Paramagnetic Collective Electronic Mode and Low Temperature Hybrid Modes in the Far Infrared Dynamics of Orthorhombic NdMnO3

TL;DR

This study investigates the infrared spectral features of NdMnO3 across a range of temperatures, revealing a collective electronic excitation linked to orbital fluctuations and its evolution into magnetic modes below the Néel temperature, indicating complex magnetoelectric interactions.

Contribution

It uncovers the origin of a collective THz excitation in NdMnO3 related to orbital fluctuations and details its transformation into magnetic modes associated with antiferromagnetic order.

Findings

Identification of a collective excitation due to eg electrons at room temperature.

Observation of two magnetic modes emerging below 78 K with power-law behavior.

Detection of phonon profile changes indicating magnetoelastic effects at the magnetic transition.

Abstract

We report on far- and mid-infrared reflectivity of NdMnO3 from 4 K to 300K. Two main features are distinguished in the infrared spectra: active phonons in agreement with the expected for orthorhombic D2h 16-Pbnm (Z=4) space group remaining constant down to 4 K and a well-defined collective excitation in the THz region due to eg electrons in a d-orbital fluctuating environment. We trace its origin to the NdMnO3 high temperature orbital disordered intermediate phase not being totally dynamically quenched at lower temperatures. This results in minute orbital misalignments that translate in randomize non-static eg electrons within orbitals yielding a room temperature collective excitation. Below TN~78 K, electrons gradually localize inducing long-range magnetic order as the THz band condenses into two modes that emerge pinned to the A-type antiferromagmetic order. They harden simultaneously down to 4 K obeying power laws with TN as the critical temperature and exponents {\beta}~0.25 and {\beta}~0.53, as for a tri-critical point and Landau magnetic ordering, respectively. At 4K they match known zone center spin wave modes. The power law dependence is concomitant with a second order transition in which spin modes modulate orbital instabilities in a magnetoelectric hybridized orbital/charge/spin/lattice scenario. We also found that phonon profiles also undergo strong changes at TN~78 K due to magnetoelasticity.