Abstract
Polarization-resolved Raman scattering measurements of MgB2 superconductor as function of excitation, temperature and magnetic field have revealed four distinct superconducting features: a clean gap below 35 cm-1 and three superconducting coherence peaks at 50 and 110 cm-1 for the E2g symmetry and at 75 cm-1 for the A1g symmetry. Their temperature and field dependences have been established. Superconductivity induced renormalization of the E2g phonon consistent with electron-phonon coupling 0.3 has been observed.