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

Variable temperature scanning-tunneling-microscopy/spectroscopy studies on single crystals and epitaxial thin films of hole-doped manganites, which sh ow colossal magnetoresistance, have been done. We have investigated the var iation 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 th e variation of density of states with temperature, from the observed data. We also report here atomic resolution images, on the single crystals and la rger range images showing the growth patterns on thin films. Our investigat ion shows unambiguously that there is a rapid variation in density of state s 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 har d gap opens up in the density of states near E-f. For temperatures much hig her than T-c, this gap most likely gives way to a soft gap. The observed ha rd 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 eventual ly, for materials with a T-c close to 400 K, the value of the gap approache s zero.