H. Schwenk

α- and β-(BEDT-TTF)2I3 - two modifications with contrasting groundstate properties: insulator and volume superconductor

Abstract The α-modification of (BEDT-TTF)2 I3 undergoes a metal-to-insulator transition at T = 135 K. Application of hydrostatic pressure shifts the transition temperature to lower values. Above 12 kbar semimetallic behavior is observed, but no superconductivity is detected up to a pressure of 17 kbar and temperatures down to l00 mK. The β-modifcation, on the other hand, exhibits ambient pressure volume superconductivity below Tc = 1.05 K. Measurements of the anisotropy of the diamagnetic shielding- as well as the Meissner-magnetization are presented.

Determination of the electron phonon coupling and the superconducting gap in β-(BEDT-TTF)2X crystals (X=I3, IAuI)

We performed point-contact and tunneling measurements on the organic superconductors β-(BEDT-TTF)2I3 and β-(BEDT-TTF)2IAuI, in the normal and superconducting states. The point-contact measurement in the normal state provides the Eliashberg function α2(ω)F (ω). This function has maxima at 4 and 15 mV, as well as a sharp peak at 1 mV which seems to indicate a very soft phonon strongly coupled to the electrons. The measurements in the superconducting state provide the superconducting gap, which for the tunneling measurements has a value 2Δ/kB Tc≅4, just slightly above the BCS value.

PECULIARITIES OF ORGANIC SUPERCONDUCTORS OF THE (BEDT-TTF)2X-FAMILY

Abstract We present evidence that in ET2AuI2, ET2IBr2 and especially in the “high Tc”-phase of ET2I3, superconductivity is a volume property. We also present first results on specific heat measurements in ET2AuI2 as well as results on deuteration and iodine-doping of ET2I3.

Evidence for Partial Diamagnetism in (TMTSF)2 FSO3

Abstract We have searched for and have found superconducting diamagnetic screening currents as well as Meissner-effects in (TMTSF)2FSO3 at different pressures. The largest signals, however, are only 2%of those expected for a perfect superconductor.

The organic superconductor (TMTSF)2ClO4: The effect of magnetic fields and anion orientational disorder

The upper critical fieldHc2 and the low field magnetoresistance of the organic super-conductor (TMTSF)2ClO4 are studied as a function of temperature, sample orientation and the degree of frozen anion disorder.Hc2-results from measurements of both the acsusceptibility and from resistivity are compared. The former might be interpreted in terms of thin film behavior of decoupled layers of organic molecules whereas the latter favor a picture of a threedimensional anisotropic superconductor. The effect of frozen anion disorder is discussed within a model of an inhomogeneous mixture of superconducting and spin density wave regions in the sample.

Quantum oscillations and field-induced transitions in (TMTSF)2ReO4

We report the magnetoresistance of (TMTSF)2ReO4 in the metallic state at low temperature and high pressure. We find evidence for a series of magnetic field induced phase transitions. We also observe Shubnikov-de Haas oscillations both above and below the threshold for the field induced transition - an anomalous result that is not explained by present theories.

Resistivity and Upper Critical Field of (TMTSF) 2clo4 in Various Intermediate States

Abstract The intermediate state of (TMTSF)2ClO4, obtained by quenching the sample from several temperatures TQ around the anion ordering temperature TAO=24 K with a rapid cooling rate (> 60 K/min), was investigated by electrical resistance, magnetoresistance and ac-suscepti-bility measurements. The effect of frozen anion disorder is discussed within a model of an inhomogeneous mixture of superconducting and spin density-wave regions in the sample.