Far-infrared conductivity and anomalous below-gap absorption in superconducting granular NbN
Abstract
The far-infrared (10-100 cm-1) complex conductivity of superconducting granular NbN films has been determined from reflection and transmission data. Moderately granular films, with R□=150 and 200 Ω□, followed Leplae-modified Bardeen-Cooper-Schrieffer-theory predictions with no absorption below the gap frequency ωg. The derived values of 2ΔkTc were 3.8 and 4.0, slightly below the results for homogeneous films. Highly granular films, with R□>=500 Ω□, displayed dc resistive tails and evidence of Josephson coupling. The real part of the far-infrared conductivity was BCS-type for frequencies above ωg but showed an anomalous absorption below ωg. This excess absorption may arise from two-dimensional fluctuations in the form of vortices or from normal conducting regions mixed in with the superconducting grains.
- Publication:
-
Physical Review B
- Pub Date:
- May 1983
- DOI:
- 10.1103/PhysRevB.27.5460
- Bibcode:
- 1983PhRvB..27.5460K