Measurements of the transmission and reflection of 12-, 8-, and 4-mm microwaves through thin (∼50 Å) superconducting films of tin and indium were made as a function of temperature below the superconducting transition temperature. A vacuum microwave cryostat with the waveguide passing straight through the system was used to make the measurements. A movable sample block made it possible to condense and anneal the films and make microwave measurements without removing the films from the vacuum. Superconducting complex electrical conductivities as formulated by Mattis and Bardeen were used to compute transmission- and reflection-coefficient ratios for comparison with experimental values. In the majority of cases the agreement between experiment and theory is quite good. In some cases the assumption of a superconducting energy gap of $2\Delta \mathrm{}\left(0\right)=2.82k{T}_c$, instead of the predicted $2\Delta \mathrm{}\left(0\right)=3.52k{T}_c$, improved the fit between experiment and theory. The temperature at which the microwave transmission and reflection differ measurably from their values in the normally conducting states can be used as a definition of the critical temperature. Transition temperatures measured in this manner are in close agreement with the usual dc definition.

• Received 20 June 1966

DOI:https://doi.org/10.1103/PhysRev.154.414