[Pw_forum] Is the phonon line-width gamma positive for a instable mode with negative freqency?
Dingfu Shao
dingfu.shao at gmail.com
Thu Feb 12 02:54:38 CET 2015
Dear QE developers and users,
When calculating the electron-phonon coupling, the phonon line-width
gamma is defined as
gamma(q,v) = 2 * pi * omega(q,v) * sum( |g|^2) * delta(E_q) *
delta(E_(k+q)). (1)
And the electron-phonon coupling strength for mode v at q is
lambda(q,v) = gamma(q,v) / (pi * h * N(E_F) * (omega(q,v))^2) (2).
(See
http://www.quantum-espresso.org/wp-content/uploads/Doc/ph_user_guide/node12.html
)
Accoding to these definitions, it seems that the phonon line-width
gamma(q,v), electron-phonon coupling strength lambda(q,v) must be BOTH
negative when a instable mode with negetive frequency omega(q,v) is
calculated.
However, I checked the electron phonon coupling I calculated before, I
found with a negetive omega(q,v), there are negative lambda(q,v) and
positive gamma(q,v). I found some similar report, for example, PHYSICAL
REVIEW B 86, 155125 (2012), in which the positive gammas were calculated
for the instable mode.
That realy puzzles me. Can anybody explain it?
Thanks a lot!
Best regards,
yours
Ding-Fu Shao
--
*Ding-Fu Shao, Ph.D *
*Institute of Solid State Physics*
*Chinese Academy of Sciences*
*P. O. Box 1129*
*Hefei 230031*
*Anhui Province*
*P. R. China*
* <dingfu.shao at gmail.com>*
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