Evidence of mobile carriers with charge-ordering gap in epitaxial Pr_0.625Ca_0.375MnO₃ thin films

Abstract

Epitaxial thin films of charge-ordered Pr_0.625Ca_0.375MnO₃ have been studied using variable-temperature scanning tunneling microscopy and spectroscopy. The as-grown films were found to be granular while the annealed films show atomic terraces at all temperatures and are found to be electronically homogeneous in 78-300 k temperature range. At high temperatures (Tgt;T_CO≈230 k) the local tunnel spectra of the annealed films show a depression in the density of states (DOS) near Fermi energy (E_F) implying a pseudogap with a significant DOS at E_F. The gap feature becomes more robust with cooling with a sharp jump in DOS at E_F at T_CO and with a gap value of ~0.3 eV at 78 k;. At low temperatures we find a small but finite DOS at E_F indicative of some delocalized carriers in the charge-ordered phase together with an energy gap. This is consistent with bulk transport, which shows weakening of the activation gap with cooling below 200 k, and indicates the presence of two types of carriers at low temperatures.