Implications of reflectance measurements on the mechanism for superconductivity in MgB${}_2$

Abstract

Recent optical studies in c-axis oriented superconducting MgB${}_2$ films indicate that the electron-phonon coupling is weak [tu01]. We reinforce this conclusion by examining the raw reflectance data; its frequency dependence is incompatible with strong electron-phonon scattering. This is further strengthened by analysis of the real part of the conductivity, and by the temperature dependence of the effective Drude scattering rate. Using a realistic electron-phonon spectral shape [kong01], we find ${\lambda }_{\mathrm{tr}}\approx 0.15$, in agreement with Tu et al. [tu01]. To the extent that ${\lambda }_{\mathrm{tr}}\approx \lambda$, this disagrees sharply with model calculations [kong01,kortus01,an01], and is far too low to provide the means for $T_c=39$ K. A simple model is constructed with coupling to a high frequency excitation, which is consistent with both the low frequency optical data and the high $T_c$.

4 pages, 4 figures

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• High $T_c$ superconductivity in MgB${}_2$ by nonadiabatic pairing
• Interpretation of the de Haas - van Alphen experiments in MgB2
• Multiband and impurity effects in infrared and optical spectra of MgB2
• Strongly Anisotropic s-Wave Gaps in Exotic Superconductors
• Electronic Structure of B-2$p\sigma$ and $p\pi$ States in MgB${}_2$, AlB${}_2$ and ZrB${}_2$ Single Crystals
• Pressure Effects and Large Polarons in Layered MgB_2 Superconductor