Origin of low-energy excitations in charge-ordered manganites

¹  1. Physikalisches Institut, Universität Stuttgart - Pfaffenwaldring 57, 70550 Stuttgart, Germany, EU
²  Prokhorov Institute of General Physics, Russian Academy of Sciences - Vavilova Str. 38, 119991 Moscow, Russia
³  Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences Hefei 230031, PRC
⁴  Faculty of Physics, Southern Federal University - Zorge Str. 5, 344090 Rostov-on-Don, Russia
⁵  Lebedev Physics Institute, Russian Academy of Sciences - Leninsky prosp., 53, 119991, Moscow, Russia

Corresponding authors: zhangtao@issp.ac.cn dressel@pi1.physik.uni-stuttgart.de

Received: 18 November 2009
Accepted: 6 April 2010

Abstract

The low-energy excitations in the charge-ordered phase of polycrystalline La_0.25Ca_0.75MnO₃ are explored by frequency-domain terahertz spectroscopy. In the frequency range from 4 cm^-1 to 700 cm^-1 (energies from 0.4 meV to 90 meV) and at temperatures down to 5 K, we do not detect any feature that can be associated with the collective response of the spatially modulated charge continuum. In the antiferromagnetically ordered phase, broad absorption bands appear in the conductivity and permittivity spectra around 30 cm^-1 and 100 cm^-1 which are assigned to former acoustic phonons optically activated due to a fourfold superstructure in the crystal lattice. Our results indicate that characteristic energies of collective excitations of the charge-ordered phase in La_0.25Ca_0.75MnO₃, if any, lie below 1 meV. At our lowest frequencies of only few wave numbers a strong relaxation is observed above 100 K connected to the formation of the charge-ordered state.