Bulk polycrystals. The highest Hc2 film also exhibits a significant upward curvature of Hc2(T), and temperature dependence of the anisotropy parameter γ(T) = Hc2 || / Hc2 ⊥ opposite to that of single crystals: γ(T) decreases as the temperature decreases, from γ(Tc) ≈ 2 to γ(0) ≈ 1.5. This remarkable Hc2 enhancement and its anomalous temperature dependence are a consequence of the two-gap superconductivity in MgB2, which offers special opportunities for further Hc2 increase by tuning of the impurity scattering by selective alloying on Mg and B sites. Our experimental results can be explained by a theory of two-gap superconductivity in the dirty limit. The very high values of Hc2(T) observed suggest that MgB2 can be made into a versatile, competitive high-field superconductor.

Effects of induced biaxial strain on the electrical transport and magnetic properties of epitaxial thin films of ${\mathrm{SrRuO}}_{\mathrm{3}}$ and ${\mathrm{La}}_{\mathrm{0.67}}{\mathrm{Sr}}_{\mathrm{0.33}}{\mathrm{MnO}}_{\mathrm{3}}$ by structural transitions of ferroelectric ${\mathrm{BaTiO}}_{\mathrm{3}}$ substrates have been studied. Large jumps of electrical resistivity (∼5% in ${\mathrm{SrRuO}}_{\mathrm{3}}$ and ∼12% in ${\mathrm{La}}_{\mathrm{0.67}}{\mathrm{Sr}}_{\mathrm{0.33}}{\mathrm{MnO}}_{\mathrm{3}})$ and low field magnetization (∼70% in ${\mathrm{La}}_{\mathrm{0.67}}{\mathrm{Sr}}_{\mathrm{0.33}}{\mathrm{MnO}}_{\mathrm{3}})$ have been observed in the films at the structural transition temperatures of ${\mathrm{BaTiO}}_{\mathrm{3}}$ substrate. The hysteretic jumps are reproducible through many thermal cycles, and they can be attributed to strain effects induced by the substrate. The use of phase transitions of ferroelectric substrates to manipulate lattice strain of epitaxial thin film heterostructures can be a useful way to modify the properties of perovskite oxides.