Strain effect on electronic transport and ferromagnetic transition temperature in La${}_{0.9}$Sr${}_{0.1}$MnO${}_3$ thin films

We report on a systematic study of strain effects on the transport properties and the ferromagnetic transition temperature $T_c$ of high-quality La${}_{0.9}$Sr${}_{0.1}$MnO${}_3$ thin films epitaxially grown on (100) SrTiO${}_3$ substrates. Both the magnetization and the resistivity are critically dependent on the film thickness. $T_c$ is enhanced with decreasing the film thickness due to the compressive stain produced by lattice mismatch. The resistivity above 165 K of the films with various thicknesses is consistent with small polaronic hopping conductivity. The polaronic formation energy $E_P$ is reduced with the decrease of film thickness. We found that the strain dependence of $T_c$ mainly results from the strain-induced electron-phonon coupling. The strain effect on $E_P$ is in good agreement with the theoretical predictions.