Atomic contrast on ultrathin La 0.7 Sr 0.3 MnO 3 films by Scanning Tunnelling Microscopy

In this work we map tunnel conductance curves and topographies with nanometric and sub-nanometric spatial resolution of fully insulating La 0.7 Sr 0.3 MnO 3 ultrathin films at room temperature. While spectroscopy shows the lack of spatial patterns, suggesting no strong correlation between topographic and spectroscopic features at room temperature, we obtained for the first time clear atomic contrast at only some surface locations. We justify our results by suggesting the presence of local intrinsic inhomogeneities, that would act on a surface with overall homogeneous electronic states as charge density reliever, by allowing atomic contrast by a tunnelling measurement. Furthermore, our findings suggest that the effect of strain is to induce a transition from weak- to strongelectron- phonon coupling regimes, making fully-strained films as interesting model-systems to investigate the nature of the electron correlation directly at nanoscale lengths.