Temperature dependence of density of states near the Fermi level in a strain-free epitaxial film of the hole-doped manganite $La_{0.7}Ca_{0.3}MnO_{3}$

摘要

Scanning tunneling spectroscopy measurements were performed on an epitaxial thin film of $La_{0.7}Ca_{0.3}MnO_{3}$ grown on a $NdGaO_{3}$ substrate. The temperature variation of the density of states (DOS) close to the ferromagnetic transition temperature $(T_c)$ was investigated. The strain-free film exhibiting a sharp metal-insulator transition at $T_p$ approximate to $T_c$ approximate to 268 K shows no phase separation as seen by the conductivity map, allowing unambiguous determination of the tunneling spectra as a function of T. The temperature dependence of the conductance (dI/dV) curves and the normalized DOS clearly indicate a depletion in DOS near $T_c$, which fills up as the sample is cooled below $T_c$. The metal-insulator transition at $T_c$ also shows up in the bias dependence of the tunneling curve as the temperature is changed across the transition. In the metallic phase we find that the DOS is similar to what is expected in a correlated metal.