Oxygen isotope effect and phase separation in the optical conductivity of (La$_{0.5}$Pr$_{0.5}$)$_{0.7}$Ca$_{0.3}$MnO$_3$ thin films

TL;DR

This study investigates how oxygen isotope substitution affects the optical conductivity and phase separation in (LaPrCa)MnO3 thin films, revealing isotope-dependent changes primarily in the ferromagnetic phase.

Contribution

It provides new insights into isotope effects on optical properties and phase separation in manganite thin films, highlighting the role of internal strain.

Findings

Oxygen isotope exchange significantly alters optical response in the ferromagnetic phase.

$^{18}$O substitution suppresses Drude response and mid-infrared peak.

Optical response indicates the presence of phase separation and effects of internal strain.

Abstract

The optical conductivities of films of (La$_{0.5}$Pr$_{0.5}$)$_{0.7}$Ca$_{0.3}$MnO$_3$ with different oxygen isotopes ($^{16}$O and $^{18}$O) have been determined in the spectral range from 0.3 to 4.3 eV using a combination of transmission in the mid-infrared and ellipsometry from the near-infrared to ultra-violet regions. We have found that the isotope exchange strongly affects the optical response in the ferromagnetic phase in a broad frequency range, in contrast to the almost isotope-independent optical conductivity above $T_C$. The substitution by $^{18}$O strongly suppresses the Drude response and a mid-infrared peak while enhancing the conductivity peak at 1.5 eV. A qualitative explanation can be given in terms of the phase separation present in these materials. Moreover, the optical response is similar to the one extracted from measurements in polished samples and other thin films, which signals to the importance of internal strain.