J.P. Ulmet

Magnetotransport Measurements on the λ-Phase of the Organic Conductors (BETS)2MCl4 (M = Ga, Fe). Magnetic-Field-Restored Highly Conducting State in λ-(BETS)2FeCl4

Magnetotransport properties of two isostructural salts, the superconducting λ-(BETS)2GaCl4 phase and the first organic conductor containing magnetic Fe3+ ions, λ-(BETS)2FeCl4, where BETS is bis(ethylenedithio)tetraselenafulvalene, have been studied up to 37 T. The previously reported sharp metal-insulator phase transition occurring at 8 K under ambient pressure for λ-(BETS)2FeCl4 is suppressed when applying magnetic fields > 10 T. This unusual phenomenon may be connected to a field-induced ferromagnetic ordering of the Fe3+-ions moments. This could result in some kind of decondensation of the carriers, initially condensed below 8 K, into a field-restored highly conducting state (FRHCS). The temperature-magnetic-field (T-H) phase diagram is presented. The angle dependence of the critical field Hc2 has been determined for the superconducting λ-(BETS)2GaCl4 phase at 4.2 K.

Angular and magnetic field dependencies of the magnetoresistance of λ-(BETS)2FeCl4 in the field-restored high conductivity state

Abstract The very sharp metal-insulator transition, which occurs at 8 K at ambient pressure in λ-(BETS) 2 FeCl 4 is suppressed when applying magnetic fields >10T. This unusual phenomenon may be connected to ferromagnetic ordering of the Fe 3+ moments, which could drive some “evaporation” of the previously condensed conduction electrons, into a field-restored highly conducting state (FRHCS) in which oscillations of the magnetoresistance are reported.

High field magnetoresistance of a new organic metal: TTF [Ni(dmit)2]2

Abstract The new synthetized compound tetrathiafulvalene bis-[bis-(4,5-dimercapto-1,3-dithiole-2-thione)nickel], TTF [Ni(dmit)2]2 exhibits a remarkable high conductivity of 300 Ω-1 cm-1 at room temperature and a metallic behaviour down to 3 K for the best samples. The transverse magnetoresistance has been investigated in fields as high as 30 T for different temperatures and angles between the B field direction and the crystalline axis. No quantum oscillations have been found but some qualitative features can be deduced which concern the relative strengths of the interactions in different directions of the crystal.

Slow and Fast Quantum Oscillations in the High-Field Magnetoresistance of (TMTSF)2NO3: Magnetic Breakthrough Linked to SDW Gap Opening

We report a study of the transverse magnetoresistance in the spin-density-wave state of the organic conductor (TMTSF)2NO3 at ambient pressure, in fields up to 37 T. For field along the lowest conductivity axis (c*), two series of quantum oscillations, with temperature-independent frequencies of 63 T and 248 T, respectively, are observed in the 2-12 K temperature range. The oscillation amplitude of both series decreases when the temperature departs from 4.2 K. This is tentatively interpreted within the framework of magnetic breakthrough and quantum interferences.

Suppression of insulating state in (BETS)2FeCl4 by magnetic field

Abstract We report on the magnetotransport properties, up to 37 T, of the first organic conductor containing magnetic Fe 3+ ions, λ-(BETS) 2 FeCl 4 , where BETS is bis(ethylenedithio)tetraselenafulvalene. The previously reported very sharp metal-insulator phase transition, which occurs at 8 K under ambient pressure, is suppressed when applying magnetic fields > 10 T. This unusual phenomenon may be connected to a field-induced ferromagnetic ordering of the Fe 3+ ion moments. This could result in some kind of “evaporation” of the previously condensed carriers, into a field-restored highly conducting state (FRHCS). In this ground state, angular dependence of the magnetoresistance is observed for current flowing perpendicularly to the best-conduction c axis.

Metal-insulator transition and metallic conductivity reentrance in the new organic metal (NHMe3) [Ni(dmit)2]2· high field magneto resistance

Abstract We present here the first data on the electron transport properties of (NHMe3)[Ni(dmit)2]2, a new organic conductor. At room temperature, the samples exhibit a low resistivity of a few 10−3Ωcm. Lowering the temperature we observe first a weakly metallic behaviour down to 100 K followed by a metal-insulator transition and then around, 10 K, by a reentrance of the metallic conductivity. At low temperature and under a high magnetic field up to 30 T a large negative magnetoresistance appears. The resistivity and magnetoresistance data could indicate we are dealing with localized carriers.

Original behaviour of the resistivity and high field magnetoresistance in the new organic metal (NHMe3) [Ni(dmit)2]2

Abstract We report here the first data on the electron transport properties of (NHMe3) [Ni(dmit)2]2 a newly synthetized organic conductor. At room temperature, most of samples exhibit a rather low resistivity of a few 10−3 Ω cm. Lowering the temperature, a metallic behaviour of the resistivity is observed down to about 100 K, then the compound undergoes a metal-insulator transition. On most samples, around 10–15 K, a reentrance of the metallic conductivity is observed but this unexpected new effect is not believed to be a superconducting transition since under a high magnetic field of 30 T a negative magnetoresistance appears in a rather narrow range of temperature (2.5–12 K) supporting the picture of strongly localized carriers.

Magnetoresistance in pulsed fields, band structure calculations and charge-density wave instability in (TSeT)/sub 2/C1

Abstract Diffuse X-rays measurements assign the 26 K metal-semimetal transition in (TSeT) 2 Cl, where TSeT = tetraselenotetracene, to the stabilization of a 2k F =c∗/4 charge-density wave ground state. Tight-binding extended Huckel band calculations suggest that 2k F yields to a partial nesting of the four conduction bands. Transverse magnetoresistance experiments performed up to 37 T confirm the previous observation of SdH oscillations. The 0 + Landau level is evidenced for the first time.

High magnetic field quantum transport in the low temperature density wave state of the purple bronze KMo6O17

Abstract The resistivity of the purple bronze KMo 6 O 17 has been studied between 1.9K and 300K with pulsed magnetic fields up to 37 T. Several anomalies are found on the curves Δϱ(B)/ϱ and ϱ(T) for different field orientations. Shubnikov-de Haas oscillations are observed at low temperatures. The results are discussed in relation with the Fermi surface topology.

Fast oscillations in (TMTSF)2X

Abstract We report on the temperature dependence of the “fast oscillation” phenomenon in quasi-one dimensional molecular conductors. Magnetoresistance and magnetization measurements to 30T in the ambient pressure spin density wave state have been taken for the X = NO 3 , PF 6 and AsF 6 members of the (TMTSF) 2 X family. Complementary data on X=AsF 6 are presented by Ulmet et al . in these proceedings. All three compounds exhibit a peak in the transverse magnetoresistances ρ xx (B//c * , I//a) and ρ yy (B//c * , I//b) and thenoscillatory components around 3K, with the oscillations vanishing at higher and lower temperatures. No magnetic moment oscillations are detected in PF 6 or AsF 6 , while only extremely weak oscillations (δm~10 −10 emu) are seen in the NO 3 salt above 25T. Also, no oscillations were observed in longitudinal magnetoresistance ρ zz (B//I//c * ) measurements performed on AsF 6 and PF 6 . Possible scenarios which have been used to explain the data include imperfect nesting at T SDW leaving behind quantizable closed orbits, and quantum interference effects arising from magnetic breakdown orbits. Another may involve a new phase transition inside the SDW state.