Dieter Schweitzer

(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.

FT-IR absorption spectroscopy of BEDT-TTF radical salts: charge transfer and donor-anion interaction

Abstract IR absorption spectra from various κ-phases of BEDT-TTF radical cation salts as well as from α-(BEDT-TTF) 2 I 3 are presented. From these spectra one special mode ν 27 representing the asymmetric ring C=C stretching vibration is assigned and the dependence of its frequency from the average charge on the donor molecule is given. The investigation of the metal-insulator phase transition of α-(BEDT-TTF) 2 I 3 at 135 K with respect to this vibrational model ν 27 gives strong indication of a charge localization in at least one of the two crystallographic different stacks. Furthermore, we correlate the frequencies of the CH 2 stretching modes of a variety of BEDT-TTF radical salts with the superconducting phase transition temperatures. These frequencies are evidently not charge dependent, but significantly reflect the strength of the interaction of the donor molecule with the respective anion. It could be shown that phases with a higher T c of their superconducting transition exhibit a smaller red shift of these frequencies, i.e., a less attractive donor-anion interaction.

Coherent versus incoherent interlayer transport in layered metals

The magnetic-field, temperature, and angular dependence of the interlayer magnetoresistance of two different quasi-two-dimensional (2D) organic superconductors is reported. For κ-(BEDT-TTF) 2 I 3 , where BEDT-TTF is bisethylenedithio-tetrathiafulvalene, we find a well-resolved peak in the angle-dependent magnetoresistance at Θ=90° (field parallel to the layers). This clear-cut proof for the coherent nature of the interlayer transport is absent for β-(BEDT-TTF) 2 SF 5 CH 2 CF 2 SO 3 . This and the nonmetallic behavior of the magnetoresistance suggest an incoherent quasiparticle motion for the latter 2D metal.

Zero-field splittings of the two lowest excited electronic states in crystalline [Ru(bpy)3]X2 with X=PF6, ClO4

For the title compounds zero-magnetic field ODMR signals have been observed on the zero-phonon lines which correspond to the two lowest excited states and which result from Ru4d→bpyπ* charge-transfer transitions. The splitting into sublevels are of the order of 0.1 cm−1 and the corresponding electronic states may be classified as doubly degenerate on the basis of the usual resolution of the optical emission spectra. Several microwave resonances are detected and assigned to different sites of the [Ru(bpy)3]2+ chromophores. Small relaxation rates between the sublevels (compared to other excited-state deactivation rates) are found to be important for a description of the spectroscopic properties at low temperatures.

Assignment of fundamental vibrations and estimation of electron-molecular vibration coupling constants for bis(ethylenedioxy)tetrathiafulvalene (BEDO)

Abstract The i.r. absorption and Raman spectra of BEDO are investigated. All fundamental vibrations are assigned using correlations between the obtained data and those for a well-known compound BEDT-TTF (ET). Comparative normal coordinate analysis has been done. The results are used for calculation of electron-intramolecular vibration coupling constants of BEDO based superconducting charge-transfer complexes. Comparison of these values with the constants obtained earlier for ET shows that BEDO is a perspective donor for searching new superconductors on its basis.

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.

FT-IR investigations of BEDO-TTF and radical salts of BEDO-TTF

Abstract We investigated powder absorption spectra of the donor BEDO-TTF and some radical salts of BEDO-TTF, which are metals even at low temperatures. After an assignment of the bands in the neutral donor we found in the radical salts different vibrational frequencies of bands correlated with Cue5f8O vibrations. These frequencies are directly related to the average charge on the donor molecule. Additionally frequencies of CH 2 -stretching vibrations exhibit differences, which can be ascribed to a varying strength of donor-anion interaction depending on the respective anion.

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.

Resonant Raman scattering on single crystals of (BEDT-TTF)2Cu(NCS)2

Resonant Raman spectra of the organic superconductor (BEDT-TTF)2Cu(NCS)2 have been measured at 1.3 K as well as at 120 K. The Raman data do not show a superconducting gap opening in the investigated two dimensional superconducting system. The vibrational modes found in the Raman spectra are discussed and assigned. Possible mechanisms for resonance conditions in such two-dimensional organic superconducting systems are proposed.