1 6 O ct 2 00 6 1 High - field magnetoresistive effects in reduced - dimensionality organic metals and superconductors

1 High-field magnetoresistive effects in reduced-dimensionality organic metals and superconductors Summary. The large charge-transfer anisotropy of quasi-one-and quasi-two-dimensional crystalline organic metals means that magnetoresistance is one of the most powerful tools for probing their bandstructure and interesting phase diagrams. Here we review various magnetoresistance phenomena that are of interest in the investigation of metallic, superconducting and charge-density-wave organic systems. Quasi-two-dimensional crystalline organic metals and superconductors are very flexible systems in the study of many-body effects and unusual mechanisms for superconductivity [1–7]. Their " soft " lattices enable one to use relatively low pressures to tune the same material through a variety of low-temperature groundstates, for example from Mott insulator via intermin-gled antiferromagnetic and superconducting states to unusual superconduc-tor [4, 6, 7]. Pressure also provides a sensitive means of varying the electron-phonon and electron-electron interactions, allowing their influence on the superconducting groundstate to be mapped [3, 4, 8]. The self-organising tendencies of organic molecules means that organic metals and superconductors are often rather clean and well-ordered systems; as we shall see below, this enables the Fermi-surface topology to be measured in very great detail using modest magnetic fields [3,9]. Such information can then be used as input parameters for theoretical models [3]. And yet the same organic molecules can adopt a variety of configurations, leading to " glassy " structural transitions and mixed phases in otherwise very pure systems [4, 10, 11]; these states may be important precursors to the superconductivity in such cases [11]. Intriguingly, there seem to be at least two (or possibly three) distinct mechanisms for superconductivity [3,12–14] in the quasi-two-dimensional organic conductors. The first applies to half-filled-band layered charge-transfer salts, such as the κ−, β− and β ′ − packing arrangements of salts of the form (BEDT-TTF) 2 X, where X is an anion molecule; the superconductiv-ity appears to be mediated by electron correlations/antiferromagnetic fluctuations [3–5]. The second mechanism applies to e.g. the β ′′ phase BEDT-TTF salts [4]; it appears to depend on the proximity of a metallic phase