Depletion of the density of states at the Fermi level in metallic colossal magnetoresistive manganites

We have performed tunneling spectroscopy measurements down to 4.2 K on single crystals of ${\mathrm{La}}_{0.75}{\mathrm{Sr}}_{0.25}{\mathrm{MnO}}_3$ and ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3,$ which show the colossal magnetoresistive behavior in the ferromagnetic metallic phase. The tunneling spectroscopy measurements give information on the density of states (DOS) at and close to the Fermi level $E_F.$ The observed energy dependence of the DOS is similar to that seen in correlated oxides with disorder, which is an important observation since it raises fundamental questions regarding the nature of the metallic state in these oxides. Measurements at low temperatures reveal that the DOS in these compounds show a depletion close to $E_F,$ $|E-E_F|<~15\mathrm{meV};$ unlike that in conventional metals. The normalized DOS, close to $E_F$ shows an energy dependence $\propto |E-E_F{|}^2$ at low temperatures but remains finite as $\overrightarrow{E}{E}_F.$ The depth of the depletion is more in ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ compared to ${\mathrm{La}}_{0.75}{\mathrm{Sr}}_{0.25}{\mathrm{MnO}}_3.$