Anomalous thermal expansion and strong damping of the thermal conductivity of NdMnO$_3$ and TbMnO$_3$ due to 4f crystal-field excitations

TL;DR

This study investigates how 4f crystal-field excitations in NdMnO₃ and TbMnO₃ cause anomalous thermal expansion and significantly reduce thermal conductivity through resonant phonon scattering, highlighting the role of 4f multiplet splitting.

Contribution

It reveals the impact of 4f crystal-field excitations on thermal properties and demonstrates resonant phonon scattering as the main mechanism in these compounds.

Findings

Schottky contributions to thermal expansion due to 4f multiplet splitting

Strong suppression of thermal conductivity caused by resonant phonon scattering

Enhanced Mn moment canting linked to Nd-Mn exchange coupling

Abstract

We present measurements of the thermal conductivity $\kappa $ and the thermal expansion $\alpha$ of NdMnO$_3$ and TbMnO$_3$. In both compounds a splitting of the $4f$ multiplet of the $R^{3+}$ ion causes Schottky contributions to $\alpha$. In TbMnO$_3$ this contribution arises from a crystal-field splitting, while in NdMnO$_3$ it is due to the Nd-Mn exchange coupling. Another consequence of this coupling is a strongly enhanced canting of the Mn moments. The thermal conductivity is greatly suppressed in both compounds. The main scattering process at low temperatures is resonant scattering of phonons between different energy levels of the $4f$ multiplets, whereas the complex 3d magnetism of the Mn ions is of minor importance.