The Density of States of hole-doped Manganites: A Scanning Tunneling Microscopy/Spectroscopy study

TL;DR

This study uses scanning tunneling microscopy/spectroscopy to analyze how the density of states in hole-doped manganites varies with temperature, revealing a transition from finite DOS to a hard gap near the Curie temperature.

Contribution

It provides detailed temperature-dependent measurements of the density of states in manganites, highlighting the evolution of a hard gap near the Curie temperature and its relation to material properties.

Findings

Rapid variation in density of states near $T_c$

Hard gap opens near $T_c$ in the density of states

Gap magnitude decreases with increasing $T_c$

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, 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.