-1070 North-Holland
Electronic structure of doped distortion?
C60" strong
correlation or lattice
T. Takahashi a, T. Morikawa a, H. Katayama-Yoshida a, S. Hasegawa b, H. Inokuchi b, K. Seki c, S. Hino d, K. Kikuchi e, S. Suzuki e, K. I k e m o t o e a n d Y. A c h i b a e "Department of Physics, Tohoku University, Sendai 980, Japan blnstitute for Molecular Science, Okazaki 444, Japan CDepartment of Image Science, Chiba University, Chiba 260, Japan ODepartment of Chemistry, Nagoya University, Nagoya 464, Japan eDepartment of Chemistry, Tokyo Metropolitan University, Tokyo 192-03, Japan It was found by photoemission and inverse photoemission that an energy gap with a finite density of states at the bottom (a pseudo-gap) opens at the Fermi level for alkali-doped solid C60 at the composition A3C60 (A = alkali metal). Strong electron correlation and lattice distortions (Jahn-Teller effect) are discussed as possible origins for the pseudo-gap.
1. Introduction The discovery of superconductivity at a relatively high transition temperature in alkali-doped solid C60 [1] has attracted much attention to the electronic structure of the host material (C6o) and its changes with alkali doping. In this paper we report a combined spectroscopic study with photoemission and inverse photoemission spectroscopies on alkali-doped solid C6o. A combination of these techniques provides detailed information on the electronic structure near the Fermi level, especially on changes with alkali doping, which causes a transfer of electronic states from unoccupied to occupied states beyond the Fermi level.
assuming a uniform distribution of alkali atoms, it reversibly decreased with further alkali deposition. This is in good agreement with a previous report [2] suggesting that alkali atoms rapidly diffuse inside a C6o film and reach thermal equilibrium. We also annealed the film at 150-200°C for 1 h and measured its photoemission spectrum, but found little change except for a slight reduction in the LUMO (lowest unoccupied molecular orbital) band probably due to re-sublimation of alkali atoms into the vacuum. Photoemission and inverse photoemission spectra were recorded at room temperature at an energy resolution of 0.15 and 0.35 eV, respectively. The Fermi level was referred to that of a gold film deposited on the sample with an accuracy within 20 meV.
2. Experimental 3. Results and discussion An alkali-doped C6o thin film was prepared for measurement in situ by vapor deposition of a certain amount of alkali atoms on a vapor-deposited C6o film. In order to characterize the film thus prepared, we monitored in situ the electrical conductivity of a monitor film simultaneously prepared on a quartz substrate placed next to the photoemission sample. The conductivity gradually increased with alkali deposition, and after reaching a maximum at the composition around A3C6o estimated with a thickness monitor
Correspondence to: T. Takahashi, Department of Physics, Tohoku University, Sendai 980, Japan.
Figure 1 shows photoemission spectra of KxC60 annealed at 150-200°C for 1 h, together with that of KC 8 (graphite intercalation compound) for comparison. The composition of KxC60 was estimated from the relative intensity of the LUMO band appearing with alkali doping near the Fermi level, since it is known that doping of K stops at the composition of K6C60 [3]. As shown in fig. 1, the K-doping produces a broad structure from the Fermi level to the HOMO (highest occupied molecular orbital) band at about 2.5 eV. This new band appearing with K-doping is expected to have
0921-4526/93/$06.00 (~ 1993 - Elsevier Science Publishers B.V. All rights reserved
T. Takahashi et al. I Electronic structure o f doped
,,,~
C6o: strong correlation or lattice distortion? Photoemission
K~Cso
I
",
y c
,.j..
;
'\
i
"..' v
4
_-...
\ M,,-" \
/-",,___
-
,
,u ! ' A / T - "
L"
h
'v"
~/
Inverse Photoemission
KCa
._ ; , f ~ . ~ . ~ . . , ~
.r'\
1069
6.0
3.1 - -
0.5 Binding
k._____ ~
0.0
energy
(eV)
Fig. 2. Comparison of photoemission and inverse photoemission spectra of C6o and K3C6o.
3 2 I EF B i n d i n g energy (¢V)
Fig. 1. Photoemission spectra of a KxC60film (x = 0.0-6.0). The film was annealed at 150-200°C for 1 h after vapor deposition of a certain amount of K atoms. The composition of film (x) was estimated with the relative intensity of the LUMO band appearing near the Fermi level with K doping. A photoemission spectrum of KC 8 (graphite intercalation compound) measured with almost the same photon energy and the energy resolution [4] is also shown for comparison. its origin in the L U M O band of a C60 molecule, since, as shown later (see fig. 2), the intensity of the corresponding unoccupied L U M O band decreases with doping. It is understood that the electrons are donated from K atoms to C60 molecules and occupy the L U M O band. However, it is noted that the photoemission spectrum at the composition around K3C6o has no sharp Fermi-edge structure as is usually observed in normal metals. This is obviously inconsistent with a simple rigid band model which predicts that the maximum of the density of states should come to the Fermi level at K3C60 since the L U M O band is threefold degenerate. We find that this absence of a sharp Fermi-edge structure is not due to the energy resolution when we compare the photoemission spectrum with that of graphite intercalation compound, KC8 (insert in fig. 1) which was measured with almost the same energy resolution [4]. This result suggests that there is an energy gap at the Fermi level for K3C60. The change in electronic structure near the Fermi level by alkali doping is clearly observed when we combine photoemission and inverse photoemission, which probe the occupied and unoccupied electronic states, respectively. Figure 2 shows a comparison of photoemission and inverse photoemission spectra of C6o and K3C6o (unannealed) [5]. As shown in fig. 2,
both the H O M O and L U M O bands approach the Fermi level with K-doping by about 0.3 eV and at the same time a new band, as described above, emerges a little away from the Fermi level. It is understood that this new band has its origin in the L U M O band since the intensity of the unoccupied L U M O band in the inverse photoemission spectrum decreases with doping. However, it is noted that the occupied and unoccupied parts of the L U M O band are well separated across the Fermi level by an energy gap with a finite density of states at the bottom (a pseudo-gap). Similar photoemission behavior has been observed for Rbdoped C6o [6]. This means that the alkali-doping does not bring about a rigid filling of the L U M O band with donated electrons, but causes splitting of the LUMO band across the Fermi level. Two possible origins of the pseudo-gap can be considered: strong electron correlation and lattice distortion. It is generally accepted that the electron correlation of C 2p electrons is small compared with that of d-electrons. However, when considering the very narrow feature of the bands of solid C6o (less than 1 eV) due to its strong molecular nature, the electron correlation of C 2p electrons becomes comparable with or larger than the band width. Lof et al. [7] estimated the on-site molecular Coulomb interaction to be about 1.6 eV from the Auger measurement. They concluded that doped C60 should be regarded as a highly correlated system and proposed that stoichiometric K3C6ois a M o r t - H u b b a r d insulator with an correlation gap of about 0.7 eV. The size of the gap predicted by Lof et al. seems similar to the present observation (fig. 2), although we observed a finite density of states at the Fermi level, contrary to their prediction. Another possible origin of the pseudo-gap is local
1070
T. Takahashi et al. / Electronic structure of doped Ceo: strong correlation or lattice distortion?
lattice distortion. It is likely that doped electrons distort the high symmetry of local structure, giving rise to release of the degeneracy of the LUMO band. A strong Jahn-Teller effect observed in a negatively charged C6o molecule [8] favors the above inference. A reported large value of a (1.4) in the isotope effect [9] suggests a strong interaction between lattices and electrons in doped C60.
4. Conclusions
We have performed photoemission and inverse photoemission spectroscopy studies of alkali-doped solid C6,, to study the electronic structure near the Fermi level. We found that alkali doping does not necessarily lead to a rigid filling of the LUMO band by the donated electrons from alkali atoms, but causes a transfer of electronic states from the LUMO band to a new band produced in the gap of the host material (C60). Comparison of photoemission and inverse photoemission spectra at the superconducting composition of A3C6o (A = alkali metal) indicates that the LUMO band splits into two sub-bands across the Fermi Level with alkali doping, producing an energy gap with a finite density of states at the bottom (a pseudo-gap) at the Fermi level. A relatively strong electron correlation of C 2p electrons and a lattice distortion through the Jahn-Teller effect should be considered as possible origins for the pseudo-gap.
References
[1] A.F. Hebard, M.J. Rosseinsky, R.C. Haddon, D.W. Murphy, S.H. Glarum, T.T.M. Palstra, A.P. Ramirez and A.R. Kortan, Nature 350 (1991) 600. [2] R.C. Haddon, A.F. Hebard, M.J. Rosseinsky, D.W. Murphy, S.J. Duclos, K.B. Lyons, B. Miller, J.M. Rosamilia, R.M. Fleming, A.R. Kortan, S.H. Glarum, A.V. Makhija, A.J. Muller, R.H. Eick, S.M. Zahurak, R. Tycko, G. Dabbagh and F.A. Thiel, Nature 350 (1991) 320. [3] R.M. Fleming, M.J. Rosseinsky, A.P. Ramirez, D.W. Murphy, J.C. Tully, R.C. Haddon, T. Siegrist, R. Tycko. S.H. Glarum, P. Marsh, G. Dabbagh, S.M. Zahurak, A.V. Makhija and C. Hampton, Nature 352 (1991) 701. [4] P. Oelhafen, P. Pfuger, E. Hauser and H.-J. Guntherodt, Phys. Rev. Lett. 44 (1980) 197. [5] T. Takahashi, S. Suzuki, T. Morikawa, H. KatayamaYoshida, S. Hasegawa, H. Inokuchi, K. Seki, K. Kikuchi, S. Suzuki, K. Ikemoto and Y. Achiba, Phys. Rev. Lett. 68 (1992) 1232. [6] T. Takahashi, T. Morikawa, S. Hasegawa, K. Kamiya, H. Fujimoto, S. Hino, K. Seki, H. Katayama-Yoshida, H. Inokuchi, K. Kikuchi, S. Suzuki, K. Ikemoto and Y. Achiba, Physica C 190 (1991) 205. [7] R.W. Lof, M.A. van Veenendaal, B. Koopmans, H.T. Jonkman, and G.A. Sawatzky, Phys. Rev. Lett. 68 (1992) 3924. [8] T. Kato, T. Kodama, M. Oyama, S. Okazaki, T. Shida, T. Nakagawa, Y. Matsui, S. Susuki, H. Shiromaru, K. Yamauchi and Y. Achiba, Chem. Phys. Lett. 186 (1991) 35. [9] T.W. Ebbesen, J.S. Tsai, K. Tanigaki, J. Tabuchi, Y. Shimakawa, Y. Kubo, I. Hirosawa and J. Mizuki, Nature 355 (1992) 620.
Physica B 186-188 (1993) 1068-1070 North-Holland
Electronic structure of doped distortion?
C60" strong
correlation or lattice
T. Takahashi a, T. Morikawa a, H. Katayama-Yoshida a, S. Hasegawa b, H. Inokuchi b, K. Seki c, S. Hino d, K. Kikuchi e, S. Suzuki e, K. I k e m o t o e a n d Y. A c h i b a e "Department of Physics, Tohoku University, Sendai 980, Japan blnstitute for Molecular Science, Okazaki 444, Japan CDepartment of Image Science, Chiba University, Chiba 260, Japan ODepartment of Chemistry, Nagoya University, Nagoya 464, Japan eDepartment of Chemistry, Tokyo Metropolitan University, Tokyo 192-03, Japan It was found by photoemission and inverse photoemission that an energy gap with a finite density of states at the bottom (a pseudo-gap) opens at the Fermi level for alkali-doped solid C60 at the composition A3C60 (A = alkali metal). Strong electron correlation and lattice distortions (Jahn-Teller effect) are discussed as possible origins for the pseudo-gap.
1. Introduction The discovery of superconductivity at a relatively high transition temperature in alkali-doped solid C60 [1] has attracted much attention to the electronic structure of the host material (C6o) and its changes with alkali doping. In this paper we report a combined spectroscopic study with photoemission and inverse photoemission spectroscopies on alkali-doped solid C6o. A combination of these techniques provides detailed information on the electronic structure near the Fermi level, especially on changes with alkali doping, which causes a transfer of electronic states from unoccupied to occupied states beyond the Fermi level.
assuming a uniform distribution of alkali atoms, it reversibly decreased with further alkali deposition. This is in good agreement with a previous report [2] suggesting that alkali atoms rapidly diffuse inside a C6o film and reach thermal equilibrium. We also annealed the film at 150-200°C for 1 h and measured its photoemission spectrum, but found little change except for a slight reduction in the LUMO (lowest unoccupied molecular orbital) band probably due to re-sublimation of alkali atoms into the vacuum. Photoemission and inverse photoemission spectra were recorded at room temperature at an energy resolution of 0.15 and 0.35 eV, respectively. The Fermi level was referred to that of a gold film deposited on the sample with an accuracy within 20 meV.
2. Experimental 3. Results and discussion An alkali-doped C6o thin film was prepared for measurement in situ by vapor deposition of a certain amount of alkali atoms on a vapor-deposited C6o film. In order to characterize the film thus prepared, we monitored in situ the electrical conductivity of a monitor film simultaneously prepared on a quartz substrate placed next to the photoemission sample. The conductivity gradually increased with alkali deposition, and after reaching a maximum at the composition around A3C6o estimated with a thickness monitor
Correspondence to: T. Takahashi, Department of Physics, Tohoku University, Sendai 980, Japan.
Figure 1 shows photoemission spectra of KxC60 annealed at 150-200°C for 1 h, together with that of KC 8 (graphite intercalation compound) for comparison. The composition of KxC60 was estimated from the relative intensity of the LUMO band appearing with alkali doping near the Fermi level, since it is known that doping of K stops at the composition of K6C60 [3]. As shown in fig. 1, the K-doping produces a broad structure from the Fermi level to the HOMO (highest occupied molecular orbital) band at about 2.5 eV. This new band appearing with K-doping is expected to have
0921-4526/93/$06.00 (~ 1993 - Elsevier Science Publishers B.V. All rights reserved
T. Takahashi et al. I Electronic structure o f doped
,,,~
C6o: strong correlation or lattice distortion? Photoemission
K~Cso
I
",
y c
,.j..
;
'\
i
"..' v
4
_-...
\ M,,-" \
/-",,___
-
,
,u ! ' A / T - "
L"
h
'v"
~/
Inverse Photoemission
KCa
._ ; , f ~ . ~ . ~ . . , ~
.r'\
1069
6.0
3.1 - -
0.5 Binding
k._____ ~
0.0
energy
(eV)
Fig. 2. Comparison of photoemission and inverse photoemission spectra of C6o and K3C6o.
3 2 I EF B i n d i n g energy (¢V)
Fig. 1. Photoemission spectra of a KxC60film (x = 0.0-6.0). The film was annealed at 150-200°C for 1 h after vapor deposition of a certain amount of K atoms. The composition of film (x) was estimated with the relative intensity of the LUMO band appearing near the Fermi level with K doping. A photoemission spectrum of KC 8 (graphite intercalation compound) measured with almost the same photon energy and the energy resolution [4] is also shown for comparison. its origin in the L U M O band of a C60 molecule, since, as shown later (see fig. 2), the intensity of the corresponding unoccupied L U M O band decreases with doping. It is understood that the electrons are donated from K atoms to C60 molecules and occupy the L U M O band. However, it is noted that the photoemission spectrum at the composition around K3C6o has no sharp Fermi-edge structure as is usually observed in normal metals. This is obviously inconsistent with a simple rigid band model which predicts that the maximum of the density of states should come to the Fermi level at K3C60 since the L U M O band is threefold degenerate. We find that this absence of a sharp Fermi-edge structure is not due to the energy resolution when we compare the photoemission spectrum with that of graphite intercalation compound, KC8 (insert in fig. 1) which was measured with almost the same energy resolution [4]. This result suggests that there is an energy gap at the Fermi level for K3C60. The change in electronic structure near the Fermi level by alkali doping is clearly observed when we combine photoemission and inverse photoemission, which probe the occupied and unoccupied electronic states, respectively. Figure 2 shows a comparison of photoemission and inverse photoemission spectra of C6o and K3C6o (unannealed) [5]. As shown in fig. 2,
both the H O M O and L U M O bands approach the Fermi level with K-doping by about 0.3 eV and at the same time a new band, as described above, emerges a little away from the Fermi level. It is understood that this new band has its origin in the L U M O band since the intensity of the unoccupied L U M O band in the inverse photoemission spectrum decreases with doping. However, it is noted that the occupied and unoccupied parts of the L U M O band are well separated across the Fermi level by an energy gap with a finite density of states at the bottom (a pseudo-gap). Similar photoemission behavior has been observed for Rbdoped C6o [6]. This means that the alkali-doping does not bring about a rigid filling of the L U M O band with donated electrons, but causes splitting of the LUMO band across the Fermi level. Two possible origins of the pseudo-gap can be considered: strong electron correlation and lattice distortion. It is generally accepted that the electron correlation of C 2p electrons is small compared with that of d-electrons. However, when considering the very narrow feature of the bands of solid C6o (less than 1 eV) due to its strong molecular nature, the electron correlation of C 2p electrons becomes comparable with or larger than the band width. Lof et al. [7] estimated the on-site molecular Coulomb interaction to be about 1.6 eV from the Auger measurement. They concluded that doped C60 should be regarded as a highly correlated system and proposed that stoichiometric K3C6ois a M o r t - H u b b a r d insulator with an correlation gap of about 0.7 eV. The size of the gap predicted by Lof et al. seems similar to the present observation (fig. 2), although we observed a finite density of states at the Fermi level, contrary to their prediction. Another possible origin of the pseudo-gap is local
1070
T. Takahashi et al. / Electronic structure of doped Ceo: strong correlation or lattice distortion?
lattice distortion. It is likely that doped electrons distort the high symmetry of local structure, giving rise to release of the degeneracy of the LUMO band. A strong Jahn-Teller effect observed in a negatively charged C6o molecule [8] favors the above inference. A reported large value of a (1.4) in the isotope effect [9] suggests a strong interaction between lattices and electrons in doped C60.
4. Conclusions
We have performed photoemission and inverse photoemission spectroscopy studies of alkali-doped solid C6,, to study the electronic structure near the Fermi level. We found that alkali doping does not necessarily lead to a rigid filling of the LUMO band by the donated electrons from alkali atoms, but causes a transfer of electronic states from the LUMO band to a new band produced in the gap of the host material (C60). Comparison of photoemission and inverse photoemission spectra at the superconducting composition of A3C6o (A = alkali metal) indicates that the LUMO band splits into two sub-bands across the Fermi Level with alkali doping, producing an energy gap with a finite density of states at the bottom (a pseudo-gap) at the Fermi level. A relatively strong electron correlation of C 2p electrons and a lattice distortion through the Jahn-Teller effect should be considered as possible origins for the pseudo-gap.
References
[1] A.F. Hebard, M.J. Rosseinsky, R.C. Haddon, D.W. Murphy, S.H. Glarum, T.T.M. Palstra, A.P. Ramirez and A.R. Kortan, Nature 350 (1991) 600. [2] R.C. Haddon, A.F. Hebard, M.J. Rosseinsky, D.W. Murphy, S.J. Duclos, K.B. Lyons, B. Miller, J.M. Rosamilia, R.M. Fleming, A.R. Kortan, S.H. Glarum, A.V. Makhija, A.J. Muller, R.H. Eick, S.M. Zahurak, R. Tycko, G. Dabbagh and F.A. Thiel, Nature 350 (1991) 320. [3] R.M. Fleming, M.J. Rosseinsky, A.P. Ramirez, D.W. Murphy, J.C. Tully, R.C. Haddon, T. Siegrist, R. Tycko. S.H. Glarum, P. Marsh, G. Dabbagh, S.M. Zahurak, A.V. Makhija and C. Hampton, Nature 352 (1991) 701. [4] P. Oelhafen, P. Pfuger, E. Hauser and H.-J. Guntherodt, Phys. Rev. Lett. 44 (1980) 197. [5] T. Takahashi, S. Suzuki, T. Morikawa, H. KatayamaYoshida, S. Hasegawa, H. Inokuchi, K. Seki, K. Kikuchi, S. Suzuki, K. Ikemoto and Y. Achiba, Phys. Rev. Lett. 68 (1992) 1232. [6] T. Takahashi, T. Morikawa, S. Hasegawa, K. Kamiya, H. Fujimoto, S. Hino, K. Seki, H. Katayama-Yoshida, H. Inokuchi, K. Kikuchi, S. Suzuki, K. Ikemoto and Y. Achiba, Physica C 190 (1991) 205. [7] R.W. Lof, M.A. van Veenendaal, B. Koopmans, H.T. Jonkman, and G.A. Sawatzky, Phys. Rev. Lett. 68 (1992) 3924. [8] T. Kato, T. Kodama, M. Oyama, S. Okazaki, T. Shida, T. Nakagawa, Y. Matsui, S. Susuki, H. Shiromaru, K. Yamauchi and Y. Achiba, Chem. Phys. Lett. 186 (1991) 35. [9] T.W. Ebbesen, J.S. Tsai, K. Tanigaki, J. Tabuchi, Y. Shimakawa, Y. Kubo, I. Hirosawa and J. Mizuki, Nature 355 (1992) 620.