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The crystal and electronic band structures as well as the transport properties of the recently synthesized layered radical cation salt -͑BETS͒ 2 Mn͓N͑CN͒ 2 ͔ 3 are investigated. It is shown that this salt is more anisotropic in the plane of conducting layers than other phases, a feature which noticeably influences its physical behavior. A phase transition resulting in the formation of an incommensurate superstructure below 102 K has been found. At temperature below 30 K a metal-insulator transition is observed. We argue that the transition is associated with electronic interactions rather than with structural changes. The T-P phase diagram has been studied under 4 He gas pressure up to 2.5 kbar. At moderately high pressure P Ϸ 0.5 kbar the metal-insulating transition is suppressed and the crystal becomes superconducting with T c Ϸ 5.7 K. In the pressurized metallic state, Shubnikov-de Haas oscillations have been found, revealing a very small closed Fermi surface with a high-effective cyclotron mass influenced by many-body interactions.

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Magnetization measurements performed on the charge-transfer salt a-(BEDT-TTF) 2 TlHg(NCS) 4 in pulsed magnetic fields reveal the existence of eddy current "resonances" in the high field state, indicating the presence of deep minima in the transverse magnetoresistivity. Their behavior can be explained qualitatively in terms of enhanced conductivity due to the quantum Hall effect.[ S0031-9007(96)00960-X] PACS numbers: 73.40.Hm, 73.61.PhSince the discovery of the quantum Hall (QH) effect, accompanied by zero transverse resistivity (r xx 0), in semiconductor-based two-dimensional electron (2DE) systems [1,2], there has been speculation over the possibility of observing such effects in bulk crystalline metals [3,4]. Thus far, the Bechgaard salts are the only bulk crystalline materials which have displayed a QH-like effect [5], albeit involving a very different mechanism. However, charge-transfer salts of the ion BEDT-TTF have recently emerged as a class of materials with quasi-twodimensional (Q2D) Fermi surfaces which satisfy the conditions required for conventional QH; i.e., the width W of the electronic band(s) in the longitudinal z direction and the broadening of the Landau levels (ht 21 ) are both significantly less than the separation between the Landau levels (hv c ) at magnetic fields B * 30 T. This seems to be especially true for salts of the form a-(BEDT-TTF) 2 MHg(SCN) 4 (M NH 4 , K, or Tl) [6]. One complication of these materials is the additional presence of quasi-one-dimensional (Q1D) states [7]. However, as will be seen below, the Q1D states provide a mechanism whereby the chemical potential m can be pinned between the Landau levels [6] thus providing a second prerequisite for the QH effect [2].A direct observation of the QH effect in BEDT-TTF salts requires the measurement of the transverse magnetoresistivities r xx and r xy at B * 30 T. Such experiments are problematic; first, the nonideal geometry of the single-crystal BEDT-TTF salt samples makes accurate determinations of r xx and r xy difficult [8]. Furthermore, the requirement B * 30 T implies the use of pulsed magnetic fields [9]; the extremely low in-plane resistivities of the samples means that noise due to induced voltages (an inevitable part of pulsed field experiments) dominates the tiny resistive signals. Magnetoresistance measurements in pulsed fields are hence restricted to the higher resistance longitudinal direction (i.e., r zz ; the current flows parallel to the magnetic field) [10]. Thus, for purely practical reasons, no direct verification of the existence of the QH effect has yet been possible in BEDT-TTF salts. Magnetization measurements provide an alternative means by which currents within the conducting planes can be investigated. Deep minima in r xx due to the QH effect cause the induction of persistent eddy currents which contribute to the magnetization [11,12]. This leads to the superposition of eddy current "resonances" on top of the conventional de Haasvan Alphen (dHvA) magnetization wave form [12,13].In this Letter, the magn...