Indium-Impregnated Porous Glass: Magnetotransport and Superconducting Transition
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INDIUM-IMPREGNATED POROUS GLASS: MAGNETOTRANSPORT AND SUPERCONDUCTING TRANSITION
M. J. GRAF', a. b. c. d.
C. A. HUBERb*,
T. E. HUBERc,
and A. P. SALZBERGd
Boston College, Dep. of Physics, Chestnut Hill, MA02167 Francis Bitter National Magnet Laboratory, M.I.T., Cambridge, MA02139 Harvard University, Lyman Laboratory of Physics, Cambridge, MA02138 University of Puerto Rico, Dep. of Physics, Rio Piedras, PR 00931
ABSTRACT Measurements of the resistive superconducting transition and magnetoresistance of indium metal in the restricted geometry of 50 A size porous Vycor glass are reported. We have mapped out the upper critical magnetic field versus temperature phase diagram for these transitions for fields up to 20 T and temperatures down to 100 mK. The results are consistent with magnetization measurements by previous authors and can be generally interpreted in terms of the description of inhomogeneous type-Il superconductors. The field-induced resistive transitions exhibit unusual behavior not probed by previous bulk-type measurements which may be related to the microstructure of the composite.
Introduction Finely dispersed metals in granular form or in filamentary networks have long been a subject of interest. One way to prepare such a material is by impregnating a microporous insulating matrix with metals. By physically impregnating a microporous glass with superconducting metals a three-dimensional nanostructure can be obtained in which the electron mean free path is signicantly smaller than the superconducting coherence length [1]. This results in an enhancement of the critical field of the superconductor. The properties of the metallic composites determined from magnetization measurements have been described in the past [2-4]. There is renewed interest in these composites in light of their possible connection to current research topics such as percolation, multiply-connected conductors, superconducting fluctuations and vortex flow. We report here on a study of the magnetic field-induced resistive superconducting transition of indium-impregnated porous glass. These measurements show new aspects of such system which have not been probed before.
Material preparation and characterization The leaching result is of pores
porous glass employed (Vycor, Corning code 7930) is made by acid the boron-rich phase in a phase-separated borosilicate glass. The a high content silica glass containing an interconnected network typically less than 100 A in diameter. Its 0.28 porosity exceeds
Mat. Res. Soc. Symp. Proc. Vol. 195. @1990 Materials Research Society
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the critical value of 0.16 for continuous percolation in three dimensions. A standard porosimetry analysis was performed on the porous glass material used in our studies. Mercury intrusion and nitrogen adsor~tion-desorption measurements indicate average pore diameters of 37A and 56 A, respectively. The main reason for the different average pore diameters is but that the pores are not cylinders, as assumed by these techniques, rather like pockets joined by gates
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