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303 SPIN SUSCEPTIBILITY OF INTERCALATED GRAPHITE AND DOPED POLYACETYLENE JAMES W. KAUFER Department of Physics, Pennsylvania 19104

University of Pennsylvania, Philadelphia,

SEIICHIRO IKEHATA Department of Physics, University of Pennsylvania, Philadelphia, Pennsylvania 19104 and Department of Physics, University of Tokyo, Tokyo 113 Japan

ABSTRACT We have examined the electronic susceptibility of intercalated graphite and doped (CH)x using ESR [li . The dopant in both cases was AsF 5 . For metallic intercalated graphite and for heavily doped (CH)x, the low frequency Schumacher-Slichter technique was used to determine the density of states at the Fermi energy. In the case of doped (CH),, susceptibility measurements as a function of temperature allowed separate determination of the Curie and Pauli contributions.

INTRODUCTION Intercalated graphite and doped polyacetylene have recently attracted a great deal of attention. Contributing to the interest is the wide variety of intercalant [23 and dopant [3-73 species available, the very high conductivities obtained with some dopants, and the structural anisotropy of these materials indicating the possibility of unusual transport mechanisms.J8,9J A large number of donor and acceptor atomic and molecular species may be intercalated in graphite. Like the parent graphite, these materials exhibit large anisotropies in conductivity. The AsF 5 - graphite 16 compounds yield particularly high anisotropies, 0a/ac aclcsa 10 , with a close to that of copper.ElO3 This anisotropy leads to the expectation that these compounds may be treated as 2-d metals, using a graphite rigid-band model. [11] By using magnetic resonance techniques, we have measured the density of states at the Fermi energy in stages I and II AsF 5 -graphite. Use of the rigid band model allowed determination of the fractional charge transfer to the intercalant. Polyacetylene in its undoped state is the simplest linear conjugated polymer. It is a direct gap semiconductor with a band gap of -1.4 eV. By doping with various donor or acceptor compounds the conductivity may -9 3 -l be varied from 10 up to >10 (0-cm) Upon doping the conductivity rises, goes through a semiconductormetal transition at a few percent doping level, after which the conductivity slowly increases up to the maximum doping level of about fourteen percent. Although the bulk properties of (CH)x show a small anisotropy[12], NMR studies show that the intrinsic anisotropy may be very large.L13]

304 This l-d character along with the broken symmetry in the ground state allows for the possibility of describing the transport in the lightly doped polymer in terms of nonlinear solitons[8,9,14] rather than in terms of Again, using magnetic resonance we traditional semiconductor doping. have measured the Curie and Pauli susceptibilities as a function of doping in order to elucidate the nature of the transport in lightly doped (CH)x and the semiconductor-metal transition. The susceptibility measurements were carried out using the SchumacherSlichter technique, whi