Effects of the Two- Gap Nature on the Microwave Conductivity of 39 K Polycrystalline MgB2 Films

Abstract

The surface resistance (Rs) and the real part (sigma_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 ~8.5 GHz as a function of temperature. Two coherence peaks were observed in the sigma_1 versus temperature curve at temperatures of ~0.5 Tc and ~0.9 Tc, respectively, providing a direct evidence for the two-gap nature of MgB2. The film appeared to have a pi-band gap energy of 1.8 meV. For the MgB2 film ion-milled down to the thickness of ~320 nm, two coherence peaks were still observed with the first conductivity peak at ~0.6 Tc. Reduction of Tc by 3 K and reduced normal-state conductivity at Tc were observed along with an enhanced pi-band gap energy of 2.1 meV and a reduced Rs at temperatures below 15 K for the ion-milled film. Calculations based on the gap energies from the weak coupling Bardeen-Cooper-Schrieffer theory and the strong coupling theory suggest that both the sigma-band and the pi-band contribute to sigma_1 of the polycrystalline MgB2 films significantly. Our results are in contrast with the observation of single coherence peak at ~0.6 Tc and dominant role of the pi-band in the microwave conductivity of c-axis oriented MgB2 films as reported by Jin et al. [Phys. Rev. Lett. 91, 127006 (2003)]. Variations in the inter-band coupling constants with the level of disorder can account for the different Tc and sigma_1 behavior for the as-grown and ion-milled films. Our results suggest that enhanced inter-band scattering can improve microwave properties of MgB2 filims at low temperatures due to the larger pi-band gap despite the reduction of Tc.

arXiv comment8 pages, 9 figures (cond-mat/0403644 is replaced by this version)

References in corpus (21)

• Beyond Eliashberg superconductivity in MgB2: anharmonicity, two-phonon scattering, and multiple gaps
• Evidence for two superconducting energy gaps in MgB2 by point-contact spectroscopy
• Specific Heat of Mg ^{11}B_2
• Thin Film Magnesium Boride Superconductor with Very High Critical Current Density and Enhanced Irreversibility Field
• Two Gap State Density in MgB${}_2$: A True Bulk Property or A Proximity Effect?
• Two-gap superconductivity in MgB${}_2$: clean or dirty?
• Multiband model for tunneling in MgB2 junctions
• Specific heat of MgB${}_2$ in a one- and a two-band model from first-principles calculations
• Temperature and field dependence of the anisotropy of MgB2
• Anisotropy of the Upper Critical Field and Critical Current in Single Crystal MgB${}_2$
• Fermi surface topology and the upper critical field in two-band superconductors - application to MgB2
• High critical current densities in superconducting MgB2 thin films
• Determination of superconducting anisotropy from magnetization data on random powders as applied to LuNi${}_2$B${}_2$C, YNi${}_2$B${}_2$C and MgB${}_2$
• Superconducting Transition and Phase Diagram of Single Crystal MgB2
• Reflection of two-gap nature in penetration depth measurements of MgB${}_2$ film
• Anomalous microwave conductivity coherence peak in c-axis MgB2 thin film
• Significant Reduction of the Microwave Surface Resistance of MgB2 Films by Surface Ion Milling
• Recent developments in the characterization of superconducting films by microwaves
• The superconducting gap of \QTR{em}{in situ} $Mg{B}_2$ thin films by microwave surface impedance measurements
• Effects of Ion Milling on the Microwave Properties of MgB2 Films
• Transition Spectra for a BCS Superconductor with Multiple Gaps: Model Calculations for MgB_2

Cited by in corpus (5)

• Effective gap at microwave frequencies in MgB2 thin films with strong interband scattering
• Multiband and impurity effects in infrared and optical spectra of MgB2
• Microwave Nonlinearity and Photoresponse of Superconducting Resonators with Columnar Defect Micro-Channels
• Microwave surface resistance of pristine and neutron-irradiated MgB2 samples in magnetic field
• Effect of disorder on the far-infrared conductivity and on the microwave conductivity of two-band superconductors