Charge and Lattice Dynamics of Ordered State in La$_{1/2}$Ca$_{1/2}$MnO$_3$: Infrared Reflection Spectroscopy Study

TL;DR

This study uses infrared reflection spectroscopy to investigate the charge and lattice dynamics in La$_{1/2}$Ca$_{1/2}$MnO$_3$, revealing phase transitions, charge ordering, and phonon behavior across magnetic states.

Contribution

It provides new insights into the charge density waves, phonon spectrum enrichment, and phase coexistence in La$_{1/2}$Ca$_{1/2}$MnO$_3$ through detailed spectroscopic analysis.

Findings

Charge density waves form below T_N.

A gap of 320 cm$^{-1}$ opens in the excitation spectrum.

Coexistence of ferromagnetic and antiferromagnetic phases at low temperatures.

Abstract

We report an infrared reflection spectroscopy study of La$_{1/2}$Ca$_{1/2}$MnO$_3$ over a broad frequency range and temperature interval which covers the transitions from the high temperature paramagnetic to ferromagnetic and, upon further cooling, to antiferromagnetic phase. The structural phase transition, accompanied by a ferromagnetic ordering at T$_C$=234 K, leads to enrichment of the phonon spectrum. A charge ordered antiferromagnetic insulating ground state develops below the N\'eel transition temperature T$_N$=163 K. This is evidenced by the formation of charge density waves and opening of a gap with the magnitude of 2$\Delta_0$ = (320 $\pm$ 15) cm$^{-1}$ in the excitation spectrum. Several of the infrared active phonons are found to exhibit anomalous frequency softening. The experimental data suggest coexistence of ferromagnetic and antiferromangetic phases at low temperatures.