In this paper we present the results of a systematic study on the magnetic field penetration depth of superconducting niobium thin films. The films of thicknesses ranging from 8 to 300 nm were deposited on a Si substrate by dc magnetron sputtering. The values of the penetration depth ͑0͒ were obtained from the measurements of the effective microwave surface impedance by employing a sapphire resonator technique. Additionally, for the films of thickness smaller than 20 nm, the absolute values of ͑0͒ were determined by a microwave transmission method. We found that the reduction of the film thickness below 50 nm leads to a significant increase of the magnetic field penetration depth from about 80 nm for 300 nm thick film up to 230 nm for a 8 nm thick film. The dependence of the penetration depth on film thickness is described well by taking into account the experimental dependences of the critical temperature and residual resistivity on the thickness of the niobium films. Structural disordering of the films and suppression of superconductivity due to the proximity effect are considered as mechanisms responsible for the increase of the penetration depth in ultrathin films.

Abstract----The temperature dependence of the real part of the microwave complex conductivity at 17.9 GHz obtained from surface impedance measurements of two c-axis oriented MgB 2 thin films reveals a pronounced maximum at a temperature around 0.6 times the critical temperature. Calculations in the frame of a two-band model based on BardeenCooper-Schrieffer (BCS) theory suggest that this maximum corresponds to an anomalous coherence peak resembling the two-gap nature of MgB 2 . Our model assumes there is no interband impurity scattering and a weak interband pairing interaction, as suggested by bandstructure calculations. In addition, the observation of a coherence peak indicates that the π-band is in the dirty limit and dominates the total conductivity of our films.

The dependences of magnetic field penetration depth at zero temperature λ(0), microwave surface resistance Rs and π-band energy gap at zero temperature Δπ(0) on the normal-state resistivity right above the critical temperature, ρ0, were studied for MgB2 thin films prepared by different techniques by employing a sapphire resonator technique. We found that the zero-temperature penetration depth λ(0) data could be well fitted by yielding a London penetration depth λL of 34.5 nm, where ξ0 is the coherence length, and is the mean free path determined from ρ0. The surface resistance Rs at 15 and 20 K increases roughly linearly with ρ0. The observed increase of Δπ(0) with ρ0 and the decrease of Tc indicate the expected effects of interband impurity scattering within an extended BCS approach. The low values of Rs and λ(0) in conjunction with the large coherence length for epitaxial films are potentially attractive for applications in electronics and microwave technology.

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