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Electronic structure of KxC60 studied by high-energy resolved photoemission spectroscopy
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Solid State Communications, Vol. 87, No. 11, pp. 1017-1021, 1993. Printed in Great Britain.
0038-1098/93 $6.00+. 00 Pergamon Press Ltd
E I . ~ ' L ' T R O N I C S T R U C T U R E O F KxCeo S T U D I E D BY H I G H - E N E R G Y RF-QOLVI~D PHOTOEMISSION SPECTROSCOPY
Takashi Morikawa and Takashi Takahashi Department of Physics, Tohoku University, Sendai 980, Japan (Received 3 June 1993, in revisedforra 3 July 1993 by 1-1.Kamiraura) High-energy resolved low-temperature ultraviolet photoemission spectroscopy has been performed on K xC60 (x=O - 6) to study the detailed electronic structure near the Fermi level (Er). The experimental result shows that a very narrow peak and a broad distribution of density of states coexist near EF in K3C60. While the narrow peak at EF is correlated to a part of the occupied LUMO (lowest unoccupied molecular orbital) band of C60 as predicted from the band structure calculation, the observed broad distribution of density of states near EF has no counterpart in the calculation. It was also found that K4C60 is a semiconductor with an energy gap of more than 0.2 eV. This shows a sharp contrast with the prediction from the band calculation that K4C60 should be a metal with four electrons in the three-fold degenerate LUMO band. Some possible origins to give these discrepancies have been discussed.
1. I n t r o d u c t i o n
observing a change of the electronic s t r u c t u r e near EF upon alkali-doping in detail. We found in this study t h a t a very narrow peak a t EF a n d a broad feature spreading from EF to about 1.5 eV coexist in the photoemission spectrum of K3C6o. We also found t h a t t h e s e s t r u c t u r e s in t h e photoemission spectrum show different i n t e n s i t y v a r i a t i o n a g a i n s t the K-doping. Although the n a r r o w p e a k a t EP is ascribable to the p a r t i a l l y occupied LUMO band of C6o as predicted from the band structure calculation [9], the observed broad d i s t r i b u t i o n of electronic s t a t e s n e a r EF is not u n d e r s t o o d in t e r m s of t h e b a n d s t r u c t u r e calculation. As for K4C6o, on the other hand, we found t h a t the photoemission i n t e n s i t y a t EF is n e g l i g i b l y small, s u g g e s t i n g t h a t K4C6o is a semiconductor with a finite energy gap of more than 0.2 eV at EF in contrast with a simple rigidband model or a band structure calculation [9,10].
The discovery of superconductivity in alkalidoped C6o [1] has brought about much attention to the electronic structure near the Fermi level (EF). I t is r e m a r k a b l e t h a t t h e s u p e r c o n d u c t i n g t e m p e r a t u r e of Cs2RbC6o (33K) is comparable to t h a t of La-Sr-Cu-O cuprate superconductors [2]. This has r a i s e d a fundamental question whether the superconducting mechanism is essentially the s a m e b e t w e e n t h e s e two a p p a r e n t l y d i f f e r e n t compounds. One of key approaches to solve this problem is to u n d e r s t a n d the role of 'doping' of alkali atoms. However it is still unknown whether t h e a l k a l i - d o p i n g causes a rigid filling of the LUMO (lowest unoccupied molecular orbital) band of Ceo or produces a d r a s t i c change of the electronic structure near EF. Spectroscopic studies such as photoemission, i n v e r s e p h o t o e m i s s i o n , x-ray a b s o r p t i o n , a n d electron-energy-loss spectroscopies [3-8] have been a c t i v e l y a p p l i e d to t h e s e novel compounds, s h o w i n g t h a t a t r a n s f e r of electronic c h a r g e (electrons) from a l k a l i atoms to C6o molecules actually takes places upon doping. However, the previous reports do not agree each other in details of the electronic structure near EF, for example, the shape of d e n s i t y of electronic s t a t e s a t EF emerging upon alkali-doping. This discrepancy, which h a s been a s c r i b e d to d i f f e r e n t s a m p l e p r e p a r a t i o n method, different energy resolution, a n d / o r different way in e s t i m a t i n g the a l k a l i content, is serious since elucidating the electronic s t r u c t u r e n e a r EF is the first step to u n d e r s t a n d the superconducting mechanism. In this Communication, we report our recent photoemission study on K-doped Ceo p e r f o r m e d with a very high energy resolution (35 meV) a n d a t very low t e m p e r a t u r e (30 - 40 K). Using the high energy-resolving power, we have succeeded in
2. E x p e r i m e n t a l A thin K-doped C6o film for photoemission m e a s u r e m e n t was p r e p a r e d in-situ by vapor deposition of K metals onto a C6o film. A pristine C6o film was a t first prepared by vapor-deposition with C6o powders (99.9 % purity) onto a stainless steel substrate held a t about 150°C. The K-doping was m a d e with a S A E S g e t t e r a t a very low deposition r a t e (about 1A/hour a t the s a m p l e surface). The substrate was kept a t 120 - 150 °C d u r i n g the alkali-deposition in order to achieve t h e r m a l equilibrium a t the sample surface. We expended great care to estimate the composition of sample since it is known t h a t the composition is one of t h e m o s t i m p o r t a n t k e y f a c t o r s in c h a r a c t e r i z i n g t h i s compound. A several e s t i m a t i o n m e t h o d s such as m o n i t o r i n g t h e electrical conductivity of a sample film have been used in the previous studies. In the p r e s e n t 1017
1018
ELECTRONIC STRUCTURE OF KxC60
study, we employed the most d i r e c t way by e s t i m a t i n g the alkali content from the m e a s u r e d photoemission spectrum itself. The composition (x in KxC6o) was e s t i m a t e d with the r e l a t i v e i n t e n s i t y of the K 3p core level a n d also the intensity-ratio between the occupied LUMO band a n d the HOMO band. This method can remove a possible uncertainty that a photoemission spectrum m a y r e p r e s e n t a sample area different from t h a t monitored by an independent estimation method s u c h as e l e c t r i c a l c o n d u c t i v i t y measurement. We u s e d t h e p h o t o e m i s s i o n spectrum of K6C6o as a s t a n d a r d since it is well established t h a t inclusion of alkali atoms into C6o stops a t K6C6o [ 1 1 ] . A c t u a l l y we o b s e r v e d s a t u r a t i o n of change in photoemission spectrum after a several times deposition of potassium on a C6o film. The r e s u l t s of two i n d e p e n d e n t estimation methods using the K 3p core level and the LUMO band agree well each other within the uncertainty in x of_+0.2. Photoemission m e a s u r e m e n t was made with helium resonance lines (21.22 and 40.8 eV) at the energy resolution of about 35 meV, being estimated by a Fermi-edge cutoff of gold metal. T e m p e r a t u r e of sample was controlled with a helium cryostat and a heater embedded in the sample-holder. The Fermi level of sample was referred to those of a gold substrate and of a thick potassium overlayer deposited on the sample. The uncertainty in the energy position of EF is less than 5 meV.
Vol. 87, No. 11
¢-
v >, ..i-, .
B
o3 (.-
{-
4
3 2 1 EF Binding energy (eV)
3. Results a n d Discussion
F i g . 1 Photoemission spectra of KxC6o m e a s u r e d with energy resolution of 35 meV a t 40 K, except for t h a t of x=0.0 which was recorded a t room temperature.
Figure 1 shows photoemission spectra of KxC6o 0 - 6). The i n t e n s i t i e s of s p e c t r a a r e normalized with respect to the total volume of HOMO (highest occupied molecular orbital) band which is located at about 2.5 eV in pristine C6o. The spectra were recorded a t 40 K except for x=0. Since the pristine C6o film showed charging-up effect when it was exposed to the He I light at 40 K, the spectrum was recorded at room temperature. As shown in Fig. 1, the K-doping causes a several d r a s t i c changes in the photoemission spectra, which a r e s u m m a r i z e d in the following. (1) additional structures a p p e a r between EF a n d the HOMO band upon the doping. (2) these structures are composed of three main components; a very narrow peak at EF, a step-like structure at 1.5 eV, and a very broad band between these two distinct structures. (3) the intensities of the narrow peak a t EF a n d the s t e p - l i k e s t r u c t u r e a t 1.5 eV concurrently increase with K-doping until x--2.5, but they gradually decrease with further doping a n d f i n a l l y d i s a p p e a r at x=4. (4) the broad s t r u c t u r e around 0.5 eV changes its shape with doping; it is almost fiat at early stage of doping (x _< 1.7) while it becomes a broad single peak centered a t about 0.5 eV for x=2.5 - 4.0. On further doping, the broad band shifts toward the high-bindingenergy direction. (5) the HOMO band and the next occupied band (HOMO-1 band) become broad upon doping. They shift t o w a r d the high-bindingenergy direction by about 0.3 eV at x=0.6 and then g r a d u a l l y approach the Fermi level with doping until x=4. However, they again move away from
EF on further doping (x=5.3 - 6). At first we discuss the electronic structure near EF of K3C6o. As shown in Fig. 1, the high-energyresolved photoemission spectrum has a distinct F e r m i - e d g e s t r u c t u r e indicative of its n o r m a l metallic nature. In many of early photoemission studies [3,4,6], however, no distinct F e r m i - e d g e s t r u c t u r e was not found a t the composition of K3C6o [11]. Rather, the early photoemission d a t a seemed to indicate the existence of a pseudo-gap (an energy gap with a finite density of states at the bottom) at EF, as pointed out by T a k a h a s h i et al.[6]. It is supposed t h a t the a p p e a r a n c e of a clear Fermi-edge s t r u c t u r e in a photoemission spectrum is largely owed to improvement of the energy resolution including the reduced t e m p e r a t u r e - b r o a d e n i n g effect. I n d e e d it was found in this study t h a t the Fermi-edge structure became less resolved and was finally covered by a large background of a prominent peak a t 0.5 eV when the energy resolution was decreased and/or the s a m p l e t e m p e r a t u r e was increased. I t is n o t e d t h a t v e r y r e c e n t l y two i n d e p e n d e n t photoemission research groups [12,13] also found a distinct Fermi-edge structure in the spectrum of K3C6o by employing a high-energy resolved lowt e m p e r a t u r e photoemission measurement. Thus it seems most likely t h a t the improved energy resolution is a key factor to observe the Fermi-edge s t r u c t u r e in the p h o t o e m i s i o n m e a s u r e m e n t .
(x=
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ELECTRONIC STRUCTURE OF KxC60
Another possible origin for the appearance of the clear Fermi-edge structure is improved quality of the sample. Indeed in all three experiments including the present one, the sample was well annealed at relatively high temperature (100 - 150 °C) to reduce the number of defects and disorder in the s a m p l e a n d t h e r e b y i m p r o v e the crystallinity. One may present a question on whether the spectrum really represents K3C6o or there are considerable contributions from other phases. It is well established by the structural study [14] that KxCeo has four distinct phases of C6o, K3C6o, K4C6o, and K6C6o. Therefore, an intermediate composition of KxCeo would be a mixture of these phases. As described above, K-doping creates some additional structures in the energy range from EF to about 2 eV. Since the narrow peak at EF a n d the step-like structure appear from the beginning of doping and show the same intensity variation before disappearing at x=4, these two structures are ascribed to K3C6o not to K4C6o. On the other hand, the broad band at 0.5 eV grows with the doping until x=4 as shown in Fig. 1, so that the broad band might be due to K4C6o. However, it is remarked that even at the early stage of doping ( x = 0 . 6 - 1.2), a broad and almost fiat structure appears in the same energy region. It is evident t h a t this fiat structure is not reproduced by a simple reduction of intensity of the broad band at x=4 since the shape itself is different. Therefore it is concluded that K3C6o has a very broad distribution of density of states spreading from EF to 1.5 eV, together with a narrow peak at EF and a step-like structure at 1.5 eV. A similar broad structure near EF is also seen in the photoemission spectrum of KaCeo reported by Weaver et a1.[12] and Merkel et al.[13]. It is also noteworthy that Dgiorgi et al.[15] found a broad optical excitation in the mid-IR region for doped Ceo and ascribed it to a broad distribution of electronic states near EF as observed in the photoemission experiment. The band structure calculation [9] has predicted that K3C6o is a metal with a half-filled LUMO band at EF, whose occupied part has the width of about 0.2 eV and the HOMO band is located at about 1.5 eV. This prediction is in good agreement with the present experimental result when we ascribe the narrow peak at E P and the step-like structure at 1.5 eV in the photoemission spectrum to the LUMO and the HOMO bands, respectively. However, the observed broad structure spreading from EF to 1.5 eV has no counterpart in the calculation, in which there is a large energy gap between the LUMO and the HOMO band. In the following, we discuss some possible origins for the observed broad structure near EF in K3C6o. We have already shown that it is not ascribable to K4C6o phase. The diluted mixed phase of C6o and potassium (a-phase Ceo) cannot be an origin of this fiat structure since the intensity increases with doping, while the relative ratio of a-phase C6oin the sample should decrease with doping. A simple background due to
1019
secondary electrons cannot produce such a large structure comparable to its main band at EF. A Jahn-Teller effect through local distortion caused by doping m a y produce polaron states in the energy gap of C 6 0 . However, a theoretical estimation [16] of the binding energy is less than a few tens meV, much smaller than the width of the observed broad structure (more than 1 eV). Electron correlation has been suggested as an origin of peculiar behavior of the electronic structure near EF in doped and undoped Ceo [3,6,17]. Lof et a1.[17] have discussed the discrepancy in the size of energy gap of undoped Ceo between the photoemission experiment (about 2 eV) and the band structure calculation (1.5 eV). They estimated the C 2p correlation energy as 1.6 eV by the Auger spectroscopy, which is much larger than the band width (0.5 eV) predicted from the band calculation. Thereby they have proposed that the strong electron correlation considerably modifies the one-electron states in solid C6o. If the strong electron-correlation dominates the electronic structure also in doped C6o, the observed broad structure near EF may be ascribed to an incoherent band (lower Mott-Hubbard band) while the narrow peak at EF being due to a coherent peak. However, this i n t e r p r e t a t i o n looks inconsistent with the present experimental result that K3Ceo is a metal with a sharp peak at EF, since the LUMO band is three-fold degenerate and as a result K3C6o with the 'half-filled' LUMO band should be a Mott-Hubbard insulator with a finite energy gap at EF. Although Lof et al. have suggested that the sharp peak observed at EF in photoemission spectrum is due to doped carriers in the Mott-Hubbard insulator (K3Ceo), we still find a finite density of states at EF for K3Ceo even when t a k i n g account of the u n c e r t a i n t y of estimated x's in the present study (~x = 0.2). We will also discuss the effect of a strong electron correlation in KxC6o in the following in connection with the observed peculiar electronic behavior of K4C6o. We find in Fig. 1 that the photoemission spectrum of K4C60 has three main structures. The first band located at 0.5 eV is ascribed to the partially occupied LUMO band of Ceo since the LUMO band is three-fold degenerate and four electrons per one Ceo molecule are expected to be donated from potassium atoms. Therefore a simple rigid band model and also a band structure calculation have predicted that K4C6o should be a metal. However, as found in Fig. 1, the density of states at EF in K4C6o is negligibly small or rather a finite energy gap of more than 0.2 eV looks to open at EF. Weaver et a1.[12] also reported a quite similar photoemission spectrum exhibiting a negligible intensity at EF for K4Ceo, suggesting its insulating character. However, Merkel et al.[13] presented a slightly different spectrum for K4.2C6o (they did not present the spectrum for K4C6o), which has a small but finite intensity at EF indicative of its metallic nature. Although the origin for this discrepancy is not known at present, it may be attributable to an experimental error in estimating the composition of samples.
1020
ELECTRONIC STRUCTURE OF KxC60
Although the three independent experiments including present one [12,13] employed essentially the same method (HOMO/LUMO intensity ratio and K 3p core-level intensity etc.) to estimate the composition, the reported uncertainity (Ax in KxC6o) ranges from 0.2 to 0.3, so that it may happen t h a t the difference in the estimated composition differes by 0.5 in x among three experiments, which may be responsible for the discrepancy. Merkel et ai.[13] suggested t h a t formation of a K overlayer and its oxidation at the sample surface is a main reason causing an u n c e r t a i n t y in e s t i m a t i n g the composition. Indeed they reported a photoemission spectrum of Kl1~C6o in average composition. However, it is unknown at present how K atoms with a relatively high vapor pressure (about 10 ~ Torr at 430 K, at which they annealed the sample) forms a thin overlayer on KeCeo and changes into the oxide under ultrahigh vacuum. These indicate that a much more precise determination of composition in photoemission measurement is necessary to extract a final conclusion, but it should not be neglected that at least two (Ref.12 and present study) of the three independent experiments agree quite well each other in spite of the reported uncertainty. The insulating or semiconducting nature of K4Ceo has been also suggested by the ~tSR experiment [18]. The energy gap at EF in K4C6o observed by the ~SR experiment (0.3 eV) is in good agreement with the present photoemission study (0.2 - 0.4 eV). Although the origin to produce such an energy gap at EF in K4C6o is not clear at present, a several possible causes are discussed in the following. It is evident that the energy gap observed in the photoemission spectrum of x=4 is not due to K6C6o since, as shown in Fig. 1, the photoemission spectrum of KeCeo has a larger energy gap at EF. The occupied LUMO band of K6C6o is located at 0.9 eV while that of K4C6o is at 0.5 eV, suggesting also that a contribution from K6C6o phase to the photoemission spectrum of x=4 is negligible. A possible contribution from K3C6o phase is rather negative, since K3Ceo has a sharp peak at EF and it completely disappears at x=4 as shown in Fig. 1. All these indicate that the energy gap observed in the photoemission spectrum of x=4 is intrinsic for K4C60. It is known that K4C6o has a body-centeredtetragonal (bct) crystal structure while K3Ceo has a face-centered-cubic (fcc) one. It seems, however, t h a t the difference in the crystal structure is not a cause for the opening of the gap, since the band structure calculation on bct K4C6o has predicted its metallic nature. A considerable local distortion, which may cause a splitting of the LUMO band through a Jahn-Teller effect, has not been observed for K4Ceo. Thus an electron correlation seems to be the most possible origin for the gap. It is noted here that since the LUMO band is three-fold degenerate, a Mott-Hubbard gap can open at each stage of doping of x=l to 5 (integer) in KxCeo if the electron correlation is sufficiently strong. However, it is not clear at
Vol. 87, No. 11
p r e s e n t why the gap opens only at x=4. Considering the experimental fact that K3C6o is a metal with a very narrow band at EF together with the following broad distribution of density of states, it is speculated that the amplitude of the effective correlation energy may change with the doping. The correlation energy may be comparable to the band width in K3CeO so that a sharp coherent peak at EF and a broad incoherent band coexist in the photoemission spectrum, while in K4C60 the correlation energy overcomes the band width and as a result a finite Mott-Hubbard gap may open at EF. In order to check this speculation, further theoretical studies for the effect of strong electron correlation in a degenerate band as in the present case are necessary. Finally, we discuss the strange behavior of the HOMO and HOMO-1 bands against the K-doping. The initial shift of the bands towards the highbinding- energy direction by about 0.3 eV may be due to filling of gap states in pristine Ceo with donated electrons. It is likely that the electrons from K atoms at first occupy the gap states and then go into the LUMO band of Ceo. The gap states, which are located around 0.3 eV above EF, may be produced by defects in the crystal. The gradual approach of the HOMO and HOMO-1 bands to EF upon further doping until x=4 is explained by lattice contraction induced by Kdoping [9]. The observed movement of the HOMO and HOMO-1 bands as well as the occupied LUMO band toward the high-binding energy-direction from x=4 to 6 is due to opening of a new energy gap between the LUMO and the LUMO+I band in KeC6o. The broadening of bands at intermediate (non-stoichiometric) compositions is due to superposition from more t h a n two phases. However, at K4C6o in Fig. 1, the HOMO band appears much broader than that of pristine C60. This may also be a consequence of the abovementioned lattice contraction introduced by Kdoping and resulting increased hybridization among molecular orbitals [9]. 4. Conclusion We found a several anomalous behaviors of the electronic structure in KxCeo by employing the h i g h - e n e r g y resolved low-temperature photoemission spectroscopy. In the photoemission spectrum of K3Ceo, we found a very narrow peak a t E F and a step-like structure at 1.5 eV as well as a broad distribution of density of states between these two structures. Although the former two structures in the spectrum are ascribed to the partially occupied LUMO band and the HOMO band, respectively, the broad distribution of density of states is not understood in terms of the band structure calculation, since there is a large energy gap between the LUMO and the HOMO band in the calculation. We also found that K4C6o is a semiconductor with an energy gap of more than 0.2 eV at EF. This presents a sharp contrast with the band structure calculation which has predicted that K4C6o should be a metal. We have discussed some possible origins for the observed anomalous behaviors in the electronic structure of
Vol. 87, No. 11
ELECTRONIC STRUCTUREOF KxC60
K=C6o and found that a strong electron correlation m a y play a key role in characterizing the electronicstructureof KxC6o.
Acknowledgment- The authors thank K. Kikuchi, S. Suzuki, K. Ikemoto, and Y. Achiba at Tokyo Metropolitan University for providing high purity
1021
C6o powders. They also thank H. KatayamaYoshida at Tohoku University and A. Fujimori at Tokyo University for useful discussions. This work was supported by grants from the Ministry of Education, Culture and Science of Japan and from the Foundation for Promotion of Material Science and Technology of Japan.
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, 60 (1991). 2. K. Tanigaki, T. W. Ehbesen, S. Saito, J. Mizuki, J. S. Tsai, Y. Kubo, and S. Kurushima, Nature 3~, 222 (1991). 3. P.J. Benning, J. L. Martins, J.H. Weaver, L.P.F. Chibante, and R.E. Smalley, Science 252, 1417 (1991); P.J. Benning, D.M. Poirier, T.R. Ohno, Y. Chen, M.B. Jost, F. Stepniak, G.H. Kroll, J.H. Weaver, J. Fure, and R.E. Smalley, Phys. Rev. B 45, 6889 (1992). 4. G.K. Wertheim, J.E. Rowe, D.N.E. Buchanan, E.E. Chaban, A.F. Hebard, A.R. Kortan, A.V. Makhija, and R.C. Haddon, Science 252, 1419 (1991). 5. C.T. Chen, L.H. Tjeng, P. Rudolf, G. Meigs, J.E. Rowe, J. Chen, J.P. McCauley, Jr., A.B. Smith, III, A.R. McGhie, W.J. Romanow, and E.W. Plummer, Nature (London) 352, 603 (1991). 6. T. Takahashi, S. Suzuki, T. Morikawa, H. Katayama-Yoshida, S. Hasegawa, H. Inokuchi, K. Kikuchi, S. Suzuki, K. Ikemoto, and Y. Achiba, Phys. Rev. Lett.{]8,1232 (1992). 7. E. Sohmen, J. Fink, and W. Kr/itschmer, Euro. Phys. Lett. 17,51 (1992). 8. J. Fink, E. Sohmen, M. Merkel, A. Masaki, H. Romberg, M. Alexander, M. Knupfer, M.S. Golden, P. Adelmann, and B. Renker, Fullerens: Status and Perspective (Proc. 1st Italian Workshop on Fullerens) (World Scientific, Singapore, 1992), p. 161. 9. S. Saito and A. Oshiyama, Phys. Rev. B 44, 11536 (1991); S.C. Erwin and W.E. Pickett, Science 254, 842 (1991); Y.-N Xu, M.-Z. Huang, and W.Y. Ching, Phys. Rev. B 44, 13171 (1991);
J.L. Martins and N. Troullier, Phys. Rev. B 46, 1766 (1992). 10. S. Saito, private communications. 11. It is guessed that a photoemission spectrum having a Fermi-edge structure reported in Ref. 5 should be attributed to an early doping stage, but not to K3C6o, as judged from the whole photoemission spectral feature. 12.J.H. Weaver, P.J. Benning, F. Stepniak, and D.M. Poirier, J. Phys. Chem. Solids 53, 1707 (1992); P.J. Benning, F. Stepniak, D.M. Poirier, J.L. Martins, J.H. Weaver, L.P.F. Chibante, and R.E. Smalley, preprint. 13. M. Merkel, M. Knupfer, M. S. Golden, J. Fink, R. Seemann, and R. L. Johnson, Phys. Rev. B 47, 11470 (1993). 14. R.M. Flemming, 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 (London) 352, 701 (1991). 15. L. Degiorgi, P. Wachter, G. Grtiner, S.-M. Huang, J. Wieley, and R.B. Kaner, Phys. Rev. Lett. (9, 2987 (1992); L. Degiorgi, G. Grttner, P. Wachter, S.-M. Huang, J. Wieley, R.L. Whetten, R.B. Kaner, K. Holczer, and F. Diederich, Phys. Rev. B 46, 11250 (1992)., 16. K. Harigaya, Phys. Rev. B 45, 13676 (1992). 17. R. Lof, M.A. van Veenendaal, B. Koopmans, H.T. Jonkman, and G.A. Sawatzky, Phys. Rev. Lett. {]8,3924 (1992). 18.R.F. Kiefl, T.L. Duty, J.W. Schneider, A. MacFarlane, K. Chow, J.W. Elzey, P. Mendels, G.D. Morris, J.H. Brewer, E.J. Ansaldo, C. Niedermayer, D.R. Noakes, C.E. Stronach, B. Hitti, and J.E. Fischer, Phys. Rev. Lett. (9, 2005 (1992).
Solid State Communications, Vol. 87, No. 11, pp. 1017-1021, 1993. Printed in Great Britain.
0038-1098/93 $6.00+. 00 Pergamon Press Ltd
E I . ~ ' L ' T R O N I C S T R U C T U R E O F KxCeo S T U D I E D BY H I G H - E N E R G Y RF-QOLVI~D PHOTOEMISSION SPECTROSCOPY
Takashi Morikawa and Takashi Takahashi Department of Physics, Tohoku University, Sendai 980, Japan (Received 3 June 1993, in revisedforra 3 July 1993 by 1-1.Kamiraura) High-energy resolved low-temperature ultraviolet photoemission spectroscopy has been performed on K xC60 (x=O - 6) to study the detailed electronic structure near the Fermi level (Er). The experimental result shows that a very narrow peak and a broad distribution of density of states coexist near EF in K3C60. While the narrow peak at EF is correlated to a part of the occupied LUMO (lowest unoccupied molecular orbital) band of C60 as predicted from the band structure calculation, the observed broad distribution of density of states near EF has no counterpart in the calculation. It was also found that K4C60 is a semiconductor with an energy gap of more than 0.2 eV. This shows a sharp contrast with the prediction from the band calculation that K4C60 should be a metal with four electrons in the three-fold degenerate LUMO band. Some possible origins to give these discrepancies have been discussed.
1. I n t r o d u c t i o n
observing a change of the electronic s t r u c t u r e near EF upon alkali-doping in detail. We found in this study t h a t a very narrow peak a t EF a n d a broad feature spreading from EF to about 1.5 eV coexist in the photoemission spectrum of K3C6o. We also found t h a t t h e s e s t r u c t u r e s in t h e photoemission spectrum show different i n t e n s i t y v a r i a t i o n a g a i n s t the K-doping. Although the n a r r o w p e a k a t EP is ascribable to the p a r t i a l l y occupied LUMO band of C6o as predicted from the band structure calculation [9], the observed broad d i s t r i b u t i o n of electronic s t a t e s n e a r EF is not u n d e r s t o o d in t e r m s of t h e b a n d s t r u c t u r e calculation. As for K4C6o, on the other hand, we found t h a t the photoemission i n t e n s i t y a t EF is n e g l i g i b l y small, s u g g e s t i n g t h a t K4C6o is a semiconductor with a finite energy gap of more than 0.2 eV at EF in contrast with a simple rigidband model or a band structure calculation [9,10].
The discovery of superconductivity in alkalidoped C6o [1] has brought about much attention to the electronic structure near the Fermi level (EF). I t is r e m a r k a b l e t h a t t h e s u p e r c o n d u c t i n g t e m p e r a t u r e of Cs2RbC6o (33K) is comparable to t h a t of La-Sr-Cu-O cuprate superconductors [2]. This has r a i s e d a fundamental question whether the superconducting mechanism is essentially the s a m e b e t w e e n t h e s e two a p p a r e n t l y d i f f e r e n t compounds. One of key approaches to solve this problem is to u n d e r s t a n d the role of 'doping' of alkali atoms. However it is still unknown whether t h e a l k a l i - d o p i n g causes a rigid filling of the LUMO (lowest unoccupied molecular orbital) band of Ceo or produces a d r a s t i c change of the electronic structure near EF. Spectroscopic studies such as photoemission, i n v e r s e p h o t o e m i s s i o n , x-ray a b s o r p t i o n , a n d electron-energy-loss spectroscopies [3-8] have been a c t i v e l y a p p l i e d to t h e s e novel compounds, s h o w i n g t h a t a t r a n s f e r of electronic c h a r g e (electrons) from a l k a l i atoms to C6o molecules actually takes places upon doping. However, the previous reports do not agree each other in details of the electronic structure near EF, for example, the shape of d e n s i t y of electronic s t a t e s a t EF emerging upon alkali-doping. This discrepancy, which h a s been a s c r i b e d to d i f f e r e n t s a m p l e p r e p a r a t i o n method, different energy resolution, a n d / o r different way in e s t i m a t i n g the a l k a l i content, is serious since elucidating the electronic s t r u c t u r e n e a r EF is the first step to u n d e r s t a n d the superconducting mechanism. In this Communication, we report our recent photoemission study on K-doped Ceo p e r f o r m e d with a very high energy resolution (35 meV) a n d a t very low t e m p e r a t u r e (30 - 40 K). Using the high energy-resolving power, we have succeeded in
2. E x p e r i m e n t a l A thin K-doped C6o film for photoemission m e a s u r e m e n t was p r e p a r e d in-situ by vapor deposition of K metals onto a C6o film. A pristine C6o film was a t first prepared by vapor-deposition with C6o powders (99.9 % purity) onto a stainless steel substrate held a t about 150°C. The K-doping was m a d e with a S A E S g e t t e r a t a very low deposition r a t e (about 1A/hour a t the s a m p l e surface). The substrate was kept a t 120 - 150 °C d u r i n g the alkali-deposition in order to achieve t h e r m a l equilibrium a t the sample surface. We expended great care to estimate the composition of sample since it is known t h a t the composition is one of t h e m o s t i m p o r t a n t k e y f a c t o r s in c h a r a c t e r i z i n g t h i s compound. A several e s t i m a t i o n m e t h o d s such as m o n i t o r i n g t h e electrical conductivity of a sample film have been used in the previous studies. In the p r e s e n t 1017
1018
ELECTRONIC STRUCTURE OF KxC60
study, we employed the most d i r e c t way by e s t i m a t i n g the alkali content from the m e a s u r e d photoemission spectrum itself. The composition (x in KxC6o) was e s t i m a t e d with the r e l a t i v e i n t e n s i t y of the K 3p core level a n d also the intensity-ratio between the occupied LUMO band a n d the HOMO band. This method can remove a possible uncertainty that a photoemission spectrum m a y r e p r e s e n t a sample area different from t h a t monitored by an independent estimation method s u c h as e l e c t r i c a l c o n d u c t i v i t y measurement. We u s e d t h e p h o t o e m i s s i o n spectrum of K6C6o as a s t a n d a r d since it is well established t h a t inclusion of alkali atoms into C6o stops a t K6C6o [ 1 1 ] . A c t u a l l y we o b s e r v e d s a t u r a t i o n of change in photoemission spectrum after a several times deposition of potassium on a C6o film. The r e s u l t s of two i n d e p e n d e n t estimation methods using the K 3p core level and the LUMO band agree well each other within the uncertainty in x of_+0.2. Photoemission m e a s u r e m e n t was made with helium resonance lines (21.22 and 40.8 eV) at the energy resolution of about 35 meV, being estimated by a Fermi-edge cutoff of gold metal. T e m p e r a t u r e of sample was controlled with a helium cryostat and a heater embedded in the sample-holder. The Fermi level of sample was referred to those of a gold substrate and of a thick potassium overlayer deposited on the sample. The uncertainty in the energy position of EF is less than 5 meV.
Vol. 87, No. 11
¢-
v >, ..i-, .
B
o3 (.-
{-
4
3 2 1 EF Binding energy (eV)
3. Results a n d Discussion
F i g . 1 Photoemission spectra of KxC6o m e a s u r e d with energy resolution of 35 meV a t 40 K, except for t h a t of x=0.0 which was recorded a t room temperature.
Figure 1 shows photoemission spectra of KxC6o 0 - 6). The i n t e n s i t i e s of s p e c t r a a r e normalized with respect to the total volume of HOMO (highest occupied molecular orbital) band which is located at about 2.5 eV in pristine C6o. The spectra were recorded a t 40 K except for x=0. Since the pristine C6o film showed charging-up effect when it was exposed to the He I light at 40 K, the spectrum was recorded at room temperature. As shown in Fig. 1, the K-doping causes a several d r a s t i c changes in the photoemission spectra, which a r e s u m m a r i z e d in the following. (1) additional structures a p p e a r between EF a n d the HOMO band upon the doping. (2) these structures are composed of three main components; a very narrow peak at EF, a step-like structure at 1.5 eV, and a very broad band between these two distinct structures. (3) the intensities of the narrow peak a t EF a n d the s t e p - l i k e s t r u c t u r e a t 1.5 eV concurrently increase with K-doping until x--2.5, but they gradually decrease with further doping a n d f i n a l l y d i s a p p e a r at x=4. (4) the broad s t r u c t u r e around 0.5 eV changes its shape with doping; it is almost fiat at early stage of doping (x _< 1.7) while it becomes a broad single peak centered a t about 0.5 eV for x=2.5 - 4.0. On further doping, the broad band shifts toward the high-bindingenergy direction. (5) the HOMO band and the next occupied band (HOMO-1 band) become broad upon doping. They shift t o w a r d the high-bindingenergy direction by about 0.3 eV at x=0.6 and then g r a d u a l l y approach the Fermi level with doping until x=4. However, they again move away from
EF on further doping (x=5.3 - 6). At first we discuss the electronic structure near EF of K3C6o. As shown in Fig. 1, the high-energyresolved photoemission spectrum has a distinct F e r m i - e d g e s t r u c t u r e indicative of its n o r m a l metallic nature. In many of early photoemission studies [3,4,6], however, no distinct F e r m i - e d g e s t r u c t u r e was not found a t the composition of K3C6o [11]. Rather, the early photoemission d a t a seemed to indicate the existence of a pseudo-gap (an energy gap with a finite density of states at the bottom) at EF, as pointed out by T a k a h a s h i et al.[6]. It is supposed t h a t the a p p e a r a n c e of a clear Fermi-edge s t r u c t u r e in a photoemission spectrum is largely owed to improvement of the energy resolution including the reduced t e m p e r a t u r e - b r o a d e n i n g effect. I n d e e d it was found in this study t h a t the Fermi-edge structure became less resolved and was finally covered by a large background of a prominent peak a t 0.5 eV when the energy resolution was decreased and/or the s a m p l e t e m p e r a t u r e was increased. I t is n o t e d t h a t v e r y r e c e n t l y two i n d e p e n d e n t photoemission research groups [12,13] also found a distinct Fermi-edge structure in the spectrum of K3C6o by employing a high-energy resolved lowt e m p e r a t u r e photoemission measurement. Thus it seems most likely t h a t the improved energy resolution is a key factor to observe the Fermi-edge s t r u c t u r e in the p h o t o e m i s i o n m e a s u r e m e n t .
(x=
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ELECTRONIC STRUCTURE OF KxC60
Another possible origin for the appearance of the clear Fermi-edge structure is improved quality of the sample. Indeed in all three experiments including the present one, the sample was well annealed at relatively high temperature (100 - 150 °C) to reduce the number of defects and disorder in the s a m p l e a n d t h e r e b y i m p r o v e the crystallinity. One may present a question on whether the spectrum really represents K3C6o or there are considerable contributions from other phases. It is well established by the structural study [14] that KxCeo has four distinct phases of C6o, K3C6o, K4C6o, and K6C6o. Therefore, an intermediate composition of KxCeo would be a mixture of these phases. As described above, K-doping creates some additional structures in the energy range from EF to about 2 eV. Since the narrow peak at EF a n d the step-like structure appear from the beginning of doping and show the same intensity variation before disappearing at x=4, these two structures are ascribed to K3C6o not to K4C6o. On the other hand, the broad band at 0.5 eV grows with the doping until x=4 as shown in Fig. 1, so that the broad band might be due to K4C6o. However, it is remarked that even at the early stage of doping ( x = 0 . 6 - 1.2), a broad and almost fiat structure appears in the same energy region. It is evident t h a t this fiat structure is not reproduced by a simple reduction of intensity of the broad band at x=4 since the shape itself is different. Therefore it is concluded that K3C6o has a very broad distribution of density of states spreading from EF to 1.5 eV, together with a narrow peak at EF and a step-like structure at 1.5 eV. A similar broad structure near EF is also seen in the photoemission spectrum of KaCeo reported by Weaver et a1.[12] and Merkel et al.[13]. It is also noteworthy that Dgiorgi et al.[15] found a broad optical excitation in the mid-IR region for doped Ceo and ascribed it to a broad distribution of electronic states near EF as observed in the photoemission experiment. The band structure calculation [9] has predicted that K3C6o is a metal with a half-filled LUMO band at EF, whose occupied part has the width of about 0.2 eV and the HOMO band is located at about 1.5 eV. This prediction is in good agreement with the present experimental result when we ascribe the narrow peak at E P and the step-like structure at 1.5 eV in the photoemission spectrum to the LUMO and the HOMO bands, respectively. However, the observed broad structure spreading from EF to 1.5 eV has no counterpart in the calculation, in which there is a large energy gap between the LUMO and the HOMO band. In the following, we discuss some possible origins for the observed broad structure near EF in K3C6o. We have already shown that it is not ascribable to K4C6o phase. The diluted mixed phase of C6o and potassium (a-phase Ceo) cannot be an origin of this fiat structure since the intensity increases with doping, while the relative ratio of a-phase C6oin the sample should decrease with doping. A simple background due to
1019
secondary electrons cannot produce such a large structure comparable to its main band at EF. A Jahn-Teller effect through local distortion caused by doping m a y produce polaron states in the energy gap of C 6 0 . However, a theoretical estimation [16] of the binding energy is less than a few tens meV, much smaller than the width of the observed broad structure (more than 1 eV). Electron correlation has been suggested as an origin of peculiar behavior of the electronic structure near EF in doped and undoped Ceo [3,6,17]. Lof et a1.[17] have discussed the discrepancy in the size of energy gap of undoped Ceo between the photoemission experiment (about 2 eV) and the band structure calculation (1.5 eV). They estimated the C 2p correlation energy as 1.6 eV by the Auger spectroscopy, which is much larger than the band width (0.5 eV) predicted from the band calculation. Thereby they have proposed that the strong electron correlation considerably modifies the one-electron states in solid C6o. If the strong electron-correlation dominates the electronic structure also in doped C6o, the observed broad structure near EF may be ascribed to an incoherent band (lower Mott-Hubbard band) while the narrow peak at EF being due to a coherent peak. However, this i n t e r p r e t a t i o n looks inconsistent with the present experimental result that K3Ceo is a metal with a sharp peak at EF, since the LUMO band is three-fold degenerate and as a result K3C6o with the 'half-filled' LUMO band should be a Mott-Hubbard insulator with a finite energy gap at EF. Although Lof et al. have suggested that the sharp peak observed at EF in photoemission spectrum is due to doped carriers in the Mott-Hubbard insulator (K3Ceo), we still find a finite density of states at EF for K3Ceo even when t a k i n g account of the u n c e r t a i n t y of estimated x's in the present study (~x = 0.2). We will also discuss the effect of a strong electron correlation in KxC6o in the following in connection with the observed peculiar electronic behavior of K4C6o. We find in Fig. 1 that the photoemission spectrum of K4C60 has three main structures. The first band located at 0.5 eV is ascribed to the partially occupied LUMO band of Ceo since the LUMO band is three-fold degenerate and four electrons per one Ceo molecule are expected to be donated from potassium atoms. Therefore a simple rigid band model and also a band structure calculation have predicted that K4C6o should be a metal. However, as found in Fig. 1, the density of states at EF in K4C6o is negligibly small or rather a finite energy gap of more than 0.2 eV looks to open at EF. Weaver et a1.[12] also reported a quite similar photoemission spectrum exhibiting a negligible intensity at EF for K4Ceo, suggesting its insulating character. However, Merkel et al.[13] presented a slightly different spectrum for K4.2C6o (they did not present the spectrum for K4C6o), which has a small but finite intensity at EF indicative of its metallic nature. Although the origin for this discrepancy is not known at present, it may be attributable to an experimental error in estimating the composition of samples.
1020
ELECTRONIC STRUCTURE OF KxC60
Although the three independent experiments including present one [12,13] employed essentially the same method (HOMO/LUMO intensity ratio and K 3p core-level intensity etc.) to estimate the composition, the reported uncertainity (Ax in KxC6o) ranges from 0.2 to 0.3, so that it may happen t h a t the difference in the estimated composition differes by 0.5 in x among three experiments, which may be responsible for the discrepancy. Merkel et ai.[13] suggested t h a t formation of a K overlayer and its oxidation at the sample surface is a main reason causing an u n c e r t a i n t y in e s t i m a t i n g the composition. Indeed they reported a photoemission spectrum of Kl1~C6o in average composition. However, it is unknown at present how K atoms with a relatively high vapor pressure (about 10 ~ Torr at 430 K, at which they annealed the sample) forms a thin overlayer on KeCeo and changes into the oxide under ultrahigh vacuum. These indicate that a much more precise determination of composition in photoemission measurement is necessary to extract a final conclusion, but it should not be neglected that at least two (Ref.12 and present study) of the three independent experiments agree quite well each other in spite of the reported uncertainty. The insulating or semiconducting nature of K4Ceo has been also suggested by the ~tSR experiment [18]. The energy gap at EF in K4C6o observed by the ~SR experiment (0.3 eV) is in good agreement with the present photoemission study (0.2 - 0.4 eV). Although the origin to produce such an energy gap at EF in K4C6o is not clear at present, a several possible causes are discussed in the following. It is evident that the energy gap observed in the photoemission spectrum of x=4 is not due to K6C6o since, as shown in Fig. 1, the photoemission spectrum of KeCeo has a larger energy gap at EF. The occupied LUMO band of K6C6o is located at 0.9 eV while that of K4C6o is at 0.5 eV, suggesting also that a contribution from K6C6o phase to the photoemission spectrum of x=4 is negligible. A possible contribution from K3C6o phase is rather negative, since K3Ceo has a sharp peak at EF and it completely disappears at x=4 as shown in Fig. 1. All these indicate that the energy gap observed in the photoemission spectrum of x=4 is intrinsic for K4C60. It is known that K4C6o has a body-centeredtetragonal (bct) crystal structure while K3Ceo has a face-centered-cubic (fcc) one. It seems, however, t h a t the difference in the crystal structure is not a cause for the opening of the gap, since the band structure calculation on bct K4C6o has predicted its metallic nature. A considerable local distortion, which may cause a splitting of the LUMO band through a Jahn-Teller effect, has not been observed for K4Ceo. Thus an electron correlation seems to be the most possible origin for the gap. It is noted here that since the LUMO band is three-fold degenerate, a Mott-Hubbard gap can open at each stage of doping of x=l to 5 (integer) in KxCeo if the electron correlation is sufficiently strong. However, it is not clear at
Vol. 87, No. 11
p r e s e n t why the gap opens only at x=4. Considering the experimental fact that K3C6o is a metal with a very narrow band at EF together with the following broad distribution of density of states, it is speculated that the amplitude of the effective correlation energy may change with the doping. The correlation energy may be comparable to the band width in K3CeO so that a sharp coherent peak at EF and a broad incoherent band coexist in the photoemission spectrum, while in K4C60 the correlation energy overcomes the band width and as a result a finite Mott-Hubbard gap may open at EF. In order to check this speculation, further theoretical studies for the effect of strong electron correlation in a degenerate band as in the present case are necessary. Finally, we discuss the strange behavior of the HOMO and HOMO-1 bands against the K-doping. The initial shift of the bands towards the highbinding- energy direction by about 0.3 eV may be due to filling of gap states in pristine Ceo with donated electrons. It is likely that the electrons from K atoms at first occupy the gap states and then go into the LUMO band of Ceo. The gap states, which are located around 0.3 eV above EF, may be produced by defects in the crystal. The gradual approach of the HOMO and HOMO-1 bands to EF upon further doping until x=4 is explained by lattice contraction induced by Kdoping [9]. The observed movement of the HOMO and HOMO-1 bands as well as the occupied LUMO band toward the high-binding energy-direction from x=4 to 6 is due to opening of a new energy gap between the LUMO and the LUMO+I band in KeC6o. The broadening of bands at intermediate (non-stoichiometric) compositions is due to superposition from more t h a n two phases. However, at K4C6o in Fig. 1, the HOMO band appears much broader than that of pristine C60. This may also be a consequence of the abovementioned lattice contraction introduced by Kdoping and resulting increased hybridization among molecular orbitals [9]. 4. Conclusion We found a several anomalous behaviors of the electronic structure in KxCeo by employing the h i g h - e n e r g y resolved low-temperature photoemission spectroscopy. In the photoemission spectrum of K3Ceo, we found a very narrow peak a t E F and a step-like structure at 1.5 eV as well as a broad distribution of density of states between these two structures. Although the former two structures in the spectrum are ascribed to the partially occupied LUMO band and the HOMO band, respectively, the broad distribution of density of states is not understood in terms of the band structure calculation, since there is a large energy gap between the LUMO and the HOMO band in the calculation. We also found that K4C6o is a semiconductor with an energy gap of more than 0.2 eV at EF. This presents a sharp contrast with the band structure calculation which has predicted that K4C6o should be a metal. We have discussed some possible origins for the observed anomalous behaviors in the electronic structure of
Vol. 87, No. 11
ELECTRONIC STRUCTUREOF KxC60
K=C6o and found that a strong electron correlation m a y play a key role in characterizing the electronicstructureof KxC6o.
Acknowledgment- The authors thank K. Kikuchi, S. Suzuki, K. Ikemoto, and Y. Achiba at Tokyo Metropolitan University for providing high purity
1021
C6o powders. They also thank H. KatayamaYoshida at Tohoku University and A. Fujimori at Tokyo University for useful discussions. This work was supported by grants from the Ministry of Education, Culture and Science of Japan and from the Foundation for Promotion of Material Science and Technology of Japan.
References
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