Discussion
Considering the BCS theory of superconductivity, which one of the following statements is not correct ?
Answer: D
The incorrect statement is that flux is quantized in units of h/e.
In BCS theory the charge carriers are Cooper pairs of charge 2e, so the magnetic flux through a superconducting ring is quantized in units of
\[ \begin{aligned} \Phi_{0} &= \frac{h}{2e} \\ &\approx 2.07 \times 10^{-15}\ \text{Wb} \end{aligned} \]
The experiments of Deaver and Fairbank and of Doll and Nabauer (1961) confirmed the factor 2. The other statements are consistent with BCS theory: it predicts an energy gap below \(T_{c}\); the isotope effect, \(T_{c} \propto M^{-1/2}\), follows from electron-phonon pairing, so different isotopes have different \(T_{c}\); and the Meissner effect is observed and accounted for.
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