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Critical field for a superconductor - why?

  1. Apr 3, 2007 #1
    Cool a cylinder made from a Type I superconducting material below the superconducting transition temperature.

    Apply a magnetic field parallel to the cylinder. The cylinder expels the flux.. up until the field reaches a critical value.

    Why does a critical value exist? That is, why doesn't the cylinder expel magnetic flux independent of the field strength?

    Many thanks.
  2. jcsd
  3. Apr 3, 2007 #2


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    In short, it is a question of energy balance. The superconducting state has lower energy due to "condensation" of electrons into Cooper pairs. In the absence of a strong field, it is energetically favorable to be in the superconducting state.

    The expelled flux, however, exerts a pressure inwards and this represents an unfavorable energy situation. As the field strength rises, the energy penalty to exclude flux exceeds the energy savings of condensation, and the material goes normal.
  4. Apr 4, 2007 #3
    Thank you marcusl, that is very helpful.

    Does a critical field exist for a zero-resistance (not superconducting) cylinder? Hall (1st ed Manchester) p264 suggests this is the case. Why?
  5. Apr 4, 2007 #4


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    Since a non-SC zero resistance material doesn't exist, this is a theoretical discussion. There is no critical field so far as I know.

    Do you know the difference in behavior between the Meissner effect (SC) and a perfect conductor?
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