Dear Ding-Fu Shao, you’re right that the eph-linewidth scales linearly with the ph frequency. But, IMO, there are no negative frequencies; they are imaginary and are displayed as negative numbers for simplicity. So gamma should be positive&real or complex for „negative“ frequencies. Maybe something strange in-routine is happening when one tries to calculate gamma for non-stable modes. Not sure, just guessing.
best regards Nicki -------------------------------------------------------- Nicki Frank Hinsche, Dr. rer. nat. Institute of physics - Theoretical physics, Martin-Luther-University Halle-Wittenberg, Von-Seckendorff-Platz 1, Room 1.07 D-06120 Halle/Saale, Germany Tel.: ++49 345 5525460 -------------------------------------------------------- > Message: 9 > Date: Thu, 12 Feb 2015 09:54:38 +0800 > From: Dingfu Shao <[email protected]> > Subject: [Pw_forum] Is the phonon line-width gamma positive for a > instable mode with negative freqency? > To: PWSCF Forum <[email protected]> > Message-ID: > <CAH0vKF03OpU17rBvjJacX11RRzP5FE=sqyahebscsi-t7yr...@mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > 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* > > > * <[email protected]>* > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: > http://pwscf.org/pipermail/pw_forum/attachments/20150212/775a62bc/attachment-0001.html > > > ------------------------------ > > _______________________________________________ > Pw_forum mailing list > [email protected] > http://pwscf.org/mailman/listinfo/pw_forum > > End of Pw_forum Digest, Vol 91, Issue 12 > **************************************** > _______________________________________________ Pw_forum mailing list [email protected] http://pwscf.org/mailman/listinfo/pw_forum
