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Resonance Raman Investigations of Low Energetic Modes in the Organic Superconductors α_{t}-(BEDT-TTF)_{2}I_{3} and β-(BEDT-TTF)_{2}IAuI

100%
Ludwig T., Schweitzer D., Keller H. J.
Acta Physica Polonica A
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1995
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vol. 87
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issue 4-5
801-806
EN
A study of low energetic modes in the resonance Raman spectra of the organic superconductors α_{t}-(BEDT-TTF)_{2}I_{3} (T_{c} = 8 K) and β-(BEDT-TTF)_{2}IAuI (T_{c} = 4 K) in the temperature range of 1.5-100 K was per­formed. In both materials, a strong vanishing of low energetic phonon bands (at 30 cm^{-1} and 40 cm^{-1} for α_{t}-(BEDT-TTF)_{2}I_{3} and at 27 cm^{-1} and 33 cm^{-1} for β-(BEDT-TTF)_{2}IAuI) below 71 was observed. For the sym­metric stretching mode of the I¯_{3} and the IAuI¯ anion at 120 cm^{-1} no change below T, was found. In addition, wavelength dependent measure­ments on α-(BEDT-TTF)_{2}I_{3} were performed, indicating that the modes around 30 cm^{-1} are probably librational modes of the BEDT-TTF molecules.
2
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Magnetic Field Dependent Effective Mass in the Organic Superconductor κ-(BEDT-TTF)_{2}I_{3}

76%
Balthes E., Schweitzer D., Heinen I., Keller H., Biberacher W., Jansen A., Steep E.
Acta Physica Polonica A
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1995
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vol. 87
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issue 4-5
767-772
EN
κ-(BEDT-TTF)_{2}I_{3} is an electronically extreme two-dimensional organic metal with a superconducting transition at around 4 K. In magnetic fields above 12 T the effective mass, as obtained from the temperature depen­dence of the amplitudes of Shubnikov-de Haas oscillations, is magnetic field dependent as long as the magnetic field is arranged perpendicular to the conducting planes. In contrast to this, by turning the magnetic field by 27° or even only 9° the observed effective mass (as obtained from Shubnikov-­de Haas and de Haas-van Alphen measurements) is field independent. We suppose that the occurrence of anyons at temperatures below 1 K in fields above 12 T is the reason for the observed field dependence of the effective mass.
3
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Investigations of Fermi Surfaces and Effective Masses of the Organic Superconductors (BEDO-TTF)_{2}ReO_{4}(H_{2}O) and κ-(BEDT-TTF)_{2}I_{3} by Shubnikov-de Haas and de Haas-van Alphen Measurements

64%
Schweitzer D., Balthes E., Karlich S., Heinen I., Keller H. J., Strunz W., Biberacher W., Jansen A. G. M., Steep E.
Acta Physica Polonica A
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1995
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vol. 87
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issue 4-5
749-759
EN
The Fermi surfaces and effective masses of (BEDO-TTF)_{2}ReO_{4}(H_{2}0) and κ-(BEDT-TTF)_{2}I_{3} were investigated by Shubnikov–de Haas (SdH) and de Haas–van Alphen (dHvA) measurements in magnetic fields up to 27 T in the temperature range from 0.5 K to 4.2 K. Two small closed pockets (0.7% and 1.5% of the first Brillouin zone) are observed in (BEDO-TTF)_{2}ReO_{4}(H_{2}0) corresponding very well with two cross-sectional areas of the Fermi surfaces obtained for a hole and an electron pocket from tight binding calculations. In contrast, in κ-(BEDT-TTF)_{2}I_{3} two rel­atively large closed sections (13% and 85% of the first Brillouin zone) of the Fermi surfaces are observed, again confirming the tight binding calcu­lations. For κ-(BEDT-TTF)_{2}I_{3} in magnetic fields above 12 T 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 (Θ = 0°). 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. We suppose that the occurrence of anyons at temperatures below 1 K and in fields above 12 T might be the reason for the observed field dependence of the effective mass in the SdH investigations under the special angle 0°.
4
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Fabrication and Characterization of Thin Films of the Organic Superconductor α_{t}-(BEDT-TTF)_{2}I_{3}

64%
Moldenhauer J., Wachtel H., Schweitzer D., Gompf B., Eisenmenger W., Bele P., Brunner H., Keller H.
Acta Physica Polonica A
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1995
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vol. 87
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issue 4-5
823-827
EN
α-(BEDT-TTF)_{2}I_{3} is a quasi-two-dimensional organic metal with a metal-insulator phase transition at 135 K. Thermal treatment at about 80°C leads to the metallic system α_{t}-(BEDT-TTF)_{2}I_{3}, which becomes supercon­ducting below 8 K. Thin films of the α-phase (thickness between 500 and 3000 Å) have been evaporated in high vacuum onto several substrates and characterized by means of X-ray diffraction, scanning electron microscopy, atomic force microscopy and low field microwave absorption. Depending on the temperature of the substrate and the evaporation rate, the films exhibit different degrees of microcrystallinity, which under certain conditions can be strongly reduced and a completely covering film can be obtained. X-ray diffraction spectra reveal a high orientation with the c-axis perpendicular to the substrate and as well the successful conversion into the α_{t}-phase by tem­pering. Scanning electron microscopy and atomic force microscopy investiga­tions prove that the conversion takes place without reducing the mechanical quality of the films. Low-field microwave-absorption experiments show that the α_{t}-films become superconducting with an onset at 9 K.
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