Linewidth for an unstable phonon mode

[dpasquie]

Dear all,

I am studying with EPW the phonon self-energy for a system that exhibits structural instabilities. I have noticed that for phonon modes with an imaginary frequency, the corresponding linewidth and coupling strength are found to be zero. I wonder, is there any reason for that? Does it signal that something went wrong with the calculations?
In the literature , for example http://arxiv.org/pdf/1103.5755.pdf Fig. 4, I can see that the linewidth is non-zero for unstable modes.


Thank you!

Diego

[sponce]

Dear Diego,

The EPW code rely on density functional perturbation theory (DFPT). The electron-phonon matrix elements are computed within that framework in QE.

Such theory implies that the system is at equilibrium. Soft phonons indicates that the system is not at equilibrium.

Since the theory is ill defined in such cases, both the QE and EPW code neglect (i.e. put explicitly to 0) any matrix elements that have negative phonon frequencies.

If your system is physically unstable (in your case because of CDW), I would be quite cautious when using a DFPT-based theory.

They seem to rely on DFPT to do the calculations in the arXiv paper. I'm personally not convince that it is fully correct but I would have to study it in more depths.

Best,

Samuel