Manifestion of structural Higgs and Goldstone modes in the hexagonal manganites

TL;DR

This study explores how Higgs and Goldstone modes manifest in hexagonal manganites through phonon analysis, predicting their temperature-dependent behavior and confirming the Higgs mode in InMnO$_3$ via Raman scattering.

Contribution

It demonstrates that Higgs and Goldstone modes in structural phase transitions are represented by combined phonon modes, with specific temperature dependencies predicted and experimentally supported.

Findings

Higgs mode softening dominates in InMnO$_3$ between 0K and room temperature.

Lattice expansion effects dominate the behavior of ErMnO$_3$.

Raman data confirm the existence of the Higgs mode in InMnO$_3$.

Abstract

Structural phase transitions described by Mexican hat potentials should in principle exhibit aspects of Higgs and Goldstone physics. Here, we investigate the relationship between the phonons that soften at such structural phase transitions and the Higgs- and Goldstone-boson analogues associated with the crystallographic Mexican hat potential. We show that, with the exception of systems containing only one atom type, the usual Higgs and Goldstone modes are represented by a combination of several phonon modes, with the lowest energy phonons of the relevant symmetry having substantial contribution. Taking the hexagonal manganites as a model system, we identify these modes using Landau theory, and predict the temperature dependence of their frequencies using parameters obtained from density functional theory. Separately, we calculate the additional temperature dependence of all phonon mode frequencies arising from thermal expansion within the quasi-harmonic approximation. We predict that Higgs-mode softening will dominate the low-frequency vibrational spectrum of InMnO$_3$ between zero kelvin and room-temperature, whereas the behavior of ErMnO$_3$ will be dominated by lattice expansion effects. We present temperature-dependent Raman scattering data that support our predictions, in particular confirming the existence of the Higgs mode in InMnO$_3$.