We have measured the Raman spectra of polycrystalline MgB 2 from 25 cm −1 to 1200 cm −1 . When the temperature was decreased below the superconducting transition temperature T c , we observed a superconductivityinduced redistribution in the electronic Raman continuum. Two pair-breaking peaks appear in the spectra, suggesting the presence of two superconducting gaps. Furthermore, we have analyzed the measured spectra using a quasi twodimensional model in which two s-wave superconducting gaps open on two sheets of Fermi surface. For the gap values we have obtained ∆ 1 = 22cm −1 (2.7 meV) and ∆ 2 = 50cm −1 (6.2 meV). Our results suggest that a conventional phonon-mediated pairing mechanism occurs in the planar boron σ bands and is responsible for the superconductivity of MgB 2 .

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The oxygen-isotope effect of the ferromagnetic transition in La 1Ϫx Ca x MnO 3 was investigated from x ϭ20% to xϭ43%. This is the range of the conducting ferromagnetic phase. We find that ␣ O ϭ Ϫ⌬ ln T c /⌬ ln m decreases from 0.36 to 0.14 with increasing Ca concentration. A large value of ␣ O ϭ0.83 was found for xϭ20%, it is possibly connected with excess oxygen content. The isotope effect decreases with increasing tolerance factor, pointing to the importance of double exchange. The isotope effect at 35% Ca is independent of magnetic field.