Insulator-Metal Transition and Conduction Processes in Trans-Polyacetylene
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INSULATOR-METAL TRANSITION AND CONDUCTION PROCESSES IN TRANS-POLYACETYLENE ESTHER M. CONWELL, HOWARD A. MIZES AND SURENDAR JEYADEV Xerox Webster Research Center, Webster, NY 14580 ABSTRACT We show by calculations for a chain of stage-1 (16.7%) potassium-doped (CH)x that the metallic state is obtained by adding to the Su-Schrieffer-Heeger Hamiltonian the Coulomb potential of the doping ions and solitons on other chains. Good agreement with the measured Pauli susceptibility is obtained. Adding interchain coupling is found to lead to essentially the same density of states. INTRODUCTION Undoped trans-polyacetylene, (CH)x, has very low conductivity and a gap of -1.8 eV. Doping causes the conductivity o to rise, very rapidly up to a concentration y -1% and then less rapidly as y increases further. The Pauli susceptibility Xp is quite small for y up to -4 to 6%, the value being sample dependent. Beyond that range Xpshows a sharp increase with increasing y [1], taken to be the signature of an insulator-metal transition. Indeed very high values of a, - 105 ohm-'cm-1 , are found in some (CH)x samples [2]. Despite the high Xp and a in samples doped beyond the transition, the conductivity generally behaves in non-metallic fashion, increasing with increasing temperature. This raises the question as to whether highly doped (CH). is really metallic or, more exactly, because doping is known to be inhomogeneous, whether there are any metallic regions in the doped samples. It is clear that the inconsistent behavior of Xp and a results from the inhomogeneous doping. Xp measurements reflect the energy level spacing around the Fermi energy EF in the metallic regions, while the a measurements are likely to be dominated by the nonmetallic regions. Although in most samples a drops by orders of magnitude from T = 300K down to 4K, say, there are some heavily doped samples for which a decreases less than an order of magnitude in this range and also varies little with T at the low temperatures. The latter feature in particular indicates that in these samples there is no gap within the doped regions and that barriers due to high resistance are small enough to be transversed by tunneling. Indeed, very good fits to a vs. T have been found using Sheng's theory [3] of thermal-fluctuation-induced tunneling between metallic particles embedded in an insulator [2]. Although strong Mat. Res. Soc. Symp. Proc. Vol. 173. ©1990 Materials Research Society
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doubts have been expressed as to the detailed applicability of the Sheng theory, the fit to the theory has yielded barrier heights and widths that appear reasonable [2]. For samples whose a changes by less than an order of magnitude from 4 to 300K the barriers are, of course, small compared to kT at 300K. It is not surprising then that for some of these samples a is found to decrease with increasing T in a small range below 300K [4], indicating the presence of phonon scattering. Although optical phonons have too high energy to scatter in this temperature range, there is no reason why acoustic phonon
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