The interlayer magnetoresistance of the quasi-two-dimensional metal ␣-͑BEDT-TTF͒ 2 KHg͑SCN͒ 4 is considered. In the temperature range from 0.5 to 10 K and for fields up to 10 T the magnetoresistance has a stronger temperature dependence than the zero-field resistance. Consequently Kohler's rule is not obeyed for any range of temperatures or fields. This means that the magnetoresistance cannot be described in terms of semiclassical transport on a single Fermi surface with a single scattering time. Possible explanations for the violations of Kohler's rule are considered, both within the framework of semiclassical transport theory and involving incoherent interlayer transport. The issues considered are similar to those raised by the magnetotransport of the cuprate superconductors. ͓S0163-1829͑98͒13219-8͔Currently a great deal of attention is being paid to the large magnetoresistance of layered materials such as magnetic multilayers 1 and manganese perovskites. 2 This is motivated by potential applications in magnetic recording and by the challenge of understanding the physical origin of the magnetoresistance, which is very different from that in conventional metals. 3 The magnetotransport of the metallic phase of the cuprate superconductors also differs significantly from conventional metals. [4][5][6] In this paper we show that the magnetoresistance of a particular organic metal may also be unconventional.Layered organic molecular crystals based on the bis͑ethylenedithia-tetrathiafulvalene͒ ͑BEDT-TTF͒ molecule are model low-dimensional electronic systems. 7,8 The family ␣-͑BEDT-TTF͒ 2 M Hg͑SCN͒ 4 [M ϭK,Rb,Tl͔ have a rich phase diagram depending on temperature, pressure, uniaxial stress, and magnetic field: metallic, superconducting, and density-wave phases are possible. 9-11 Band-structure calculations predict coexisting quasi-one-dimensional ͑open͒ and quasi-two-dimensional ͑closed͒ Fermi surfaces. 12 At ambient pressure these materials undergo a transition at a temperature T DW ͑8 K in the MϭK salt͒ into a low-temperature metallic phase that has been argued to be a density wave ͑DW͒. This phase is destroyed in high magnetic fields. There is currently controversy as to whether this phase is a spin-density wave, a charge-density wave, or a mixture of both. 9,[13][14][15][16] The following picture of the low-temperature phase has been proposed. 17,18 The nesting of the quasi-one-dimensional Fermi surface leads to a density-wave instability at T DW . Below T DW a gap opens on the quasi-one-dimensional Fermi surface and the associated carriers no longer contribute to the transport properties. The density wave introduces a new periodic potential into the system resulting in reconstruction of the quasi-two-dimensional Fermi surface. One of the proposed Fermi surface reconstructions involves large open sheets. 17 Semiclassical transport theory can then explain the large magnetoresistance and its angular dependence in the low-temperature phase. 18 The complete field dependence of the resistance can also be explained if ...

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We consider the magnetoresistance oscillation phenomena in the Bechgaard salts (TMTSF) 2 X, where X ϭClO 4 , PF 6 , and AsF 6 in pulsed magnetic fields to 51 T. Of particular importance is the observation of a new magnetoresistance oscillation for XϭClO 4 in its quenched state. In the absence of any Fermi-surface reconstruction due to anion order at low temperatures, all three materials exhibit nonmonotonic temperature dependence of the oscillation amplitude in the spin-density-wave ͑SDW͒ state. We discuss a model where, below a characteristic temperature T* within the SDW state, a magnetic breakdown gap opens.

We present a unique high magnetic field phase of the quasi-one dimensional organic conductor (TMTSF) 2 ClO 4 . This phase, termed "Q-ClO 4 ", is obtained by rapid thermal quenching to avoid ordering of the ClO 4 anion.The magnetic field dependent phase of Q-ClO 4 is distinctly different from that in the extensively studied annealed material. Q-ClO 4 exhibits a spin density wave (SDW) transition at ≈ 5 K which is strongly magnetic field dependent. This dependence is well described by the theoretical treatment of Bjelis and Maki. We show that Q-ClO 4 provides a new B-T phase diagram in the hierarchy of low-dimensional organic metals (one-dimensional towards two-dimensional), and describe the temperature dependence of the of the quantum oscillations observed in the SDW phase.