Browsing by Author "Swietlik, Roman"

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    Existence of two phases in β-(BEDT-TTF)2I3: proof by resonance Raman spectroscopy
    (1987) Swietlik, Roman; Schweitzer, Dieter; Keller, Heimo J.
    Among various organic metals and superconductors based on the BEDT-TTF molecule [bis(ethylenedithiololtetrathiafulvalene] and different polyhalide anions the most intensively studied is β-(BEDT-TTF)2I3. Partially it is caused by the fact that these crystals exhibit the superconductivity with the highest critical temperature reported for organic materials, but the main reason of this interest is the existence of two superconducting states in these crystals. Under ambient pressure the superconductivity in β-(BEDT-TTF)2I3 is observed below Tc≈1,3K1-4 (low-Tc phase) but after a particular pressure-temperature cycling procedure the superconductivity can be stabilized under ambient pressure at Tc≈8,1K5,6(high-Tc phase). α-(BEDT-TTF)2I3 is another modification of the salt formed hetween BEDT-TTF and I3-; at ambient pressure α-phase crystals undergo a metal-insulator phase transition at T=135K 4,7. Recently, it was discovered that by tempering α-phase crystals at about 75°C for several days crystals can be obtained with similar properties as the high-Tc state of the β-crystals 8,9 (these crystals are further denoted as α t-(BEDT-TTF)2I3).
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    Far infrared response of α- and αt-(BEDT-TTF)2I3
    (1990) Zelezný, Vladimir; Petzelt, Jan; Swietlik, Roman; Gorshunov, Boris P.; Volkov, Alexander; Kozlov, Gennadij; Schweitzer, Dieter; Keller, Heimo J.
    The temperature dependence of the reflectivity of α- and αt-(BEDT-TTF)2I3 was determined in the 10-700 cm-1 range. Moreover the transmission spectra of α-phase between 10 and 33 cm-1 were studied. Using Kramers-Kronig analysis the IR conductivity and permittivity were calculated. The similarity of α t and β-phases was confirmed. The spectra in the metallic phase are dominated by a broad electronic peak between 200 and 500 cm-1 which can be understood as due to transitions across the small gap whose origin is discussed. In the spectra a rich phonon peak structure is observed, mostly of vibronic origin. It is suggested that non totally symmetric modes of BEDT-TTF molecule can couple with electrons.
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    Infrared spectra of the BEDT-TTF and DIMET salts with NO3- and ClO4- anions
    (1988) Swietlik, Roman; Schweitzer, Dieter; Keller, Heimo J.
    The infrared absorption spectra of the salts α-(BEDT-TTF)3(N03)2, (BEDT-TTF)3(Cl04)2, (DIMET)2Cl04xTHF and (DIMET)xx(N03)yx(TRF)z have been investigated in the temperature range 80-300 K. The vibrational bands in the BEDT-TTF crystals result from the coupling between electrons and a g modes of the BEDT-TTF molecule. The infrared spectra of the DIMET g salts suggest that the electron-molecular-vibration coupling is not limited to totally symmetric modes.
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    Raman studies of the organic superconductor β-(BEDT-TTF)2IAuI
    (1987) Swietlik, Roman; Grimm, Hans; Schweitzer, Dieter; Keller, Heimo J.
    The resonance Raman spectra of (IAuI)-anions in the organic superconductor. The resonance Raman spectra of (IAuI)-anions in the organic superconductor β-(BEDT-TTF)IAuI as well as in the electrolyte salt n-Bu4AuI2 are investigated. In contrast to β-(BEDT-TTF)2I3 in the crystals of β-(BEDT-TTF)2IAuI at low temperature only one Raman line at 120cm-1 corresponding to the symmetrical stretching mode of the (IAuI)-anion is observed. No distinct change in the Raman spectra is observed at the superconducting transition.
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    Resonance Raman investigations of the symmetric stretching mode of I3- anions in α and β phases of di-bis(ethylenedithio)tetrathiafulvalene tri-iodide
    (1987) Swietlik, Roman; Schweitzer, Dieter; Keller, Heimo J.
    The resonance Raman spectra of I3- anions in the organic superconductor β-(BEDT-TTF)2I3 [BEDT-TTF represents bis(ethylenedithio)tetrathiafulvalene] were studied and discussed in comparison with the nonsuperconducting phase of α-(BEDT-TTF)2I3. In both modifications at low temperature a splitting of the Raman mode assigned to the symmetric stretching mode of the I3- anions was discovered. The splitting of this mode in the β-(BEDT-TTF)2I3 crystals is related to the commensurate superstructure developed below 125 K in contradiction to the α phase where the splitting is related to a crystal-field effect. It is shown that an electronic excitation of the β-phase crystals by laser light can induce a structural transformation at least in a thin layer of the surface. This transformation is observed by a disappearance of the splitting of the symmetric stretching mode. The structural change can be related to a transformation from the commensurate superstructure with a superconducting transition temperature Tc=1.3 K into a more ordered and symmetric structure which becomes superconducting at Tc=8.1 K.
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    A stable superconducting state at 8K and ambient pressure in αt-(BEDT-TTF)2I3
    (1987) Schweitzer, Dieter; Bele, Petra; Brunner, Hermann; Gogu, Emil; Haeberlen, Ulrich; Hennig, Ingolf; Klutz, Thomas; Swietlik, Roman; Keller, Heimo J.
    We report bulk superconductivity at 8 K and ambient pressure in crystals of α t (BEDT-TTF)2I3. In contrast to the earlier observed metastable superconducting state at 8 K in crystals of β-(BEDT-TTF)2I3 here the superconducting state is stable and the crystals can be prepared by tempering α-(BEDT-TTF)2I3 above 70 °C for several days. ac-susceptibility measurements show that the observed superconducting state at 8 K is a bulk property of the crystals. Resistivity measurements indicate a sharp superconducting transition at 8 K with an onset temperature of about 9 K. The upper critical fields Hc2 at 1.3 K lie between 3 and 11 T depending on the direction of the magnetic field with respect to the crystal axes. ESR- as well as NMR-measurements indicate a total transformation of the α-phase crystals into the new superconducting α t -crystals after tempering.
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    X-ray investigations of the low-temperature phases of the organic metals α- and β-(BEDT-TTF)2I3
    (1986) Endres, Helmut; Keller, Heimo J.; Swietlik, Roman; Schweitzer, Dieter; Angermund, Klaus; Krüger, Carl
    The structure of single crystals of the organic metals α- and β-(BEDT-TTF) was determined at 100 K, well below the phase transitions indicated by resistivity and thermopower measurements as well as by differential thermal analysis. In the α-phase no unusual change of the room temperature unit cell but a slight variation in the triiodide network and especially a more pronounced dimerization in one of the two donor stacks have been found. The β-phase develops a superstructure with a unit cell volume three times as large as that at room temperature and with pronounced distortions of the I3--ions.