Browsing by Author "Wang, H. Hau"

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    Analysis of the infrared conductivity of the organic superconductor α-(BEDT-TTF)2(NH4)Hg(NCS)4
    (1992) Dressel, Martin; Eldridge, John E.; Wang, H. Hau; Geiser, Urs; Williams, Jack M.
    An optical study of the α phase superconducting compound, α-(ET)2(NH4)Hg(NCS)4 has been performed. The absolute reflectivity of a deuterated crystal has been measured as a function of temperature and polarization, from which the conductivity was obtained. The overall shapes of these spectra are discussed and compared with those obtained from compounds of different phases. In particular we were able to perform a Drude fit of the low-temperature conductivity for one polarization, rather than of the reflectivity dip. The plasma frequency and damping were extracted and these yielded a reasonable effective mass and mean free path. The isotopic vibrational frequency shifts were obtained by repeating the measurements on a protonated sample, which was, however, too small to provide absolute data. The vibrational features are assigned and discussed.
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    ItemOpen Access
    The correlation of lattice phonon frequencies with the superconducting transition temperatures of some (BEDT-TTF) salts
    (1992) Dressel, Martin; Eldridge, John E.; Williams, Jack M.; Wang, H. Hau
    The far-infrared powder absorption spectra, between 10 cm-1 and 350 cm-1, of seven BEDT-TTF salts that exhibit superconductivity at or near ambient pressure, have been measured as a function of temperature, between 5 K and 300 K. The temperature dependences of the peak frequencies, along with the measured deuterium isotopic frequency shifts, separated the lattice from the internal modes. The frequencies of the observed lattice modes were found to increase with Tc in the three κ-phase salts in agreement with the reduced volume of the unit cell, and to have a mixed response in the three β-phase compounds. Furthermore, the general range and magnitude of the frequencies in the higher-Tc κ-phase compounds were found to be larger than in the lower Tc β-phase compounds. Consequently, no evidence was found for a correlation between higher values of Tc and a softer lattice in these organic superconductors.
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    Electrodynamics of the organic superconductors κ-(BEDT-TTF)2Cu(NCS)2 and κ-(BEDT-TTF)2Cu[N(CN)2]Br
    (1994) Dressel, Martin; Klein, Olivier; Grüner, George; Carlson, K. Douglas; Wang, H. Hau; Williams, Jack M.
    We have performed measurements of the surface impedance in the normal and superconducting states of the title compounds in the millimeter wave-frequency range (1–3 cm-1), and have evaluated the complex conductivity for different crystallographic orientations. Above the transition temperature, the materials behave like metals with a scattering rate of approximately 20 cm-1. In the superconducting state the electrodynamics is in good agreement with calculations based on a BCS ground state: while the penetration depth and the coherence length are anisotropic, the superconducting energy gap shows no indications of line nodes.
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    ItemOpen Access
    The infrared optical properties of the α-phase organic superconductor, α-(BEDT-TTF)2(NH4Hg(NCS)4)
    (1993) Dressel, Martin; Eldridge, John E.; Wang, H. Hau; Geiser, Urs; Williams, Jack M.
    The title compound is the only known organic superconductor (Tc = 0.8 K) with an α-phase crystal structure, in which the conducting plane contains single BEDT-TT F molecules with alternating orientation, rather than the pairs of molecules found in the higher Tc k-phase compounds. We have measured the polarized reflectivity of protonated and deuterated (BEDT-TTF)2(NH4)Hg(NCS)4., from 80 cm-1 to 8000 cm-1, at temperatures between 6.5 K and 300 K. The overall shape of these spectra are discussed. The optical conductivity obtained after a Kramers-Kronig analysis shows no maximum in the mid-infrared as known from the compounds of the κ-phase due to the lack of dimerization of the BEDT-TTF molecules. This causes an accentuation of the far-infrared intraband conductivity. The vibrational features are assigned and discussed.
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    ItemOpen Access
    Low-temperature microwave surface impedance of the conventional organic superconductor κ-(BEDT-TTF)2Cu(NCS)2
    (1993) Dressel, Martin; Bruder, S.; Grüner, George; Carlson, K. Douglas; Wang, H. Hau; Williams, Jack M.
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