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).