title: Paramagnetic collective electronic mode and low temperature hybrid modes in the far infrared dynamics of orthorhombic NdMnO₃.
abstract: We report on the far- and mid-infrared reflectivity of NdMnO3 from 4 to 300 K. Two main features are distinguished in the infrared spectra: active phonons in agreement with expectations for the orthorhombic [Formula: see text]-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 into randomized 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 antiferromagnetic order. They harden simultaneously down to 4 K, obeying power laws with TN as the critical temperature and exponents β ∼ 0.25 and β ∼ 0.53, as for a tri-critical point and Landau magnetic ordering, respectively. At 4 K 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.
authors.affiliation: Laboratorio Nacional de Investigación y Servicios en Espectroscopía Óptica-Centro CEQUINOR, Universidad Nacional de La Plata, C.C. 962, 1900 La Plata, Argentina.
authors.forename: Néstor E
authors.initials: NE
authors.lastname: Massa
authors.forename: Leire
authors.initials: L
authors.lastname: Del Campo
authors.forename: Domingos
authors.initials: D
authors.lastname: De Sousa Meneses
authors.forename: Patrick
authors.initials: P
authors.lastname: Echegut
authors.forename: María Jesús
authors.initials: MJ
authors.lastname: Martínez-Lope
authors.forename: José Antonio
authors.initials: JA
authors.lastname: Alonso