The application of a sufficiently strong magnetic field to a superconductor will, in general, destroy the superconducting state. Two mechanisms are responsible for this. The first is the Zeeman effect, which breaks apart the paired electrons if they are in a spin-singlet (but not a spin-triplet) state. The second is the so-called 'orbital' effect, whereby the vortices penetrate into the superconductors and the energy gain due to the formation of the paired electrons is lost. For the case of layered, two-dimensional superconductors, such as the high-Tc copper oxides, the orbital effect is reduced when the applied magnetic field is parallel to the conducting layers. Here we report resistance and magnetic-torque experiments on single crystals of the quasi-two-dimensional organic conductor lambda-(BETS)2FeCl4, where BETS is bis(ethylenedithio)tetraselenafulvalene. We find that for magnetic fields applied exactly parallel to the conducting layers of the crystals, superconductivity is induced for fields above 17 T at a temperature of 0.1 K. The resulting phase diagram indicates that the transition temperature increases with magnetic field, that is, the superconducting state is further stabilized with magnetic field.

On the basis of the extended Hückel molecular orbital calculation, the intermolecular overlaps of the highest occupied molecular orbitals are calculated for α-(BEDT-TTF)2I3 reported by Bender et al., and for superconducting β-(BEDT-TTF)2I3 reported by Yagubskii et al. (BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene). α-(BEDT-TTF)2I3 is a two-dimensional semimetal or a narrow gap semiconductor. β-(BEDT-TTF)2I3 is a two-dimensional metal which has an almost isotropic closed Fermi surface.

The preparation, crystal structures, and electric and magnetic properties of (BETS)2MX4 molecular conductors (BETS = bis(ethylenedithio)tetraselenafulvalene; M = Mn2+, Fe3+, Co2+, Ni2+, Cu2+; X = Cl, Br, CN) are reported. Resistivity measurements down to 2 K reveal the coexistence of the BETS π conduction electrons and the localized magnetic moments of the anions in the κ-(BETS)2FeCl4 and κ-(BETS)4(CoCl4)(C2H3Cl3) salts. Another FeCl4 phase, λ-(BETS)2FeCl4, undergoes a sharp metal-insulator (MI) transition around 8 K. At the same temperature, a magnetic transition of the FeCl4-anions takes place cooperatively. A superconducting transition is observed at 4.6 K in λ-(BETS)2(FeCl4)0.5(GaCl4)0.5, where half of the anion sites are occupied by magnetic ions. The crystals prepared from 1,1,2-trichloroethane solutions with the NiCl4 2- and MnCl4 2- anions exhibit the behavior of a semimetal down to ≈100 K. The (BETS)4(Cu2Cl6) salt remains metallic down to 4.2 K. ESR studies show that the Fe3+ ions in κ- and λ-(BETS)2FeCl4 are in a high-spin state. The temperature dependencies of the spin susceptibilities of κ- and λ-(BETS)2FeCl4 indicate antiferromagnetic interactions between the Fe3+ ions. The crystal structure analyses of κ- and λ-(BETS)2FeCl4 have been carried out at 298 and 10 K. Closer BETS···FeCl4 contacts in λ-(BETS)2FeCl4 are observed, which is consistent with the larger Weiss temperature of this compound.

Based on the extended Hückel molecular orbital calculations, the relation between the anisotropy of the band structure and the arrangement of the organic molecules is investigated for two organic donors, tetrathiafulvalene (TTF) and bis(ethylenedithio)tetrathiafulvalene (BEDT–TTF). The intermolecular overlaps of their HOMO are calculated while the intermolecular arrangements are varied. The maps of the overlaps thus obtained are then used to estimate the band-structure parameters of (TMTTF)2X and (BEDT–TTF)2ClO4(C2H3Cl3)0.5. The Fermi surface of (TMTTF)2X is quasi-one-dimensional and not closed. On the contrary, (BEDT–TTF)2ClO4(C2H3Cl3)0.5 is regarded as a two-dimensional semimetal.

Molecular metals normally require charge transfer between two different chemical species. We prepared crystals of [Ni(tmdt)2] (tmdt, trimethylenetetrathiafulvalenedithiolate) and carried out crystal structure analyses and resistivity measurements. The analyses and measurements revealed that these single-component molecular crystals are metallic from room temperature down to 0.6 kelvin. Ab initio molecular orbital calculations suggested that pi molecular orbitals form conduction bands. The compact molecular arrangement, intermolecular overlap integrals of the highest occupied and lowest unoccupied molecular orbitals, and tight-binding electronic band structure calculation revealed that [Ni(tmdt)2] is a three-dimensional synthetic metal composed of planar molecules.

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We present the synthesis, characterization by IR, TGA, single crystal X-ray structure and magnetic properties of a novel series of NaCl-type frameworks of [AmineH(+)][Mn(HCOO)(3)(-)], templated by alkylammonium. The anionic NaCl-framework of [Mn(HCOO)(3)(-)] is counter-balanced by the alkylammonium cations located in the cavities of the framework to which they are hydrogen-bonded. The divalent manganese ions have octahedral geometry and are bridged by the formate in an anti-anti mode of coordination. All the compounds exhibit long-range antiferromagnetism below 9 K with a slight non-collinear arrangement of the moments. The canting, likely due to second-order spin-orbit coupling, is via a Dzyaloshinski-Moriya antisymmetric exchange mechanism. A spin-flop is observed in each case at fairly low fields. An orthorhombic to monoclinic transformation was observed for the protonated cyclotrimethyleneamine that is accompanied by localization of the cations into two positions below 240 K from the rapid dynamic flipping of the ring observed at room temperature.

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