Scanning tunneling spectroscopy across the insulator-to-metal transition in strained manganite films

The transition from insulating to metallic-like behavior in La[1-x]Ca[x]MnO[3] (X~0.33) films is studied by scanning tunnelling spectroscopy (STS). The dependence on substrate-induced strain on the transition temperature and the apparent gap measured by transport is characterized for different film thickness. STS measurements on highly and weakly strained films using a home-made setup showed no evidence of a bimodal distribution of zero-bias conductance (ZBC) values, precluding a static phase separation over lengthscales of 10-100 nm. ZBC follows the trend of macroscopic conductivity, increasing (decreasing) on cooling at low (High) temperatures. At high temperatures, ZBC follows an activated behavior and normalized conductance presents a depletion at low energies, features consistent with a gapped density of states. However, the depletion does not disappear on cooling below the transition temperature. A possible explanation in terms of antiferromagnetic/charge order (AFCO) correlations, which are present both in the ferromagnetic and paramagnetic phases, is provided