Libration-torsional mode as order parameter in H-ErMnO3

NESTOR E. MASSA; AURÉLIEN CANIZARÈS; LEIRE DEL CAMPO; VINH TA PHUOC,; KARSTEN HOLLDACK; PAULA KAYSER; JOSÉ ANTONIO ALONSO

Montevideo

Congreso; VIII Congreso Nacional de Sólidos y Primer Encuentro Bi-Nacional (Argentina-Uruguay),; 2019

Comite ad-hoc

Multiferroic hexagonal ErMnO3 is a Mott insulator that has Mn3+ ions at the center of MnO5 bipyramids. They are joined by in plane corner-sharing oxygens making up seven neighbor cages within which are Rare Earth separating Mn-O planes. It is non-centrosymmetric (space group P63cm (C36v)) ferroelectric below the Curie lock-in temperature at ~830 K. Between this and ~1200 K, it sustains an intermediate phase with negative thermal expansion along c and is paraelectric between ~1200 K and ~1600K. Here we present measurements of absorption, reflectivity, and emission of H-ErMnO3 in the THz to the mid-infrared from 2 K to 1600 K. We will put emphasis in the temperature evolution of a phonon centered at ~290 cm-1 corresponding to out of the plane motions of the Mn and O. On heating, it has a marked softening denoting a latent structural lattice instability, and it is almost totally screened when entering the intermediate phase. This mode is assimilated to a torsion-librational mode giving rise to the negative thermal expansion and a c axis complex hybridization of O-2p with Er3+ levels. This order parameter undergoes remarkable profile changes as the transverse (TO) and longitudinal optical (LO) mode split increases associated with the ferroelectric polar condensation below Tc ~830 K. Since origin of the LO macroscopic field electronic screening implies an strength compatible with phonon dynamic coupling to charge fluctuations we also used a Raman probe exciting with above band gap λexc= 355 nm line. This brings up a remarkable Fröhlich resonance at the macroscopic field frequencies due to the interaction between electron charge and the macroscopic Coulomb potential responsible of the TO-LO split. It implies a univocal detection of a huge electron-phonon interaction concomitant with finding small polaron correlations for which this phonon mediates in the formation of high temperature H-ErMnO3 mid-infrared bipolarons.