It has been suggested that the maximum magnitude of colossal magnetoresistance occurs in mixed-valent manganites with a tolerance factor t = 0.96 ͓Zhou, Archibald, and Goodenough, Nature ͑London͒ 381, 770 ͑1996͔͒. However, at t Ϸ 0.96 most manganites have relatively low values of the metal-insulator transition temperature T MI ͑ϳ60-150 K͒. Here, we report that a 50 Å La 0.9 Sr 0.1 MnO 3 thin film with t = 0.96 grown on a ͑100͒ SrTiO 3 substrate has a metal-insulator transition above room temperature, which represents a doubling of T MI compared with its value in the bulk material. We show that this spectacular increase of T MI is a result of the epitaxially compressive strain-induced reduction of the Jahn-Teller distortion.
We discuss the effects of hydrostatic pressure on the superconducting properties of MgB2 within the framework of Eliashberg theory. By considering the pressure dependences of all parameters appearing in the McMillan formula, we show that the calculated pressure derivative of Tc as well as the variation of Tc with pressure are in good agreement with recent measurements. The pressure dependences of the energy gap ∆0, the effective interaction strength N (EF )v, the critical magnetic field Hc(0), and the electronic specific heat coefficient γ are also predicted for this system. A comparison of pressure effect in non-transition elements clearly suggests that MgB2 is an electronphonon mediated superconductor.
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