Figure 160: In the superconducting state the entropy is decreased with respect to the normal state.Figure 161: Schematic illustration of the dependence of the heat capacity on temperature in a
superconductor.
within the BCS theory. In difference to normal insulators, the energy gap in superconductivity is not
a result from electron-lattice interactions, but from electron-electron interactions.
16.2.4 Isotope Effect
The experimental observation thatTcvaries with the isotopic mass is referred to as the isotope effect.
For instance, in case of Hg one measures a change ofTcfrom 4. 185 K to 4. 146 K under a variation
of the isotopic massMfrom 199. 5 to 203. 4 atomic mass units. The experimentally observed relation
betweenTcandMreads
MαTc=const., (267)where some selected values forαare given in Tab. 4. This observation includes a further, highly
important information about the underlying physical processes: Since the transition temperature of
the superconductor varies with the atomic mass rather then with the electronic structure we can
conclude that electron phonon interactions might play a crucial role.