FERMI SURFACE, TRANSPORT AND SUPERCONDUCTING PROPERTIES OF GRAPHITE INTERCALATION COMPOUNDS

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FERMI SURFACE, TRANSPORT AND

SUPERCONDUCTING PROPERTIES OF GRAPHITE INTERCALATION COMPOUNDS

S. Tanuma, K. Higuchi, H. Suematsu, Y. Koike

To cite this version:

S. Tanuma, K. Higuchi, H. Suematsu, Y. Koike. FERMI SURFACE, TRANSPORT AND SUPER-

CONDUCTING PROPERTIES OF GRAPHITE INTERCALATION COMPOUNDS. Journal de

Physique Colloques, 1978, 39 (C6), pp.C6-1104-C6-1105. �10.1051/jphyscol:19786489�. �jpa-00217971�

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JOURNAL DE PHYSIQUE Colloque C6, supplt+ment au no 8, Tome 39, aoat 1978, page ~ 6 - 1 1 0 4

FERMI SURFACE, TRANSPORT A N D SUPERCONDUCTING PROPERTIES OF GRAPHITE INTERCALATIONCOMPOUNDS

S. Tanuma, K. Higuchi, H. Suematsu and Y. Koike

I n s t i t u t e f o r S o l i d S t a t e P h y s i c s , U n i v e r s i t y o f Tokyo, Rappongi, Minatoku, Tokyo, 106 Japan

R6sum6.- On a construit la surface de Fermi du compos6 intercalaire graphite-potassium C,,K et C,,K afin qu'elle soit adapt6e au cycle observd de Haas-van Alphen. L16tude de la rdsistance magn6tique a permis de prdsenter l'orbite ouverte pr6vue. On observe la supra-conductivit6 du compos6 C,K P 134 mK ; on trouve une forte anisotropie du Hc2. On dtudie aussi les composds graphite-brome par des m6thodes similaires.

Abstract.- The Fermi surfaces of graphite-potassium intercalation compounds, C,,K and C,,K, were composed in such a way that the observed de Haas-van Alphen periods were fitted. The predicted open orbits were proved by the magneto-resistance study. Superconductivity of C,K compound was observed at 134 mK, A large anisotropy'of Hc2 was found. Graphite-bromine compounds were also studied by a similar method.

Intercalation of atoms or molecules into graphite attracts recent attention /l/ in the wiew points of electronic structure, transport properties, electronic phase changes of superconductivity of CDW possibility and phonon modes. We have investigated low temperature properties of C K and CBr as the

X Y

typical intercalation compounds of donor and accep- tor types respectively. We prepared pseudo-single crystals of CIznK (n = 4,3,2) and CBr0.021- 0.045 using highly oriented pyrolytic graphite given by

Dr. Moore of Union Carbide Co.. The number n means that n hexagonal carbon layers exists between adja- cent intercalated layers to make a superlattice structure along the c-axis, and n is called the stage number.

The de Haas-van Alphen (dHvA)eff ect was observed at 1.3 K in most of the prepared crystals. Figu- re 1 shows the field angle dependence of the dHvA periods. The dashed lines (cos 8 ) correspond to cylindrical Fermi surfaces (FS). The lines are adjus- ted with the experimental points at 9 = 0'. For the 4th stage compound C,, K, five branches a, B, y, 6 and E are found and their values at O0 are smaller by two orders of magnitudes than the values of pure graphite (1.60 X 1 0 - ~ 0e-' for electron and

2.16 X 1 0 - ~ 0e-' for hole). Except the E-branch which has not enough observed points, every branch

suggests rather thin FS. For the most intercalated compound C,K only one value of 3.49 X 10" 0e-' is observed at present, which agrees fairly with the calculated value 2.8 X 10-' 0e-l corresponding to the cylindrical FS in the band calculation by

9

(angle between c-axis and H)-,

Inoshita et al. 1 2 1 . No less dHvA period than the a-branch is considered to exist in C,,K because

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19786489

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much l e s s p e r i o d i s found i n C8K sample o f which q u a l i t y should b e worse than high s t a g e compounds ; t h e r e f o r e t h e a-branch should correspond t o t h e l a r - g e s t FS i n Ch8K. We adopted t h e r i g i d band model of g r a p h i t e and l e t t h e observed p e r i o d 2.96 X 1oe7 0e-' a t 8 = 0' i n t h e a-branch t o f i t t h e s e c t i o n a l a r e a a t K-point of t h e B r i l l o u i n Zone (BZ) and we o b t a i - ned t h e Fermi energy of 0.42 eV which i s compared

t o 0.018 eV i n pure g r a p h i t e . The c a l c u l a t e d FS i s much l i k e a n e l l i p s o i d i n t h e extended BZ, and t h e h o l e FS i n g r a p h i t e disappears.The 4 t h s t a g e compound has twice t h e c-axis p e r i o d o f t h e pure gra- p h i t e , i . e . , t h e BZ h a s h a l f t h e t h i c k n e s s of t h e o r i g i n a l one. We made t h e FS of C r 8 K by f o l d i n g t h e r i g i d band FS i n t o t h e h a l f t h i c k BZ and i n t r o d u - c i n g energy gaps a t t h e new zone edge. F i g u r e 2 shows t h e e l e c t r o n FS t h u s c o n s t r u c t e d . The f i g u r e i s drawn a s t h e extended zone scheme and t h i c k l i n e

FEPJll SURFACE OF Co8K

shows t h e FS i n t h e u n i t zone fiI(ii. The t h r e e k i n d s o f FS a p p e a r s ; t h e s m a l l e s t one i s a closed FS and t h e o t h e r two a r e open. By f i t t i n g t h e s e c t i o n a l a r e a S1 a t K-point, t h e observed v a l u e s of S2 t o S5 a r e f a i r l y compatible w i t h t h e r i g i d band model i f we i g n o l e t h e s e p a r a t i o n s of S2 and S3 and of S, and Ss, a s i s shown i n t h e i n s e r t e d t a b l e i n f i g u r e 2.

The FS of C 3 6 K was c o n s t r u c t e d i n a s i m i l a r way.The- r e were p r e d i c t e d open o r b i t s i n C K(n=4,3,2) a s

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w e l l a s i n C8K 121. We measured t h e f i e l d a n g l e dependence o f t h e t r a n s v e r s e magneto-resistance a t low temperatures and found a hamp o r a peak cente- r e d a t 8 = 90' (HL c - a x i s ) i n c o n t r a s t t o t h e z e r o magneto-resistance a t 8 = 90° f o r pure g r a p h i t e .

temperature was 134 and 130 mK f o r two samples. The samples a r e s q u a r e shaped l a m e l l a r p l a t e s of which edge l e n g t h do d e p t h r a t i o a r e about 4. The v a l u e s of Hcl, (Hel w i t h o u t t h e c o r r e c t i o n of demagnetiza- t i o n f a c t o r ) and Hc2 a r e 6 . 0 Oe and about 50 Oe f o r H // c-axis, and 20 Oe and

<

^Ik o e f o r

HL

^c-axis

r e s p e c t i v e l y (Fig. 3 ) . The measured Tc i n t h e s e r a t h e r w e l l d e f i n e d C8K compounds i s lower t h a n t h e

e a r l y r e p o r t e d v a l u e 550 mK i n t h e sample wet with potassium m e t a l /3/ and h i g h e r t h a n t h e v a l u e 80 mK i n t h e powder samples 141.

By t h e s i m i l a r experimental i n v e s t i g a t i o n s on CBr i n t e r c a l a t i o n compoitnds, we concluded t h a t t h e r e s i d u e compounds a r e m u l t i p a s e . The main compound Y

i s probably CQ2(Br2) i r r e s p e c t i v e t o t h e y-value.

This compound makes i s l a n d s i n t h e o r i g i n a l h i g h l y o r i e n t e d g r a p h i t e , and t h e y-value determines t h e m01 . r a t i o of i n t e r c a l a t e d t o pure g r a p h i t e .

References

/ l / Conference on G r a p h i t e I n t e r c a l a t i o n Compounds, 1977, La Napoule

/2/ I n o s h i t a , T., Nakao, K. and Kamimura,H., J . Phys.

Soc. J p n

9

(1977) 1237

/ 3 / Hannay, N.B. e t a l . , Phys. Rev. L e t t .

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(1965) 225

T h i s f a c t proves t h e e x i s t e n c e of t h e open o r b i t s / 4 / Kobayashi, M. and Tsujikawa, I., Meeting of

a t t h i s c o n f i g u r a t i o n . Phys. Soc. Japan (1977)

The s u p e r c o n d u c t i v i t y of C,K was observed on s i m i l a r samples t o t h e above one. The t r a n s i t i o n