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  \underline{EFFECT OF CARRIER DOPING ON IRREVERSIBILITY
    FIELD OF}\\ \underline{Bi-2212 SUPERCONDUCTORS}\\ \ \\ 
  \underline{M. Kiuchi}, M. Tagomori, H. Yamato, T. 
  Matsushita and M. Wakata$^{\ast}$\\ Department of Computer
  Science and Electronics, Kyushu Institute of Technology,\\ 
  680--4 Kawazu, Iizuka 820, Japan\\ $^{\ast}$ Materials \&
  Electronic Devices Laboratory, Mitsubishi
  Electric Corp.,\\ 1--1--57 Miyasita, Sagamihara, Kanagawa,
  229, Japan\\ 
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It has been shown that the irreversibility field of high
temperature superconductors is affected by carrier doping
and improved drastically when those are overdoped. This comes
from the variation in the dimensionality of the
superconductor. It is also known, however, that the
irreversibility characteristics depend on the flux pinning
strength which may also be influenced by the carrier doping. 
Hence, it is desired to understand the effect only of the
variation in the dimensionality by the carrier doping on the
irreversibility field. In this paper, the irreversibility
field and the critical current density are measured for
Bi-2212 superconductors prepared under the condition of
various oxygen pressures. The contributions from the
dimensionality and the flux pinning strength to the
irreversibility field are separated by a theoretical
analysis using the flux creep theory. It is shown that the
improvement of the irreversibility field by overdoping can be
attributed to the both effects.

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