[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 14:46:43 CET 2015
Dear Ion,
Thanks for your reply. Now I understand it.
Best regards,
Yours Ding-Fu Shao
2015-02-12 15:44 GMT+08:00 Ion Errea <ion.errea at ehu.eus>:
> Dear Ding-Fu,
>
> Considering that the g contains a omega(q,v)^-1/2, gamma(q,v) is
> independent of the phonon frequency and, thus, always positive.
>
> On the contrary, calculating lambda for a mode with imaginary frequencies
> makes no sense, as the crystal is unstable in the harmonic approximation.
>
> Bests,
>
> Ion Errea
>
> Donostia International Physics Center (DIPC),
> Manuel de Lardizabal Pasealekua 4,
> 20018 Donostia-San Sebastián, Basque Country, Spain
>
> Email: ion.errea at ehu.eus
> Tel: +34 943 01 54 23
> Fax: +34 943 01 56 00
> Web: http://ionerrea.wordpress.com/
>
> 12/02/15 02:54(e)an, Dingfu Shao igorleak idatzi zuen:
>
> 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*
>
>
>
>
> _______________________________________________
> Pw_forum mailing listPw_forum at pwscf.orghttp://pwscf.org/mailman/listinfo/pw_forum
>
>
>
--
*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*
*Email: dingfu.shao at gmail.com <dingfu.shao at gmail.com>*
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