Tunneling study of the charge-ordering gap on the surface of La_0.350Pr_0.275Ca_0.375MnO₃ thin films

Abstract

Variable temperature scanning tunneling microscopy and spectroscopy studies on (110) oriented epitaxial thin films of La_0.350Pr_0.275Ca_0.375MnO₃ are reported in the temperature range of 77-340 K. The films, grown on lattice matched NdGaO₃ substrates, show a hysteretic metal-insulator transition in resistivity at 170 k. The topographic scanning tunneling microscopy images show step-terrace morphology, while the conductance images display a nearly homogeneous surface. The normalized conductance spectra at low temperatures (Tlt;150 K) show an energy gap of 0.5 eV, while for T≥180 k, a gap of 0.16 eV is found from the activated behavior of the zero bias conductance. The presence of energy gap and the absence of phase separation on the surface over more than 2×2 μm² area contradict the metallic behavior seen in resistivity measurements at low temperatures. We discuss the measured energy gap in terms of the stabilization of the insulating CO phase at the film surface.