Polarized and Resonant Raman scattering (PRS, RRS) have been carried out in microcrystals of MgB2 at room temperature in order to study the symmetry of the broad E2g phonon and its coupling to electronic states. We observe a breakdown of the polarization selection rules when the medium is excited with the 2.41 eV laser line. The RRS experiments show that this breakdown is associated to the resonant behavior of the E2g mode. These results may be understood in terms of band structure calculations that predict a strong coupling between the E2g modes and the B-σ bands. Unpolarized Raman scattering measurements between 10 −300 K in polycrystalline MgB2 using the resonant line of 2.41 eV show the expected anharmonic decay of the linewidth for the E2g mode and its energy remains the same below and above the superconducting critical temperature. These results suggest that the E2g phonon near the Γ point may not play an important role in the 1 electron-phonon coupling mechanism of the superconductivity in MgB2
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