Density of states of hole-doped manganites: a scanning-tunneling-microscopy/spectroscopy study

Abstract

Variable temperature scanning-tunneling-microscopy/spectroscopy studies on single crystals and epitaxial thin films of hole-doped manganites, which show colossal magnetoresistance, have been done. We have investigated the variation of the density of states (DOS), at and near the Fermi energy (E_f), as a function of temperature. Simple calculations have been carried out, to find out the effect of temperature on the tunneling spectra and extract the variation of density of states with temperature, from the observed data. We also report here atomic resolution images, on the single crystals and larger range images showing the growth patterns on thin films. Our investigation shows unambiguously that there is a rapid variation in density of states for temperatures near the Curie temperature (T_c). While for temperatures below T_c, a finite DOS is observed at E_f, for temperatures near T_c a hard gap opens up in the density of states near E_f. For temperatures much higher than T_c, this gap most likely gives way to a soft gap. The observed hard gap for temperatures near T_c is somewhat higher than the transport gap for all the materials. For different materials, we find that the magnitude of the hard gap decreases as the T_c of the material increases and eventually, for materials with a T_c close to 400 K, the value of the gap approaches zero.