Search for E_2g phonon modes in MgB_2 single crystals by point-contact   spectroscopy
  The electron-phonon interaction in magnesium diboride MgB_2 single crystals is investigated by point-contact (PC) spectroscopy. For the first time the electron coupling with E_2g phonon modes is resolved in the PC spectra. The correlation between intensity of the extremely broad E_2g modes in the PC spectra and value of the superconducting gap is established. Our observations favor current theoretical models for electron-phonon mediated superconductivity in MgB_2 and they better match the harmonic phonons model. 
Yu. G. Naidyuk, I. K. Yanson, O. E. Kvitnitskaya S. Lee, and S. Tajima