[en] This paper presents some aspects of the current controversial understanding on the nature of the superconducting state in quasi-two-dimensional organic superconductors. A brief introductory review is given on the not yet conclusive experimental situation with indications in favor for unconventional superconductivity as well as results in line with standard behavior. Some recent results on the isotope effect, the magnetic-phase diagrams, fluctuation-controlled properties, and of specific-heat measurements exemplifying the fascinating physical properties of organic superconductors are presented

[en] We report systematic de Haas-van Alphen (dHvA) investigations in the normal and superconducting state of RNi₂B₂C (R = Y and Lu). The observed rich frequency spectrum of the dHvA signals results from a rather complex electronic band structure with different open and closed Fermi-surface sheets. From our data in combination with full-potential local-orbital calculations we are able to extract the angular-resolved mass-enhancement factors, l, for different bands. We find a strong anisotropy and band dependence of l, clearly reflecting the multiband character of the superconductivity in RNi₂B₂C. We further were able to resolve dHvA oscillations deep into the superconducting state. The observed additional damping of the dHvA amplitudes is much less than expected from most theories. This hints at a reduced or even zero superconducting gap for the detected Fermi surface.

[en] The specific heat near the phase transition of the diluted ferromagnetic system Eu_xSr_1-xS has been studied for 0.59≤x≤1. Our data yield that the critical exponent α describing the specific-heat anomaly decreases from the nearly 3d Heisenberg value α = 0.14±0.02 for EuS to α ≅ - 0.9±0.2 for samples near the limiting concentration of ferromagnetic ordering x_c = 0.51. These results are in every respect consistent with recent theoretical predictions of concentration-dependent effective exponents of disordered magnetic systems

No abstract available

[en] The critical exponent α of the diluted Heisenberg ferromagnet Eu_xSr_1-xS, derived directly from specific heat data, decreases from the nearly pure Heisenberg value α = -0.14 for EuS to α = -0.48 for Eu_0.7Sr_0.3S. These results are in qualitative agreement with theoretical predictions of concentration-dependent effective exponents of strongly disordered magnetic materials

[en] Full text: In the so-called FFLO state, named after Fulde, Ferrell, Larkin, and Ovchinnikov, the superconducting state can survive even at high magnetic fields above the Pauli paramagnetic limit. The quasi-two-dimensional (2D) organic superconductors have been suggested as good candidates for exhibiting the FFLO state. When applying the magnetic field exactly parallel to the conducting layers the orbital pair breaking is greatly suppressed and the Pauli limit is reached. We performed high-resolution specific-heat and torque-magnetization experiments in magnetic fields up to 32 T for such 2D organic superconductors. In a very narrow angular region close to parallel-field orientation we observe additional anomalies below the upper critical field signaling the existence of an additional superconducting phase. The specific-heat data for □-(BEDT-TTF)₂Cu(NCS)₂ with T_c = 9.1 K show that the superconducting transition becomes first order for fields above 21 T indicating that the Pauli limit is reached. Below about 3 K, the upper critical field increases sharply and a second first-order transition appears within the superconducting phase. These results are corroborated by magnetic-torque data which allowed to follow the phase diagram to lower temperatures and higher fields. Our results give strong evidence for the realization of the FFLO state in organic superconductors. Work done in cooperation with R. Lortz, B. Bergk, Y. Wang, A. Demuer, I. Sheikin, G. Zwicknagl, and Y. Nakazawa. (authors)

[en] In this short review, we report on the recently found growing experimental evidence for the existence of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states in quasi-two-dimensional organic superconductors. At high magnetic fields aligned parallel to the conducting organic layers, we observe an upturn of the upper critical field beyond the Pauli limit, as evidenced by specific-heat and torque-magnetization measurements. Inside the superconducting state a second thermodynamic transition emerges. These features appear only in a very narrow angular region close to parallel-field orientation.

[en] Superconducting radical cation salts derived from the donor molecule bis(ethylenedithio)tetrathiafulvalene (ET) are an important class of molecular organic conductors. Superconducting ET-iodine phases, owing to relatively high transition temperatures, are playing a key role in the development of prototypic devices. Here is reported a novel redox preparation of the organic metal α-(ET)₂I₃, as well as a study of the thermally induced conversion of this material and the structural characterization and superconducting properties of the transformation product. (orig.)

[en] Electrical-transport measurements of the semimetal CeBiPt in magnetic fields up to 60 T reveal a drastic change of the electronic band structure. The oscillating Shubnikov-de Haas signal vanishes above about 25 T although the quantum limit is not yet reached. Above this field the magnetoresistance rises strongly independent of angle and temperature. These unique features are caused by the Ce 4f electrons as evidenced by the absence of any unconventional behavior in the sister compound LaBiPt

[en] The current status of the pulsed-magnet program of the Dresden High Magnetic Field Laboratory (HLD) is reported. The non-destructive pulsed magnets for the HLD include a wide spectrum of coils designed for energies between 1 and 46MJ, magnetic fields of 60 - 100T, and pulse durations of 10 - 1000 ms. Various experimental techniques at pulsed magnetic fields will be available for external users soon. Some user magnets for first scientific experiments have been installed and tested. A 8.5MJ/70 T mono-coil magnet has been built and first test results for this magnet are presented. The design of a two-coil 46 MJ magnet for magnetic fields above 85T has been completed. This magnet is under construction now. Important issues of the coil design are numerical simulations of the pulsed-magnet performance. Both analytical approaches and finite-element analysis are used for these simulations at the HLD