Magnetically-Related Properties of Bismuth-Based High Tc Superconductors
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MAGNETICALLY-RELATED PROPERTIES OF BISMUTH-BASED HIGH Tc SUPERCONDUCTORS
G.C.VEZZOLI*, M.F.CHEN*, F. CRAVER*, T. BURKE**, B.M. MOON*** A. SAFARI***, B. LALEVIC***, W. STANLEY**** *U. S. Army Materials Technology Laboratory, Watertown, MA "**U.S. Army Pulsed Power Laboratory, Ft. Monmouth, NJ ***Rutgers University, Busch Campus, Piscataway, NJ ****Decision Software Company, Cambridge, MA
ABSTRACT The effect of magnetic fields to IST on the electrical resistance (R) of Bi-Sr-Ca-Cu-O superconductors has been measured at precise temperatures during the transition to the superconducting state. The results show that the temperature at which the externally-applied magnetic field causes a divergence of resistance (R) as a function of inverse temperature is approximately at the same temperature where the positive Hall coefficient begins its steep descent to zero. At slightly higher temperature the Hall coefficient shows a singularity peak akin to a delta function. Internal electric field calculations show that the structure of the superconducting oxides, having more than a single building block polyhedron, gives rise to strong electrostatic fields in the unit cell which in turn causes charge separation or polarization. The charge separation is in accord with the importance of high oxidation states of the multivalence cations and suggests the use of high oxygen overpressure during processing. The magnitude of Tc scales closely with the number of bound holes (associated with the charge transfer excitations) per unit cell. Extensive computer calculations using this model indicate attractive pairing of electrons at inter-electron separations of about 10-15A. INTRODUCTION Our Hall effect studies as a function of temperature (1) were carried out on ceramic Y Ba Cu3 0 72), and were motivated by the work of Hundley et al 13)-on ceramic LaI Sr CuO (4)that had shown a delta-funttion-like singularity in -xR x Our observations of a similar anomaly in the 123 compoung*(1,2) had in turn motivated a similar study in the Bi-based high Tc superconductors. Recent work has shown this type of peaked behavior in Hall studies of single crystal high-Tc oxide superconductor materials (5) as a function of temperature similar to Ref. 1. EXPERIMENTAL PROCEDURES Samples of ceramic Bi-Sr-Ca-Cu-O and Bi-Sr-Ca-Pb-Cu-O high Tc superconductors were prepared by synthesis and processing techniques described elsewhere (6). Materials characterization techniques showed that these powders contained 90 to 95 per cent 2223 phase. All magnetic propertiesand Hall measurements (bar geometry) were performed using a Bitter magnet at the Francis Bitter National Magnet Laboratory at MIT. Mat. Res. Soc. Symp. Proc. Vol. 169. 91990 Materials Research Society
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EXPERIMENTAL RESULTS In Fig 1 we plot the electrical resistance (R) during the transition zone of the Bi-based high-Tc superconductor vs 1000/T for differing values of magnetic field. These data show that the magnetic field causes a divergence of the R vs IOO/T data due to its action on breaking apart Cooper
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