Evidence of mobile carriers with charge-ordering gap in epitaxialPr0.625Ca0.375MnO3thin films

Abstract

Epitaxial thin films of charge-ordered Pr$_{0.625}$Ca$_{0.375}$MnO$_{3}$ have been studied using variable temperature Scanning tunneling microscopy and spectroscopy (STM/STS). 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 78-300K temperature range. At high temperatures (T$>$T$_{CO}\approx$ 230 K) the local tunnel spectra of the annealed films show a depression in the density of states (DOS) near Fermi energy 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 $\sim$0.3 eV at 78K. At low temperatures we find a small but finite DOS at E$_F$ indicative of some delocalized carriers in the CO phase together with an energy gap. This is consistent with bulk transport, which shows weakening of the activation gap with cooling below 200K, and indicates the presence of two types of carriers at low temperatures.