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Item Open Access Temperature dependent resistivity under pressure and magnetoresistance data of the organic superconductor (BEDO-TTF)2ReO4(H2O)(1993) Kahlich, Siegfried; Schweitzer, Dieter; Auban-Senzier, Pascale; Jerome, Denis; Keller, Heimo J.The temperature dependence of the resistivity under pressure (up to 6 kbar) of the organic metal (BEDO-TTF)2ReO4(H20) is reported. An increase of the resistivity below 35 K, which is observed at ambient pressure, is already suppressed at 1 kbar. In addition at this pressure the superconducting transition sharpens and the onset temperature of 2.3 K is nearly the same as at ambient pressure. Magnetoresistance data, observed at 1.3 K and 6.7 Tesla show a strong angle dependence. In addition at 1.3 K first SdH -oscillations in the magnetoresistance are found already at such low fields as 5 Tesla.Item Open Access Characterisation of the Fermi surface and phase transitions of (BEDO-TTF)2 ReO4·(H2O) by physical property measurements and electronic band structure calculations(1994) Kahlich, Siegfried; Schweitzer, Dieter; Rovira, Concepcio; Paradis, Jeffrey A.; Whangbo, Myung-Hwan; Heinen, Ilsabe; Keller, Heimo J.; Nuber, Bernhard; Bele, Petra; Brunner, Hermann; Shibaeva, Rimma P.The electronic properties of the organic superconductor (BEDO-TTF)2 ReO4·(H2O) were investigated by temperature dependent resistivity, ESR, Hall effect and magnetoresistance measurements. Shubnikov-de Haas (SdH) oscillations were observed in magnetic fields up to 24 T in the temperature range 0.5 K to 4.2 K. The electronic band structure of (BEDO-TTF)2 ReO4·(H2O) was calculated by employing the extended Hückel tight binding method on the basis of its room temperature crystal structure. The two observed SdH frequencies of 75 T and 37 T correspond very well with two cross-sectional areas of the hole and electron Fermi surface pockets obtained from the tight binding calculation. From the temperature dependence of the SdH oscillation amplitudes, the cyclotron effective mass (mc) belonging to the larger and smaller pockets were found to be 0.9 m0 and mc=1.15 m0 respectively. Measurements of the angular dependence of the SdH frequencies show no deviation from that expected for a cylindrical Fermi surface. In terms of our tight binding calculations and experimental measurements, probable causes for the 213 K and ∼35 K phase transitions are discussed. The calculations show that (BEDO-TTF)2 ReO4·(H2O) is a two dimensional semimetal but possesses a hidden nesting. The latter is likely to cause an SDW instability leading to the ∼35 K transition. The resistivity drop associated with the 213 K transition is likely to be induced by an abrupt increase in the relaxation time. The excellent agreement between the calculated and experimentally observed Fermi surface implies that, with decreasing temperature below 35 K, (BEDO-TTF)2 ReO4·(H2O) gradually gets out of the SDW state and re-enters the "original" metallic state, in which it becomes superconducting below 2.4 K.Item Open Access Investigations of the transport properties of the organic superconductors β-(BEDT-TTF)2I3 and βt-(BEDT-TTF)2I3(1990) Kahlich, Siegfried; Schweitzer, Dieter; Keller, Heimo J.The temperature dependence of the resistivity, thermopower and ac-susceptibility of the organic superconductors β-(BEDT-TTF )2I3 and βt-(BEDT-TTF)2I3 were investigated. A metal-metal phase transition in β-(BEDT-TTF)2I3 at around 125K, which was found earlier already in thermopower measurements, was observed in resistivity measurements. βt-(BEDT-TTF)2I3 crystals were prepared by annealing β-(BEDT-TTF)2I3 at 85 K for several weeks. The superconducting transition temperatures of the βt-(BEDT-TTF)2I3 crystals were found between 4.5K and 7K, that means at much higher temperatures than the observed Tc = 1.3K of β-(BEDT-TTF)2I3 crystals. Ac-susceptibility measurements indicate that the superconducting state is a bulk effect in βt-(BEDT-TTF)2I3.Item Open Access Pressure dependence of the resistivity of (BEDO-TTF)2ReO4·(H2O)(1992) Kahlich, Siegfried; Schweitzer, Dieter; Auban-Senzier, Pascale; Jérome, Denis; Keller, Heimo J.The temperature dependence of the resistivity under pressure (up to 6 kbar) of the organic metal (BEDO-TTF)2ReO4(H2O) was investigated. The increase of the resistivity below 35 K, which is observed at ambient pressure, is already totally suppressed at 1 kbar. In addition at this pressure the superconducting transition sharpens drastically and the onset temperature of 2.3 K is nearly the same as at ambient pressure.Item Open Access (BEDO-TTF)2ReO4·(H2O): a new organic superconductor(1991) Kahlich, Siegfried; Schweitzer, Dieter; Heinen, Ilsabe; Song, En Lan; Nuber, Bernhard; Keller, Heimo J.; Winzer, Klaus; Helberg, Hans W.The structure and the temperature dependence of the resistivity, thermopower and ac-susceptibility of the new organic metal (BEDO-TTF)2ReO4(H2O) was investigated. The resistivity and thermopower data indicate phase transitions at 213K, around 90K and 35K. Below 2.5K an onset to superconductivity is observed in the resistivity data. Superconductivity was suppressed in the resistivity at 1.3K by applying a magnetic field of about 0.2T. Ac-susceptibility data indicate that superconductivity is a bulk effect in (BEDO-TTF)2ReO4(H2O) but the onset for superconductivity observed in the ac-susceptibility is only at 0.9K and the transition seems to be complete only at temperatures below 50 mK. This broad transition might be due to some disorder in the structure created by the low temperature phase transitions.Item Open Access Superconductivity at 7.5 K and ambient pressure in polycrystalline pressed samples of βp-(BEDT-TTF)2I3(1989) Schweitzer, Dieter; Gogu, Emil; Grimm, Hans; Kahlich, Siegfried; Keller, Heimo J.Recently, bulk superconductivity at ambient pressure in polycrystalline pressed samples of an organic metal - α-(BEDT-TTF)2I3 - was observed. This is a remarkable fact because this finding shows that organic superconductors can in principle be used for the production of electronic devices, such as squids, and might even be suitable for the preparation of superconducting cables similar to the high temperature superconductors of the copper oxides.Item Open Access Organic metals from chiral BEDT-TTF donors(1991) Chen, Benming; Deilacher, Frank; Hoch, Matthias; Keller, Heimo J.; Wu, Peiji; Armbruster, Kurt; Geiger, Rolf; Kahlich, Siegfried; Schweitzer, DieterWe have recently described our project to prepare methylated BEDT-TTF derivatives in order to obtain different radical ion salts of these donors (1). Meanwhile we have obtained 7,7'-Dimethyl-BEDT-TTF ("DIET"), 7,8'-Dimethyl-BEDT-TTF ("DIET") and 7,7',8,8'-Tetramethyl-BEDT-TTF ("TMET") as isomeric mixtures and some of them in an "optically pure" form. The isomeric mixtures have been prepared as reported earlier (1) corresponding to the "classical" synthesis of BEDT-TTF (2). The optically pure compounds have been obtained starting from the optically pure 1,2-propanediol and 2,3-butanediol respectively, depicted on page 4.Item Open Access Bulk superconductivity in polycrystalline pressed samples of αt-(BEDT-TTF)2I3 and βp-(BEDT-TTF)2I3(1990) Schweitzer, Dieter; Kahlich, Siegfried; Gärtner, Stephan; Gogu, Emil; Grimm, Hans; Zamboni, Roberto; Keller, Heimo J.Bulk superconductivity at ambient pressure in pressed samples of polycrystalline αt-(BEDT-TTF)2I3 and βp-(BEDT-TTF)2I3 is reported. This finding is remarkable because it demonstrates that organic superconductors can be used in principal for applications such as the preparation of electronic devices and superconducting cables. In addition the observation of bulk superconductivity in large pressed samples of crystallites of a typical diameter of 1μm of organic metals indicates that superconductivity in organic polymers should be observable as well.Item Open Access Superconductivity in polycrystalline pressed pellets of k-(BEDT-TTF)2I3(1993) Balthes, Eduard; Breining, Angela; Kahlich, Siegfried; Moldenhauer, Jörg; Schweitzer, Dieter; Bele, Petra; Brunner, Hermann; Heinen, Ilsabe; Nuber, Bernhard; Keller, Heimo J.Investigations of the transport properties of single crystals and polycrystalline pressed samples of K-( BEDT-TTF)2I3 are presented. In contrast to polycrystalline pressed samples of K-(BEDT-TTF)2Cu(NCS)2 and K-(BEDT-TTF)2Cu [N(CN)2]Br those of K-(BEDT-TTF)2I3 show bulk superconductivity indicating that the loss of superconductivity in the first two types of samples is not due to the special structure of the K-phase, but probably due to a disproportionation of Cu1+ under the anisotropic pressure during the preparation of the samples. Annealing the single crystals as well as the polycrystalline pressed samples of K-(BEDT-TTF)2I3 at 75°C for a few days results in a structural transfomation with an increase in the transition temperature for superconductivity (about 8 K for the crystals). IR-spectra indicate a transfomatlon into a structure which is identical with αt-(BED-TTF)2I3 which seems to be the thermodynamically most stable phase of the several (BEDT-TTF)2I3 phases.Item Open Access New radical salts of BEDO-TTF: structures and electronic properties of organic metals and superconductors(1993) Schweitzer, Dieter; Kahlich, Siegfried; Heinen, Ilsabe; Song, En Lan; Nuber, Bernhard; Keller, Heimo J.; Winzer, Klaus; Helberg, Hans W.Structural data as well as some measurements on the electronic properties of a few organic metals of the donor BEDO-TTF are reported.