09magnets - P461 Semiconductors 1 Superconductivity •...

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Unformatted text preview: P461 - Semiconductors 1 Superconductivity • Resistance goes to 0 below a critical temperature T c element T c resistivity (T=300) Ag --- .16 mOhms/m Cu -- .17 mOhms/m Ga 1.1 K 1.7 mO/m Al 1.2 .28 Sn 3.7 1.2 Pb 7.2 2.2 Nb 9.2 1.3 • many compounds (Nb-Ti, Cu-O-Y mixtures) have T c up to 90 K. Some are ceramics at room temp Res. T P461 - Semiconductors 2 Superconductors observations • Most superconductors are poor conductors at normal temperature. Many good conductors are never superconductors • b superconductivity due to interactions with the lattice • practical applications (making a magnet), often interleave S.C. with normal conductor like Cu • if S.C. (suddenly) becomes non-superconducting (quenches), normal conductor able to carry current without melting or blowing up • quenches occur at/near maximum B or E field and at maximum current for a given material. Magnets can be “trained” to obtain higher values P461 - Semiconductors 3 Superconductors observations • For different isotopes, the critical temperature depends on mass. ISOTOPE EFFECT • again shows superconductivity due to interactions with the lattice. If M b infinity, no vibrations, and T c b • spike in specific heat at T c • indicates phase transition; energy gap between conducting and superconducting phases. And what the energy difference is • plasma b gas b liquid b solid b superconductor M K E Sn t cons T M vibrations c ∝ = ) ( tan 119 , 117 , 115 5 . P461 - Semiconductors 4 What causes superconductivity? • Bardeen-Cooper-Schrieffer (BCS) model • paired electrons (cooper pairs) coupled via interactions with the lattice • gives net attractive potential between two electrons • if electrons interact with each other can move from the top of the Fermi sea (where there aren’t interactions between electrons) to a slightly lower...
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This note was uploaded on 10/02/2009 for the course PHYS 460 taught by Professor Johnson,c during the Spring '08 term at Northern Illinois University.

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09magnets - P461 Semiconductors 1 Superconductivity •...

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