S. Kahlich

(BEDO-TTF)2 ReO4·(H2O): A new organic superconductor

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.

New radical salts of BEDO-TTF: Structures and electronic properties of organic metals and superconductors

Structural data as well as some measurements on the electronic properties of a few organic metals of the donor BEDO-TTF are reported.

Superconductivity in polycrystalline pressed pellets of k-(BEDT-TTF)2I3

Abstract Investigations of the transport properties of single crystals and polycrystalline pressed samples of k -(BEDT-TTF) 2 I 3 are presented. In contrast to polycrystalline pressed samples of k -(BEDT-TTF) 2 Cu(NCS) 2 and k -(BEDT-TTF) 2 Cu[N(CN) 2 ]Br those of k -(BEDT-TTF) 2 I 3 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 Cu 1+ 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) 2 I 3 at 75°C for a few days results in a structural transformation with an increase in the transition temperature for superconductivity (about 8K for the crystals). IR-spectra indicate a transformation into a structure which is identical with α t -(BEDT-TTF) 2 I 3 , which seems to be the thermodynamically most stable phase of the several (BEDT-TTF) 2 I 3 phases.

Characterisation of the Fermi surface and phase transitions of (BEDO-TTF)2 ReO4·(H2O) by physical property measurements and electronic band structure calculations

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.

Pressure dependence of the resistivity of (BEDO-TTF)2ReO4·(H2O)

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.

Investigations of the transport properties of the organic superconductors β-(BEDT - TTF)2I3 and βt-(BEDT - TTF)2I3

Abstract The temperature dependence of the resistivity, thermopower and ac - susceptibility of the organic superconductors β-( BEDT - TTF ) 2 I 3 and β t -( BEDT - TTF) 2 I 3 were investigated. A metal-metal phase transition in β-( BEDT - TTF ) 2 I 3 at around 125K, which was found earlier already in thermopower measurements, was observed in resistivity measurements. β t -( BEDT - TTF ) 2 I 3 crystals were prepared by annealing β-( BEDT - TTF ) 2 I 3 at 85 K for several weeks. The superconducting transition temperatures of the β t -( BEDT - TTF ) 2 I 3 crystals were found between 4.5K and 7K, that means at much higher temperatures than the observed T c = 1.3K of β-( BEDT - TTF ) 2 I 3 crystals. Ac - susceptibility measurements indicate that the superconducting state is a bulk effect in β t -( BEDT - TTF ) 2 I 3 .

Investigations of Fermi surfaces and cyclotron effective masses of organic superconductors by Shubnikov-de Haas (SdH)- and de Haas-van Alphen (dHvA)-measurements

Abstract The Fermi surfaces (FS) and effective masses of the organic superconductors (BEDO-TTF) 2 ReO 4 (H 2 O) ( 1 ) and κ-(BEDT-TTF) 2 I 3 ( 2 ) were investigated by SdH- and dHvA-measurements in magnetic fields up to 27T in the temperature range from 0.5K to 4.2K. In 1 two small closed pockets (0.7% and 1.5% of the first Brillouin zone [FBZ] are observed corresponding very well with two cross-sectional areas of the FS obtained for a hole and an electron pocket from tight binding calculations. In contrast to 1 in 2 two relatively large closed sections (13% and 85% of the FBZ) of the FS are observed, again confirming the tight binding calculations. For 2 in magnetic fields above 12T the effective mass for the larger orbit, as obtained from the temperature dependence of the SdH-oscillation amplitudes, is magnetic field dependent as long as the field is arranged perpendicular to the conducting planes. In contrast, from dHvA-measurements — which were performed by turning the magnetic field by 27° with respect to the SdH-experiments — the observed effective mass is field independent (m ∗ = 3.9m O ). We suppose that the occurrence of particles with fractional statistics at temperatures below 1K and in fields above 12T might be the reason for the observed field dependence of the effective mass in the SdH investigations.

Magnetic field dependence of the cyclotron effective mass in the organic superconductor k-(BEDT-TTF)/sub 2/I/sub 3/

Summary form only given. k-(BEDT-TTF)/sub 2/I/sub 3/ is an organic superconductor [1] with a T/sub c/ = 4 K. High quality single crystals of x-(BEDT-TTF)/sub 2/I/sub 3/ can be grown by electrochemical methods [2] and show already in magnetic fields from 5 to 12 Tesla giant quantum oscillations in Shubnikov de Haas (SdH)- and de Haas van Alphen (dHvA)- experiments. Therefore the cyclotron effective mass m* and the Fermi-surface can be determined in such fields with high accuracy [3]. In order to investigate whether m* is enhanced due to many body correlations such as electron-phonon or electron-electron interactions SdH- and dHvA- oscillations have been measured in fields up to 23.5 Tesla. Using the Lifshitz - Kosevich (LK) formula, from the temperature dependence of the SdH - amplitudes a relative strong magnetic field dependence of in* (m*=3.8m/sub 0/ at 10 Tesla, in*= 2.9m/sub 0/ at 23 Tesla) is obtained. In order to discriminate whether many body interactions are important or not, corrections on the LK-formula have to be discussed taking into account the quasi-two dimensional electronic properties of the material

Organic Metals from Chiral BEDT-TTF Donors

The chemistry and physics of solids with layered structures has been of central interest in the solid state sciences for several decades [1]. One of the many scientifically and technically important aspects connected with these materials is the possibility of pronounced two-dimensional delocalized interatomic and/or intermolecular electronic interactions, which lead to unusual magnetic and electric properties in the bulk. Especially remarkable are a variety of phase transitions, which occur in selected materials of this type at different temperatures. Typical, and very well-known, examples of such specimens are graphite, the binary sulfides MoS2 and TaS2, together with several of their intercalation compounds, ternary or quarternary chalcogenides and/or halides of the transition elements, and so on. Inorganic ternary systems, composed of anionic transition metal oxide layers and different kinds of countercations located between these anion sheets, are especially well-suited for systematic variations in composition and - as a consequence thereof - in solid state properties. Varying stages of band fillings can be achieved “chemically,” depending on the number of electrons per metal ion in the different sheets. The systematic work on these materials culminated in the discovery of superconductivity above 40 K by Muller and Bednorz in layered copper oxide structures [2].

Temperature dependent resistivity under pressure and magnetoresistance data of the organic superconductor (BEDO-TTF)2ReO4(H2O)

The temperature dependence of the resistivity under pressure (up to 6 kbar) of the organic metal (BEDO-TTFhReO.(H 2 0) is reported . AD increase of the resistivity below 35 K, whicb 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 6.7Tesla show a strong angle dependence. In addition at 1.3 K first SdH -oscillations in the magnetoresistance are found already at such Jow fields as 5 Tesla.