The Discovery of Superconductivity

Index Superconductivity concepts Reference Rohlf,Ch 15  

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Lead as SuperconductorEvidence for zero resistanceLead is a Type I superconductor with a critical temperature of 7.2 K. Although such superconductors can conduct currents with zero resistance, their usefulness is limited because of low critical magnetic fields. Above a certain current, the magnetic field created by the current drives the material into a normal resistive state. If a current is generated in a superconducting lead ring, it will persist because of the zero resistivity. Currents have been maintained in lead rings for several years to test the zero resistance condition. An induced current in an ordinary metal ring would decay rapidly from the dissipation of ordinary resistance, but superconducting rings had exhibited a decay constant of over a billion years! An exactly zero resistance implies a quantum effect  an energy gap. If the charge carriers do not interact with their environment to reduce their energy even a little bit, it must be because they can't  they are forbidden to by conservation of energy. This implies that there are no available quantum states within reach of the energy they have. The evidence for an energy gap was one of the steps which led to the BCS theory of superconductivity. 
Index Superconductivity concepts Reference Rohlf,Ch 15  

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