Lattice dynamics reveals a local symmetry breaking in the emergent dipole phase of PbTe

TL;DR

This study uses inelastic neutron scattering to reveal local symmetry breaking and emergent dipole behavior in PbTe, showing a new phonon mode that indicates dynamic lower symmetry without a structural phase transition.

Contribution

It demonstrates the use of INS to detect local symmetry breaking and introduces the concept of 'emphanisis' as an intrinsic, non-dopant related dipole emergence in PbTe.

Findings

Observation of a new zone center phonon mode with increasing temperature

Detection of local Pb off-centering dipoles without a structural transition

Identification of 'emphanisis' as an intrinsic phenomenon in PbTe

Abstract

Local symmetry breaking in complex materials is emerging as an important contributor to materials properties but is inherently difficult to study. Here we follow up an earlier structural observation of such a local symmetry broken phase in the technologically important compound PbTe with a study of the lattice dynamics using inelastic neutron scattering (INS). We show that the lattice dynamics are responsive to the local symmetry broken phase, giving key insights in the behavior of PbTe, but also revealing INS as a powerful tool for studying local structure. The new result is the observation of the unexpected appearance on warming of a new zone center phonon branch in PbTe. In a harmonic solid the number of phonon branches is strictly determined by the contents and symmetry of the unit cell. The appearance of the new mode indicates a crossover to a dynamic lower symmetry structure with increasing temperature. No structural transition is seen crystallographically but the appearance of the new mode in inelastic neutron scattering coincides with the observation of local Pb off-centering dipoles observed in the local structure. The observation resembles relaxor ferroelectricity but since there are no inhomogeneous dopants in pure PbTe this anomalous behavior is an intrinsic response of the system. We call such an appearance of dipoles out of a non-dipolar ground-state "emphanisis" meaning the appearance out of nothing. It cannot be explained within the framework of conventional phase transition theories such as soft-mode theory and challenges our basic understanding of the physics of materials.