Highly-angle-resolved ultraviolet photoelectron spectroscopy of C8Cs

Solid State Communications,Vo1.59,No.3, pp.105-109, 1986. Printed in Great Britain.

0038-1098/86 $3.00 + .OO Pergamon Journals Ltd.

HIGHLY-ANGLE-RESOLVEDULTRAVIOLET PHOTOELECTRON SPECTROSCOPY OF C&s T. Takahashi, N. Gunasekara, and T. Sagawa Department of Physics, Tohoku University, Sendai 980, Japan H. Suematsu Department 6f Materials Sciences, Tsukuba UniversitY, Ibaraki (Received

305,

14 April

1986

Japan by T.

Tsuzuki)

y-angle-resolved ultraviolet photoelectron spectroscopy out for a C Cs single crystal to study the electronic was &?&I charge‘“tr&fer in alka B1 metal graphite intercalation compounds. The dispersive n*-band at the R point in the Brillouin zone was observed for the first time. The electron occupation in the n*band was estimated to be 0.45M.05 unit electronic charge. This strongly suggests that a substantial part of an interlayer band exists below the Fermi level at the r point, forming a spherical Fermi surface on the center of the Brillouin zone.

at the r point. However, it seems very peculier that either of the two w-OUPS did not observe any dispersive features of the x*-band at the”K point, although the x*-band should have a substantial volume below the Fermi level when nearly one unit electronic charge is transferred into it from the s states of alkali atoms. In this letter, we report the first direct observation of the energy dispersion of the n*band of C8Cs. BY estimating quantitatively the electron occupation in the x*-band, we discuss and re-examine the charge transfer in C8Cs as well as the shape of the Fermi surface. The volume of the x*-band was proved to be the most sensitive measure to the charge transfer in GICs. Before proceeding to the experimental results, we should remark a recent re-interpretation of the band calculation of Ohno, Nakao, and Kamimura ClOl. According to it, the nearlyfree-electron-like band just below the Fermi level at the r point, which forms a spherical Fermi surface, has a strong character of interlayer states of graphite different from the alkali s states. Thus, the main discrepancy between the two models C2,41 is not the charge transfer from the alkali atom to graphite layer, but the balance of the electronic charge between the $-band and the interlayer band. Anyway, whether the spherical Fermi surface exists I21 or not E41 at the r point is a key problem in

The electronic structure of the first stage alkali metal graphite intercalation compounds (GICsl has been the subject of recent theoretiInoshita, Nakao, cal and experimental debate. and Kamimura Cl1 carried out the first tightbinding band calculation for C8K and Predicted a Fermi surface consisting of a large nearly-freeelectron-like portion centered on the r point of the Brillouin zone together with almost cylindrical pieces at the zone boundary. A subsequent self-consistent calculation by Ohno, Nakao, and Kamimura I21 yielded a very similar band structure to that of Inoshita et al. In spite of the massive experimental support for this band structure C31, a number of papers have recently questioned its validity on its charge transfer ratio. The KKRband calculation by DiVincenzo and Rabii C41 placed the bottom of the K 4s band about 1.8 eV ahove the Fermi level, implying a complete charge transfer. The Fermi surface proposed by DiVincenzo and Rabii I41 consists of only cylindrical parts at the zone boundary without a spherical one on the Brillouin zone Some electron C5.61 and photoemission center. C7.8.91 spectroscopies were subsesuently presented in support of the calculation by Recent two angle-resolved DiVincenzo and Rabii. ultraviolet photoelectron spectroscopies fARUPS) C8,91 presented experimental two-dimensional band structure of C8Cs and both of them claimed that there are no nearly-free-electron-like band 105

106

ULTRAVIOLET PHOTOELECTRON SPECTROSCOPY

understanding basic properties (e.g. superconductivity [II] etc.) of alkali metal GICs. In the latter part of this letter, we call the nearly-free-electron-like band at the r point as an interlayer band. A single crystal of C8Cs (about 3x3xO.5 mm 3) was prepared by the two-temperature method with a synthesized single crystalline graphite (kish graphite). Photoemission spectra were measured using a highly-angle-resolved ultraviolet photoelectron spectrometer constructed at our laboratory [12]. The base pressure is about IxlO -I0 Tort and its working pressure during the lamp o p e r a t i o n is about 6x10 -10 T o r r . The a n g u l a r and energy r e s o l u t i o n s a r e about i . 5 ° and 0 . ! eV, r e s p e c t i v e l y . The C8Cs sample was p e e l e d o f f a l o n g t h e basal p l a n e i n t h e spect r o m e t e r by an a d h e s i v e t a p e i n o r d e r t o o b t a i n a c l e a n and a t o m i c a l l y f l a t s u r f a c e of t h e C8Cs crystal. The sample was k e p t a t a b o u t -80°C d u r i n g t h e measurement t o p r e v e n t c e s i u m atoms from s u b l i m i n g i n t o vacuum. The r e l a t i v e o r i e n t a t i o n o f t h e c r y s t a l axes of t h e s a m p l e t o t h e e l e c t r o n e n e r g y a n a l y z e r was d e t e r m i n e d by t h e a z i m u t h a l and p o l a r a n g l e d e p e n d e n c e o f ARUPS spectra. The m i s o r l e n t a t i o n of t h e a z i m u t h a l a n g l e was e s t i m a t e d t o b e w i t h i n 0 . 5 ° . The u n p o l a r i z e d l i g h t (He I , 2 1 . 2 eV) was i n c i d e n t a t a b o u t 45 ° o n t o t h e s a m p l e s u r f a c e . F i g u r e l shows t h e B r i l l o u l n zone of C8Cs t o g e t h e r w i t h t h a t of g r a p h i t e . P h o t o e m i s s i o n m e a s u r e m e n t s were c a r r i e d o u t a l o n g t h e l ~ direction since the n~-band ls located at the point. F i g u r e 2 shows a normal e m i s s i o n s p e c t r u m of C8Cs. We f i n d f o u r p r o m i n e n t s t r u c t u r e s A - D l o c a t e d a t a b o u t 0 . 3 , 4 . 3 , 6 . 1 , and 9 . 7 eV, respectively. The b a n d s A - D a r e a s s i g n e d t o the interlayer band, the folded n-band, the u n f o l d e d a - b a n d , and t h e u n f o l d e d ~ - b a n d , respectively. S i n c e i t was found t h a t t h e band A ( t h e i n t e r l a y e r band) d i d n o t show any r e m a r k a b l e e n e r g y d i s p e r s i o n i n t h e He I p h o t o e m i s s i o n s p e c t r a as r e p o r t e d i n t h e p r e v i o u s s t u d i e s of C8Cs [9] and CsK [ 1 3 ] , i t i s t o o h a r d t o d i s c u s s the charge balance only with the observed feat u r e of t h e i n t e r l a y e r b a n d . T h e r e f o r e , we r e m a r k e d t h e n ~ - b a n d a t t h e K p o i n t , which i s an a l t e r n a t i v e s e n s i t i v e measure t o t h e c h a r g e b a l a n c e between t h e ~ - and t h e i n t e r l a y e r b a n d . F i g u r e 3 shows ARUPS s p e c t r a of C8Cs measured a l o n g t h e FK d i r e c t i o n . Polar angle r e f e r r e d t o t h e s u r f a c e normal i s i n d i c a t e d on each spectrum. ARIJPS s p e c t r a of t h e p o l a r a n g l e 0 : 21.5 ° t o 35 ° c o r r e s p o n d t o t h e r e g i o n o f t h e

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(b) F l g . 1 (a) g r i l l o u l n zone o f C8Cs and (b) twod i m e n s i o n a l B r i l l o u i n z o n e s of C8¢s ( b r o k e n l i n e ) and g r a p h i t e ( s o l i d l i n e ) .

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EF

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]07

ULTRAVIOLET PHOTOELECTRON SPECTROSCOPY

folded ~ - b a n d while spectra of 8 = 45 ° to 70 ° r e p r e s e n t the range of the unfolded ~ - b a n d {see the B r l l l o u i n zone in Fig. l ) . In Fig. 3, we !

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ARUPS s p e c t r a o f 8 = 45 ° - 70 = i n t h e h i g h e r b i n d i n g energy ( 1 . 5 - 3.5 eV} r e g i o n . This d i s p e r s i v e f e a t u r e corresponds t o t h e u n f o l d e d n-band. The present ARUPS study Is the first to

observe the dispersive feature of the ~*-band. The reason why the earlier ARIPS studies [8,9] could not find the dispersive ~ - b a n d is not known at present, but might be mlsorientation of the azimuthal angle, lower anqular resolution, or deterioration of the sample surface. In Fig. 4, we show the two-dlmensional band Binding

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Fig. 3 Angle-resolved ultraviolet photoemission spectra of C8Cs measured with the He I resonance llne alonq the l-1~direction in the Brillouin zone. Polar angle referred to the surface normal is indicated on each spectrum. clearly f i n d the dispersive feature of the unfolded ~-band, which adruptly appears at the Fermi level in the ARUPS spectrum of 0 = 45 °, then shows a prominent energy dispersion by about I eV with increase of the polar angle, and again crosses the Fermi level at 8 = 70 ° . Another dispersive feature is also found in the

Fig. 4 Experimental band structure of C8Cs in the ~-band reqlon determined from ARUPS spectra in Fig. 3. Filled circles and crosses represent strong and weak structures in the ARUPS spectra, respectively. The calculated ~*-bands of Ohno, Nakao, and Kamimura (ONK, solid line) and DiVincenzo and Rabll (DR, broken llne) are also shown for comparison.

s t r u c t u r e of C8Cs in the n*-band reglon determined from the p r e s e n t ARUPS s p e c t r a using the following formula, ~ik, = [ 2m( R o - E~ - e@ )]I/2sin0 , (I) where k, is the wave vector parallel to the surface, f~othe photon energy, E B the binding energy of peaks in the ARUPS spectra, e~ the work function of C8Cs, and 0 the polar angle referred to the surface normal. Filled circles and crosses in Fig. 4 represent strong and weak peaks in the ARUPS spectra, respectively. The calculated n*-bands of Ohno, Nakao, and Kamimura (ONK, solid line)[2] and DiVincenzo and Rabll (DR, broken llne)[4] are also shown in Fig. 4

I08

ULTRAVIOLET PHOTOELECTRON SPECTROSCOPY

for comparison. (Note that whole the n*-band is not shown for the DR calculation because they did not present it in their paper [4].) As is seen in Fig. 4, the observed unfolded ~-band shows a prominent energy dispersion by about I eV at the K point (at k, = 1.7 ~-I). The unfolded n-band also shows a notable energy dispersion at the same K point. Similar structures, although they are weak, are seen at the point of the first Brillouin zone of CsCS (at k, = 0.85 ~-I). These structures are due to the folded n*-and n-bands. The Y point is the center of the mirror symmetry between the folded and unfolded bands (see the Brillouin zone in Fig. I). This observation of folding of the bands indicates that the surface region of the sample probed by ARUPS has the same 2x2 superstructure as in the bulk. Next we discuss the number of electrons which occupy the ~-band. The electron population is estimated by the slope of the energy dispersion of the ~-band and the shift of the Fermi level referred to that of pristine graphite [14]. In the numerical estimation, we used 5.4 eV-~ as the slope of the energy dispersion, which is the average of the two values for the two different slopes directed from the ~ point to the Y and F points {5.8 and 5.0 eV-~}. As for the shift of the Fermi level, we referred to the middle point between the bottom of the n*-band and the top of the n-band at the K point (1.3 - 1.4 eV}. Using these parameters to the calculated density of states of the two-dimensional band structure of graphite [15l, we obtained 0.45.-~0.05 unit electronic charge as the electron occupation of the experimental ~*-band. We find that this value is fairly reasonable when we compare the experimental ~-band with the two calculated ~ bands (Fig. 4), because the ~*-bands of the DR and ONK calculations accommodate 1.0 and 0.57 unit electronic charge, respectively. Thus, the present experimental result definitely shows that nearly a half of the Cs 6s electrons is transferred to the n*-band of graphite. This presents a sharp contrast to the earlier ARUPS studies [8,9]; they claimed that all the Cs 6s electrons are moved to the x*-band although they did not observe any dispersive features of the n*-band. The present experimental result strongly suggests that another half of unit electronic charge occupies the interlayer band at the F point in order to keep the charge neutrality of the system. This indicates that a

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spherical Fermi surface formed by the interlayer band does exist on the center of the Brillouin zone as predicted by Kamimura and his co-workers [1,2]. It is noteworthy here that a similar experimental result has been also obtained for C8K [131. Finally we will c o m e n t on the observed non-dispersive feature of the i n t e r l a y e r band, which should have a n e a r l y - f r e e - e l e c t r o n - l i k e dispersion. At present, we have attributed it to the large interlayer energy dispersion. In ARUPS measurements with a fixed photon-energy as in t h e p r e s e n t c a s e , we can know o n l y t h e wave v e c t o r p a r a l l e l t o t h e s u r f a c e (k~t) b u t n o t p e r p e n d i c u l a r t o t h e s u r f a c e (k±}. We t h i n k t h a t i n t h e p r e s e n t measurement w i t h t h e He I r e s o n a n c e l i n e , t h e kx of t h e i n t e r l a y e r band may be accidentally located close to the Z point not t o t h e F p o i n t ( s e e F i g . 1). S i n c e t h e i n t e r l a y e r band h a s a n e a r l y - f r e e - e l e c t r o n - l i k e energy dispersion, it disperses very close to t h e Fermi l e v e l a t t h e zone b o u n d a r y (Z p o i n t ) . T h e r e f o r e , i f t h e k± i n t h e p h o t o e m i s s i o n measurement is located close to the Z Point, it is very hard to observe its energy dispersion. Another possible process to smear out the dispersive feature of the interlayer band may be an overlapping from the indirect transition from the n*-band. Anyway, the observed non-dlsperslve feature of the interlayer band is a future problem to be solved. In conclusion, we have carried, out a highly angle-resolved ultraviolet photoelectron spectroscopy for a CsCs single crystal and observed the dispersive ~-band at the K point for the first time. The electron occupation of the n*band was estimated to be 0.4Rk0.05 unit electronic charge. This indicates that another half of the unit electronic charge occupies the interlayer band at the F point. The present experimental result prefers the band calculation by Kamlmura and his co-workers [1,2] who predicted a spherical Fermi surface on the center of the Brillouln zone toqether with the cylindrical pieces at the zone boundary.

Ackncywledgeraent - We thank Mr. Minagawa for his collaboration in preparing the C8Cs sample. We also thank Dr. Saito for giving us his calculation of the density of states of graphite. This work was financially supported by the grants from Ministry of Education and The Kurata Foundation.

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ULTRAVIOLET PHOTOELECTRON SPECTROSCOPY ~h-EI~CES

I T. Inoshita, K. Nakao, and H. Kamimura, J. Fnys. Soc. Japan, 41, 1480 (1976). 2 T. Ohno, K. Nakao, and H. Kamimura, J. Phys. Soc. Japan, 47, 1125 (1979}. 3 For example, see M.S. Dresselhaus and G. Dresselhaus, Adv. Phys. 30, 139 (1981). 4 D.P. DiVincenzo and S. Rabii, Phys. Rev. B 25, 4110 (1982). 5 J.J. Ritsko and C.F. Brucker, S o l i d State Commun. 44, 889 (1982). 6 L.A. Grunes and J.J. Ritsko, Phys. Rev. B 28, 3439 (1983}. 7 M.E. Pretl and J.E. Fischer, Phys. Rev. Lett. 52, I141 (1984).

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C. Fretigny, D. Marchand, and M. Lagu~s, Phys. Rev. B 32, 8462 (1985). 9 M.T. Johnson, H.I. Starnberg, and H.P. Hughes, Solid State Commun. 57, 545 (1986). I0 H. Kamimura, Annal. Phys. (1986), in press. II R. AI-Jishi, Phys. Rev. B 28, I12 (1983}. 12 S. Suzuki, K. Furusawa, M. Terakura, a. Yoshida, and T. Sagawa, Sci. Rep. Tohoku Univ. Ser. 8, I, 16 (1980). 13 T. Takahashi, N. Gunasekara, T. Sagawa, and H. Suematsu, to be published. 14 R. Saito, Doctor Thesis (The University of Tokyo),1984. 15 P.R. Wallace, Phys. Rev. 71, 622 (1947).

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