Polaronic Pseudogap in the Metallic Phase of La$_{0.625}$Ca$_{0.375}$MnO$_3$ Thin Films

TL;DR

This study investigates the electronic density of states in La$_{0.625}$Ca$_{0.375}$MnO$_3$ thin films, revealing a polaronic pseudogap in the metallic phase and the presence of free carriers linked to correlated polaronic states.

Contribution

It provides direct experimental evidence of a polaronic pseudogap and free carriers in the metallic phase of LCMO thin films using scanning tunneling microscopy and spectroscopy.

Findings

Identification of a pseudogap in the DOS between 78-310 K.

Increase in zero bias conductance with cooling indicating more free carriers.

Correlation of pseudogap features with Jahn-Teller polarons and metallic behavior.

Abstract

The electronic density of states (DOS) of La$_{0.625}$Ca$_{0.375}$MnO$_3$(LCMO) strain free epitaxial thin films with an insulator-metal transition temperature (T$_{IM}$) of 250 K was probed using variable temperature scanning tunneling microscopy and spectroscopy. We find a depression in the DOS with a finite zero bias conductance (ZBC) signifying a pseudogap (PG) in 78-310 K temperature range. With cooling, the ZBC is found to increase indicating an increased DOS near E$_F$. We interpret PG as a signature of Jahn-Teller polarons while ZBC change, in agreement with the bulk insulator-metal transition, optical Drude peak, and photoemission experiments indicates the presence of free carriers at the Fermi energy in the metallic phase. The free carriers are discussed in terms of correlated polaronic states.