Optical response to magnetic ordering in PrMnO_3: the relevance of the double exchange interaction

The optical conductivity of the undoped PrMnO${}_3$ manganite has been investigated in details at various temperatures between 300 and 4 K. Its low energy spectrum exhibits an optical gap, and is characterized by a single broad peak centered at $\sim$ 2 eV. This peak is interpreted in terms of an indirect interband transition between the split bands $e_{\text{g}}$ caused by a strong electron Jahn-Teller phonon coupling. The spectral weight of this transition is found to be related to the magnetic ordering, which consists of ferromagnetic planes coupled antiferromagnetically. We show that such a 2D ferromagnetism plays, via the double exchange interaction, an essential role in the electronic properties of PrMnO${}_3$, which is a 3D antiferromagnetic compound. Finally, an excess of optical spectral weight is found above the N'eel temperature, and is attributed to ferromagnetic fluctuations. A signature of such fluctuations is equally found from electron spin resonance experiments.