Local Symmetry Breaking in the Bulk and in Domain Boundaries: Breit–Wigner Damping of Phonons and Acoustic Resonances

Abstract

It is proposed that Breit-Wigner damping and the appearance of Fano profiles in Raman, Brillouin and resonant ultrasound spectroscopy can be fingerprints for structural disorder and dissipation with a broad excitation spectrum (e.g. heterogeneous distributions of nanodomains), which typically occurs in multiferroic materials. Changes in line shapes from Lorentzian occur by coupling of the normal phonon mode or mechanical resonance with a broad excitation to give a characteristic asymmetry. The detailed features of the resulting Fano profile functions are derived and examples are given for bulk coupling in Raman spectra and interface related couplings in resonant ultrasound spectra related to the dynamics of domain walls. Examples of fano-profiles in BaTiO₃ and SrTiO₃ are discussed and the broad Breit-Wigner spectrum is associated with the disordering of Ti and Sr, respectively.

Acknowledgments

RUS facilities in Cambridge were established with support from the Natural Environment Research Council of Great Britain under grant no. NE/B505738/1 and subsequently under grant number NE/F017081/1.