Effect of strain on the electrical conduction in epitaxial films of ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$

We have used scanning tunneling microscopy and potentiometry to study the microstructure and nanoscopic current transport in thin films of ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ grown on ${\mathrm{SrTiO}}_3$ and ${\mathrm{NdGaO}}_3$ substrates. Thin (50 nm) films, which are strained, show step-terrace growth, whereas relatively thicker film (200 nm), which is strain relaxed, shows well-connected grains. Charge transport in these films is inhomogeneous on the scale of nanometers. There are large variations in the local potential at grain boundaries as well as step edges. The value of the average field distribution at grain boundaries and step edges is found to be dependent on the strain in the film. Within the terraces local variations in potentials correlate with the nature of the strain in the film and this gives rise to inhomogeneous current flow in them.