Electrical nonlinearity in colossal magnetoresistance manganite films: Relevance of correlated polarons

Abstract

The metal-insulator (MI) transition in epitaxial thin films of ${\text{La}}_{0.75}{\text{Ca}}_{0.25}{\text{MnO}}_{3}$ (LCMO) is accompanied by the appearance of an intrinsic electrical nonlinearity. The latter, probed by electrical third harmonic voltage, ${U}_{3\ensuremath{\omega}}$, or resistance, ${R}_{3\ensuremath{\omega}}=d{U}_{3\ensuremath{\omega}}/dJ$, is drastically enhanced in the vicinity of the MI transition, ${T}_{\text{MI}}=267\text{ }\text{K}$. Applied magnetic field, $B=5\text{ }\text{T}$, suppresses the nonlinearity, resulting in a huge ``nonlinear'' ${\text{CMR}}_{3\ensuremath{\omega}}({T}_{\text{MI}})\ensuremath{\sim}{10}^{5}%$. ${R}_{3\ensuremath{\omega}}$ shows a peculiar low-frequency $(\ensuremath{\omega}\ensuremath{\le}1\text{ }\text{kHz})$ dependence, ${R}_{3\ensuremath{\omega}}\ensuremath{\sim}{(\ensuremath{\omega}\text{\ensuremath{-}}{\ensuremath{\omega}}_{0})}^{n}$, with exponent, $n$, changing across the MI transition from $n\ensuremath{\sim}1,5--2$ for $T\ensuremath{\ge}{T}_{\text{MI}}$ to $n=1$ $(Tl{T}_{\text{MI}})$. The observed electrical nonlinearity in LCMO reflects the behavior of correlated polarons, the number of which dramatically enhances in the vicinity of ${T}_{\text{MI}}$. We argued that correlated polarons, considered as electric-elastic quadrupoles, provide a nonlinear (quadratic) coupling to the electric field, yielding a third harmonic electric nonlinearity in LCMO. The reference film of ${\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_{3}$ (LSMO), a prototypic double exchange system with second-order phase transition, is characterized as a linear metallic material in the whole range of temperatures $(T=10--400\text{ }\text{K})$, magnetic fields $(B=0--5\text{ }\text{T})$, and frequencies $(\ensuremath{\omega}=1--1000\text{ }\text{Hz})$.