Effects of the two-gap nature on the microwave conductivity of polycrystalline MgB2 films with a critical temperature of 39 K

Published

December 1, 2002

Author(s)

James C. Booth, S. Y. Lee, J. H. Lee, Jung Hoon Han, S. H. Moon, H. N. Lee, John H. Claassen

Abstract

The surface resistance !RS" and the real part !!1" of the microwave complex conductivity of a #380-nm-thick polycrystalline MgB2 film with the critical temperature !TC" of 39.3 K were investigated at frequency #8.5 GHz as a function of temperature. Two distinct coherence peaks were observed in the !1 vs temperature curve at T#0.5TC and #0.9TC, respectively, providing a direct evidence for the two-gap nature of MgB2. The film is shown to have a "-band gap energy of #1.7 meV. For the MgB2 film ion milled down to the thickness of #320 nm, two coherence peaks were still observable with the first conductivity peak at #0.6 TC. The inferred "-band gap energy of #2.0 meV is higher than before ion milling. Reduced normalstate conductivity at TC and RS at temperatures below 15 K were found for the ion-milled film. Calculations based on the weak-coupling BCS theory and the strong coupling theory suggest that both ! and " bands contribute to !1 of the polycrystalline MgB2 films significantly. Our results are in contrast with the observation of single coherence peak, ascribed to the dominant role of the " band, in the microwave conductivity of c-axis-oriented MgB2 films reported by Jin et al. $Phys. Rev. Lett. 91, 127006 !2003"%. Variations in the interband coupling constants with the level of disorder can account for the different TC and !1 behavior of the as-grown and ion-milled films.

Citation

IEEE Xplore Digital Library

Pub Type

Journals

Keywords

superconductivity

Citation

Booth, J. , Lee, S. , Lee, J. , , J. , Moon, S. , Lee, H. and Claassen, J. (2002), Effects of the two-gap nature on the microwave conductivity of polycrystalline MgB2 films with a critical temperature of 39 K, IEEE Xplore Digital Library (Accessed October 5, 2024)