- How do transverse optical (TO) phonons contribute to the emergence of exotic quantum phenomena? Do they have a role in applications and technology?
Main Question or Discussion Point
Phonons can be acoustic or optical in their character and transverse or longitudinal in their polarization. While the importance of longitudinal acoustic (LA), transverse acoustic (TA), and longitudinal optical (LO) phonons is clear to me for the emergence of different quantum phenomena and for applications, less clear is the role played by transverse optical (TO) phonons. In particular, a subset of them at the Brillouin zone center can be directly excited by light (infrared-active modes, those possessing an intrinsic dipole moment). Besides the role played by these modes in ferroelectricity (with the soft phonon at the Brillouin zone center), I could not find any other example in condensed matter theory where these modes contribute to the emergence of some kind of electronic/structural order, nor any significant effect in applications and technology. Do you have any other example? Thank you in advance for your help! Camillo