Penetration depth measurements of single-crystal NbN films at millimeter-wave region

The zero temperature penetration depth λ(0) of single-crystal NbN thin films has been measured at 82 GHz using an open resonator. The NbN films used in this study have a (200) single-crystal structure and Tc and ρ20 (normal state resistivity at 20 K) are 16 K and 62 mΩ cm, respectively. λ(0) was obtained by applying a least-squares fit to the change of penetration depth using the BCS theory. We obtain λ(0)=194 nm, which shows good agreement with the calculated value from Tc,ρ20 and Δ(0).