Effect of strain and tetragonal lattice distortions in doped perovskite manganites

A series of high-quality, coherently strained [La2∕3(Ca0.3em0exor0.3em0exBa)1∕3MnO₃∕SrTiO₃]×N superlattices has been prepared on (100)0.3em0exSrTiO₃ and NdGaO₃ substrates by laser molecular beam epitaxy. The manganite layers are biaxially strained due to lattice mismatch. A quadratic decrease of the metal-to-insulator transition temperature Tₚ with increasing biaxial distortion εbi² both for tensile and compressive in-plane strain is found. For T>Tₚ, the resistivity versus temperature curves could be well described by the small polaron hopping model with the polaron binding energy increasing with increasing εbi². Furthermore, the magnetoresistance of the manganite films was found to strongly increase with increasing εbi² or decreasing Tₚ, respectively, following a universal behavior. An anomalous upturn of resistivity in the low-temperature regime (T<250.3em0exK) was detected, which may be attributed to enhanced Coulomb interaction of the charge carriers resulting from disorder due to the lattice distortion. Our analysis clearly demonstrates the importance of biaxial strain and Jahn-Teller-type lattice distortions for the physics of the doped manganites. It is shown that epitaxial coherency strain can be used to deliberately modify the materials properties.