Violation of Kohler’s rule by the magnetoresistance of a quasi-two-dimensional organic metal

Abstract

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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emperature Dependence of the Magnetoresistance Effect in Metals

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