High-field magnetoresistive effects in low-dimensional organic metals and superconductors

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, 2, 3, 4, 5, 6, 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 intermingled antiferromagnetic and superconducting states to unusual superconductor [4, 6, 7]. Pressure also provides a sensitive means of varying the electron-phonon and electronelectron 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, enabling the Fermisurface 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, 13, 14] in the quasi-two-dimensional organic conductors. The first applies to half-filled-band layered chargetransfer salts, such as the κ−, β− and β′− packing arrangements of salts of the form (BEDT-TTF)2X, where X is an anion molecule; the superconductivity appears to be mediated by electron correlations/antiferromagnetic fluctuations [3, 4, 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 to charge order [11, 12, 13]. Finally, there may be some instances of BCS-like phonon-mediated superconductivity [14]. The main purpose of this paper is to discuss the role that high magnetic fields play in unravelling the magnetoresistive properties of quasi-two-dimensional organic metals and superconductors; hence we shall spend some time discussing the high-field magnetotransport that have helped measure the Fermi surface of charge-transfer salts or molecules such as BETD-TTF and BETS. In addition to their invaluable role in mapping the bandstructure, high magnetic fields allow one to tune some of the organic conductors into some new and intriguing phases; examples include field-induced superconductivity [15] and exotic states such as the Fulde-FerrellLarkin-Ovchinnikov (FFLO) phase [16] (Fig. 1). Later in this paper, we shall describe other recent observations of field-induced phases in crystalline organic metals, which is related to the FFLO but which results in an insulating state.