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126 results on '"Geiser, U."'

12. Vortex pinning in layered organic superconductors: κ -(BEDT-TTF)2Cu[N(CN)2]Br.

Author

Khizroev, S., Zuo, F., Alexandrakis, G. C., Schlueter, J. A., Geiser, U., and Williams, J. M.

Subjects
ORGANIC semiconductors, VORTEX motion, MAGNETIZATION, TEMPERATURE
Abstract

Focuses on a magnetization study which examined field and temperature dependence of vortex pinning of organic single crystal superconductors with the field H parallel to the axis perpendicular to the conducting plane. Illustration of an overlay of the magnetization as a function of field at different temperatures; Description of the normalized relaxation rate as a function of field; Graph of irreversibility field as a function of temperature.

Published
1996
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13. Low Temperature Penetration depth of \kappa-(ET)_2Cu[N(CN)_2]Br and \kappa-(ET)_2Cu(NCS)_2

Author

Carrington, A., Bonalde, I. J., Prozorov, R., Giannetta, R. W., Kini, A. M., Schlueter, J., Wang, H. H., Geiser, U., and Williams, J. M.

Subjects
Condensed Matter::Superconductivity, Condensed Matter - Superconductivity
Abstract

We present high precision measurements of the penetration depth \lambda of single crystals of \kappa-(ET)_2Cu[N(CN)2]Br and \kappa-(ET)_2Cu(NCS)_2 at temperature down to 0.4 K. We find that, at low temperatures, the in-plane penetration depth varies as a fractional power law, \lambda\sim T^1.5. W hilst this may be taken as evidence for novel bose excitation processes, we show that the data are also consistent with a quasi-linear variation of the superfluid density, as is expected for a d-wave superconductor with impurities. Our data for the interplane penetration depth show similar features and give a direct measurement of the absolute value, \lambda(0)=100\pm 20 \mu m., Comment: 4 pages, 4 postscript figures

Published
1999

26. Anodized Aluminum Oxide Membranes As Templates for Nanoscale Structures.

Author

Han, C. Y., Xiao, Z. L., Wang, H. H., Willing, G. A., Geiser, U., Welp, U., Kwok, W. K., Bader, S. D., and Crabtree, G. W.

Subjects
ALUMINUM oxide, OXIDES, NANOSCIENCE, SCIENCE, SUPERCONDUCTIVITY, NANOWIRES, ELECTRIC wire, NANOSTRUCTURED materials, NANOTUBES
Abstract

Porous anodic aluminum oxide (AAO) membranes have attracted significant interest during recent years because AAO can be made conveniently and inexpensively. Yet they are extremely useful in nanoscience studies. Pore diameters (5 to 300 nm), and pore-to-pore distances (25 to 500 nm) can be easily controlled by varying the anodizing conditions. Highly ordered, straight nanopores in hexagonally close-packed arrays with domain sizes of approximately 2.5 x 2.5 µm² and aspect ratios as high as 1000:1 can be achieved. The nanopores within the AAO membranes are used as templates for fabricating various nanoscale structures, such as the first successful fabrication of superconducting lead nanowires with diameters of 60 and 200 nm. Lead was electrodeposited into the nanopores of AAO, and bismuth quantum nanotubes were fabricated, utilizing a newly developed high-temperature sintering method. It is also possible to construct highly ordered anti-dot arrays by coating the surfaces of the porous AAO membranes with magnetic materials. Significant enhancement of the critical field in superconducting Pb nanowires and an abnormal magnetoresistance in Ni magnetic anti-dot arrays are found in these samples. The preparation and characterization of AAO and its application in fabricating various nanowires, nanotubes and nanoscale antidots will be presented. INSET: Nuts & Bolts: What This Paper Means to You. [ABSTRACT FROM AUTHOR]

Published
2004

28. BEDT-TTF salts with fluorinated sulfonate anions

Author

Geiser, U., Schlueter, J.A., Kini, A.M., Wang, H.H., Ward, B.H., Whited, M.A., Mohtasham, J., and Gard, G.L.

Subjects
*ORGANIC superconductors, *CRYSTALS
Abstract

A number of layered conducting BEDT-TTF, bis(ethylenedithio)tetrathiafulvalene, salts with heavily fluorinated organosulfonate anions have been prepared and characterized. Of particular interest are the salts containing SF5RSO3− anions, where R is a partially fluorinated aliphatic backbone. While structurally similar —the β″ packing type predominates—the ground state of these salts varies from superconducting in the case of β″-(BEDT-TTF)2SF5CH2CF2SO3 to insulating. Many of the salts with insulating ground states are metallic at room temperature, but charge localization and disproportionation over crystallographically non-equivalent sites occurs at low temperature. The organosulfonate group exhibits a propensity to bind to lithium ions, thus ternary salts incorporating Li+ into the complex anion layer are often found. The fluorophilic effect in organofluorine compounds may be exploited to form salts where the conducting BEDT-TTF layers are separated by extremely bulky anion bilayers. The crystal structure of one such system, (BEDT-TTF)3[(CF3)2CFC2H4SO3]4(H5O2)2, is described here. [Copyright &y& Elsevier]

Published
2003
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30. Electronic structure of some β-(C10H8S8)2X compounds as studied by infrared spectroscopy

Author

Jacobsen, Claus Schelde, Tanner, D. B., Williams, Jack M., Geiser, U., and Wang, H. H.

Abstract

Polarized reflectance measurements have been made on two isostructural conducting compounds of bis(ethylenedithio)tetrathiafulvalene [BEDT-TTF or ET, (C10H8S8)]: β-(ET)2AuI2 and β-(ET)2I2Br. The former is superconducting at ambient pressure with Tc=5 K, whereas the latter retains normal-metal conductivity to low temperatures. The reflectance measurements, made at temperatures of approximately 30 and 300 K, spanned 80 cm-1 (0.01 eV) through 33 000 cm-1 (∼4 eV); they were made for polarization along the ET molecular stacking axis and transverse to it in the sheets or layers of ET molecules. Band-structure parameters determined from the plasmon frequencies imply that the anisotropy is rather low for organic conductors, with t?≊0.22 eV and t⊥≊0.09 eV for β-(ET)2AuI2; t?≊0.18 eV and t⊥≊0.07 eV for β-(ET)2I2Br. At 300 K, the spectra differ substantially from the expectations of simple one-electron models, suggesting that both electron-phonon and electron-electron interactions play important roles in the electronic structure of these materials. At low temperatures, the materials display basically metallic characteristics, yet deviate significantly from simple Drude-model behavior. At 30 K, no effect attributable to superconducting fluctuations could be observed.

Published
1987
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