Surface impedance of superconducting Nb3Sn

Microwave cavities with a resonant frequency of 8 GHz are coated with Nb
3
Sn by the vapour deposition technique. The surface resistance and the change of the penetration depth were determinded by measuring the quality factor and the shift of the resonant frequency of the cavity in the temperature range from 2 K to 20K. The temperature dependence of the surface resistance can be described well by the BCS-theory in the temperature range
T < 0.5 T_{c}
, however, the value of the reduced energy gap
\Delta_{0}/kT_{c}
has to be increased from 1.76 to 2.15. The temperature dependence of the penetration depth shows significant deviations from the predictions of the BCS-theory for temperature
T < 0.5 T_{c}
. The increase of the reduced energy gap is not sufficient to fit the data but one has to treat the effects of strong electron-phonon coupling consistently. Therefore, we calculated the surface impedance for strong coupling superconductors using an Einstein model for the phonon density of states. The absolute value and the temperature dependence of the surface impedance in the whole temperature range
T < T_{c}
are discussed and a comparison with the experimental data is given.