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 ...

We report comprehensive (magneto)transport studies of the two-phase state in (TMTSF) 2 ClO 4 , where superconducting (SC) phase coexists with spin-density wave insulator (SDW). By tuning the degree of ClO 4 anion ordering in controlled manner we smoothly suppress the SDW state and study resulting evolution of the SC phase spatial texture. We find that as SDW is suppressed, SC regions initially appear inside the SDW insulator in a form of filaments extended in the interlayer direction and further merge into the two-dimensional sheets across the most conducting axis of the crystal. We demonstrate that almost all our results can be explained within the soliton phase model, though with several assumptions they can also be related with the creation of non-uniform deformations. We believe that the anisotropy is intrinsic to SC/SDW coexistence in various quasi one-dimensional superconductors.

We report detailed angle-dependent studies of the microwave (ν = 50 to 90 GHz) interlayer magneto-electrodynamics of a single crystal sample of the organic charge-density-wave (CDW) conductor α-(BEDT-TTF) 2 KHg(SCN) 4 .Recently developed instrumentation enables both magnetic field (B) sweeps for a fixed sample orientation and, for the first time, angle sweeps at fixed ν/B.We observe series' of resonant absorptions which we attribute to periodic orbit resonances (POR) − a phenomenon closely related to cyclotron resonance.The angle dependence of the POR indicate that they are associated with the low temperature quasi-one-dimensional (Q1D) Fermi surface (FS) of the title compound; indeed, all of the resonance peaks collapse beautifully onto a single set of ν/B versus angle curves, generated using a semiclassical magnetotransport theory for a single Q1D FS. We show that Q1D POR measurements provide one of the most direct methods for determining the Fermi velocity, without any detailed assumptions concerning the bandstructure; our analysis yields an average value of v F = 6.5 × 10 4 m/s. Quantitative analysis of the POR harmonic content indicates that the Q1D FS is strongly corrugated. This is consistent with the assumption that the low-temperature FS derives from a reconstruction of the high temperature quasi-two-dimensional FS, caused

We present angular dependent magneto-transport and magnetization measurements on α-(ET) 2 MHg(SCN) 4 compounds at high magnetic fields and low temperatures. We find that the low temperature ground state undergoes two subsequent field-induced density-wave type phase transitions above a critical angle of the magnetic field with respect to the crystallographic axes.This new phase diagram may be qualitatively described assuming a charge density wave ground state which undergoes field-induced transitions due to the interplay of Pauli and orbital effects.