Temperature-dependent and anisotropic optical response of layered Pr$_{0.5}$Ca$_{1.5}$MnO$_{4}$ probed by spectroscopic ellipsometry

TL;DR

This study investigates how the optical conductivity of layered Pr$_{0.5}$Ca$_{1.5}$MnO$_{4}$ varies with temperature and direction, revealing multiple phase transitions and the influence of spin, charge, and lattice interactions.

Contribution

It provides a detailed analysis of temperature-dependent optical anisotropy and charge ordering in Pr$_{0.5}$Ca$_{1.5}$MnO$_{4}$ using spectroscopic ellipsometry, supported by theoretical modeling.

Findings

Charge ordering peak shifts to higher energy as temperature decreases.

Optical anisotropy emerges below the charge-ordering temperature.

Spectral weight anomalies around the 2D-AFM transition suggest oxygen-$p$ orbital involvement.

Abstract

We study the temperature dependence as well as anisotropy of optical conductivity ($\sigma_1$) in the pseudocubic single crystal Pr$_{0.5}$Ca$_{1.5}$MnO$_{4}$ using spectrocopic ellipsometry. Three transition temperatures are observed and can be linked to charge-orbital ($T_{\rm CO/OO}$ $\sim$ 320 K), two-dimensional-antiferromagnetic (2D-AFM) ($\sim$ 200 K), and three-dimensional AFM ($T_{\rm N} \sim$ 125 K) orderings. Below $T_{\rm CO/OO}$, $\sigma_1$ shows a charge ordering peak ($\sim$0.8 eV) with a significant blue shift as the temperature decreases. Calculations based on a model that incorporates a static Jahn-Teller distortion and assumes the existence of a local charge imbalance between two different sublattices support this assignment and explain the blue shift. This view is further supported by the partial spectral weight analysis showing the onset of optical anisotropy at $T_{\rm CO/OO}$ in the charge-ordering region ($0.5-2.5$ eV). Interestingly, in the charge-transfer region ($2.5-4$ eV), the spectral weight shows anomalies around the $T_{\rm 2D-AFM}$ that we attribute to the role of oxygen-$p$ orbitals in stabilizing the CE-type magnetic ordering. Our result shows the importance of spin, charge, and lattice degrees of freedom in this layered manganite.