High-field magnetoresistive effects in reduced-dimensionality 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; 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, 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 chapter is to discuss the role that high magnetic fields and magnetoresistance measurements, can play in unravelling the above-mentioned properties of quasi-oneand quasi-two-dimensional organic metals and superconductors. Hence, we shall spend some time discussing the high-field magnetotransport experiments that have helped to measure the Fermi surfaces of charge-transfer salts of molecules such as BETSTTF 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; magnetoresistance phenomena can then be used to delineate the phase boundary of the new state. Examples include field-induced superconductivity [15] and exotic states such as the Fulde-FerrellLarkin-Ovchinnikov (FFLO) phase [16]. The phase diagram of the latter state in κ-(BEDT-TTF)2Cu(NCS)2 is shown in Fig. 1; its derivation is a good illustration of the general utility of high fields and magnetoresistive phenomena. First, a conventional (∼ 30 Hz) measurement of the magnetoresistance was used to very precisely orient the sample in the magnetic field and to measure the superconducting-to-resistive transition [16]. Subsequently, high-frequency (MHz) magnetoresistance measurements that are sensitive to changes in dissipation within the zero-resistance state, allowed the FFLO to type II superconductivity boundary to be measured [16]. In view of recent doubts about the proposed FFLO state in CeCoIn5 [17], organic conductors such as κ-(BEDTTTF)2Cu(NCS)2 [16] and λ-(BETS)2GaCl4 [18] are perhaps as yet the only systems in which the FFLO has been truly observed. Later in this paper, we shall describe other recent observations of field-induced phases in crystalline organic metals, which are related to the FFLO but which result in insulating states.