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.

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It is generally believed that the electrical transport in magnetoresistive manganites above T C is governed by small polarons associated with large lattice distortions. We present evidence which confirms the existence of magnetic clusters ͑polarons͒ associated with minimal lattice distortions above the Curie temperature. We take advantage of the fact that the Mössbauer spectra of the relatively large and slowly fluctuating magnetic polarons consist of incompletely collapsed magnetic component with extensive wings.