What is the role of transverse optical (TO) phonons?

[camillomorin]

TL;DR Summary

How do transverse optical (TO) phonons contribute to the emergence of exotic quantum phenomena? Do they have a role in applications and technology?

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

 

[Dr_Nate]

Could you tell me what you know about the roles the LA, TA, and LO phonons play in quantum phenomena?

 

[camillomorin]

Hi Dr_Nate,
As far as I know, LO phonons are responsible for the Frohlich interaction that mediates the formation of large polarons in many materials. LO phonons also play an important role in the formation of Cooper pairs in BCS superconductivity. TA and LA phonons play an important role in charge-density wave formation in the Peierls mechanism displaying the Kohn anomaly at finite momentum. Let me know if something else comes to your mind!

 

[Fred Wright]

One prospect would be finding a material candidate for time reversal invariant topological superconductivity. That is, finding a candidate monolayer material sandwich which exhibits quasi-one dimensional dispersion. Transverse optical phonons can induce a paraelectric to ferroelectric phase transition and induce a strong electronic interaction. See https://arxiv.org/pdf/1907.04304.pdf

 

[camillomorin]

Thank you very much, Fred Wright!