Temperature Dependence and Low-Temperature Value of the Electromagnetic Goherence Length in Superconducting Pb andPb0.9Bi0.1

The electromagnetic coherence length of Pb and of ${\mathrm{Pb}}_{0.9}$ ${\mathrm{Bi}}_{0.1}$ at a number of temperatures has been calculated by using the microscopic theory of strong electron-phonon coupling. The results show that the coherence length cannot, in general, be correctly obtained from BCS theory by a scaling argument. Near zero temperature, the coherence length is substantially larger than the BCS value $\frac{\overline{\hslash }{v}_{f^{*}}}{\pi {\Delta }_0\mathrm{}\left(0\right)\mathrm{}}$.