STM studies of electronic inhomogenieties in La__{0.35}Pr__{0.275}Ca__{0.375}MnO__{3} thin films

Abstract

We report on temperature dependent STM/S measurements of Laser ablated epitaxial thin films ( ≅ 400nm thick) of La_0:35Pr_0:275Ca_0:375MnO₃ (LPCMO) on NdGaO₃ (NGO) substrate. Four-probe resistivity measurement on this film shows a sharp transition near 145K (T_MI ) and a significant hys- teresis with temperature between 90K and 160K. The topographic STM images show a clean terraced surface at all temperatures (77-350K) with mono-atomic steps and terrace width of 300-400nm. Some inhomogeni- eties are observed in the conductance images on a length scale of ≅20nm and more apparent near the terrace steps. Although spatial variations in spectra are seen at all temperatures but with some common features that evolve with temperature. The spectra become gap-like below 210K (i.e. T_CO) and with cooling this charge-ordering (CO) gap becomes more pronounced with a magnitude of 0.4-0.5eV. This shows an increase in the strength of the CO order parameter and makes us believe that with cooling the CO fraction in the phase-separation scenario is not decreas- ing with temperature. In this case the large resistivity change at T_MI may have to invoke a third phase (other than metallic and CO phases) that becomes metallic with cooling rather than CO fraction melting into metallic phase.