Polaronic pseudogap in the metallic phase ofLa0.625Ca0.375MnO3 thin films

Abstract

The electronic density of states (DOS) ofLa0.625Ca0.375MnO3 (LCMO) strain-free epitaxial thin films with an insulator–metal transition temperature (TIM) of 250 K was probed using variable-temperature scanning tunneling microscopyand spectroscopy. We find a depression in the DOS with a finite zero biasconductance (ZBC) signifying a pseudogap in the 78–310 K temperature range.With cooling, the ZBC is found to increase, indicating an increased DOS nearEF. We interpret the pseudogap as a signature of Jahn–Teller polarons while the ZBC change,in agreement with the bulk insulator–metal transition, optical Drude peak andphotoemission experiments, indicates the presence of free carriers at the Fermi energy inthe metallic phase. The free carriers are discussed in terms of correlated polaronic states.