Magnetic Resonance as a Probe of Anisotropic Conductors
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MAGNETIC RESONANCE AS A PROBE OF ANISOTROPIC CONDUCTORS
W. G. CLARK Physics Department, University of California at Los Angeles, California 90024
Los Angeles,
ABSTRACT A review is given of the wide range of magnetic resonance methods used to study the special properties of quasi onedimensional (l-d) conductors. Specific examples are presented which show how magnetic resonance methods have been exploited (a) to tell if the conduction is l-d, (b) to locate the conduction electrons on a molecular distance scale, (c) to probe the unusual effects of disorder in l-d conductors, (d) to study the charge density wave transition, and (e) to verify exotic electrical transport mechanisms, such as charged solitons in trans-(CH)x
INTRODUCTION In the past few years, a large number of highly anisotropic, or quasi onedimensional (l-d) conductors have been discovered. They have evoked strong interest on the part of researchers because many of them are organic metals and because l-d conductors exhibit special properties and exotic conduction mechanisms which are particular to their one-dimensionality. Some examples of these are: (a) their instability against a Peierls, or charge density wave (CDW) transition [11 from a high temperature metallic state to a low temperature (modified) metallic, semiconducting, or insulating state, (b) nonlinear and frequency dependent conduction by the CDW's 12], (c) conduction by charged solitons in lightly doped polyacetylene, (CH)x [3,41, and (d) the unusual importance of disorder on the properties of l-d conductors 15-7]. A wide range of magnetic resonance methods have been employed to investigate some of the most basic questions posed by l-d conductors. In this paper, several examples are described which demonstrate how magnetic resonance experiments have been used to study the basic properties of l-d conductors. Even though this session of the Symposium is formally on NMR, I shall include as well such topics as ESR and three types of double resonance, as all have played a key role in research on l-d conductors. The rest of this paper is organized in terms of several key questions regarding l-d conductors. In order to keep this paper from being unduly long, some topics which could be reported have been left out, and the goal of a truly complete bibliography has not been met. Instead, the more recent or comprehensive references are emphasized, as they are most useful at leading the interested reader to a more complete bibliography. IS THE CONDUCTION REALLY 1-D? The essential feature of a l-d conductor is that the motion of the electrons be along only one direction in the solid 18]. For any material proposed to be a l-d conductor, it is important that this point be established and, if possible, the microscopic transport times be established. The traditional and most widely used method to investigate this question has been to measure the electric conductivity (a) parallel and perpendicular to the high conductivity
46 axis. Such measurements are done over macroscopic distances and may be misl
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