Implications of reflectance measurements on the mechanism for superconductivity in MgB₂

Original · EN

Recent optical studies in c-axis oriented superconducting MgB₂ 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 λ tr ≈ 0.15, in agreement with Tu et al. [tu01]. To the extent that λ tr ≈ λ, this disagrees sharply with model calculations [kong01,kortus01,an01], and is far too low to provide the means for Tc = 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 Tc.