Tunneling Study of the Charge-Ordering Gap on the Surface of La$_{0.350}$Pr$_{0.275}$Ca$_{0.375}$MnO$_3$ Thin Films

TL;DR

This study uses scanning tunneling microscopy and spectroscopy to investigate the charge-ordering gap on the surface of La$_{0.350}$Pr$_{0.275}$Ca$_{0.375}$MnO$_3$ thin films, revealing a surface energy gap and its relation to the insulating charge-ordered phase.

Contribution

It provides the first detailed surface tunneling measurements of the charge-ordering gap on these manganite thin films, showing a stable energy gap without phase separation.

Findings

Energy gap of 0.5 eV at low temperatures

Absence of phase separation on the surface

Contradiction between surface gap and bulk metallic behavior

Abstract

Variable temperature scanning tunneling microscopy/spectroscopy studies on (110) oriented epitaxial thin films of La$_{0.350}$Pr$_{0.275}$Ca$_{0.375}$MnO$_3$ are reported in the temperature range of 77 to 340 K. The films, grown on lattice matched NdGaO$_3$ substrates, show a hysteretic metal-insulator transition in resistivity at 170 K. The topographic STM images show step-terrace morphology while the conductance images display a nearly homogeneous surface. The normalized conductance spectra at low temperatures (T$<$150 K) show an energy gap of 0.5 eV while for T$\geq$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 $\mu$m$\times$2 $\mu$m area contradicts 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.