[Pw_forum] ph.x and Nesting Fermi Surfaces

[W2AGZ]

Miguel, thanks for getting back so quickly.

Actually, I use what the ph.x docs call "line of input" feature failrly
often to do "quicky" calculations of the "gamma point" e-p interaction.  I'm
familiar with example 2, but note, for Si, G and X are two separate
calculations.  I don't think you can specify two different q-points, e.g.,
at -kf, +kf as separate lines of input (I could be wrong).  What I sort of
have in mind is whether one could somehow specify as input to ph.x a
"q-vector" spanning the points -kf to kf and calculate the net e-p coupling.
For example, suppose kf = (0.25, 0.25, 0)(2pi/a units) and 2kf "connects"
two mostly planar Fermi surfaces on opposite sides of the BZ.  Physically,
this an extremely unstable situation would result in a CDW phase transition
doubling the unit cell (dimerization) and gapping the Fermi surface and
should in principle yield a very large e-p coupling constant beforehand.  I
suspect ph.x, at present, doesn't have the capability to simulate this
situation.

Right now, I have a run going with a "line of input" for q(or kf) = (0.37,
0.37, 0) but it's taking a long time to converge.  I will also try later 2kf
(0.74, 0.74, 0) just to see what happens.  Note these are not "high
symmetry" points.

Again, thanks for your feedback.

-Paul

-----Original Message-----
From: pw_forum-bounces at pwscf.org [mailto:pw_forum-bounces at pwscf.org] On
Behalf Of Miguel Martínez
Sent: Sunday, March 06, 2011 3:22 AM
To: pw_forum at pwscf.org
Subject: Re: [Pw_forum] ph.x and Nesting Fermi Surfaces

Dear Paul,

If I understood you correctly, you want to run a ph.x calculation at a q of
your choice such that q=2kf, right? In that case, you only need to specify
ldisp =.false., not specify any nq1, nq2, nq3 and *after* the slash in the
ph.x input, specify the q points at which you want the calculations done.

Example 02 has ph.x calculate the phonon frequencies at X (and gamma) for
Fm-3d Si and C, and fcc Ni at X.

Hope it helps,

Miguel

On Sat, 5 Mar 2011 18:08:30 -0800 "W2AGZ" <w2agz at w2agz.com> wrote:
> To All:
> 
>  
> 
> I'm running some exercises on electron-phonon interactions in narrow 
> band metal monolayers (actually tetragonal unit cells with "large" c/a 
> parameters...and think doped transition metal oxides).  In this 
> quasi-2D system, several of the Fermi surfaces approach planarity 
> (actually, "Fermi lines"), suggesting  strong electron-phonon 
> interaction should be driven by the "nesting" wave vector (think CDW 
> and/or Peierls-Froehlich).
> 
>  
> 
> However, I'm having difficulty seeing how to structure the input to 
> ph.x to select this wavevector alone (or indeed if this is even 
> possible with ph.x). Imagine a square-planar BZ with dimensions (0.5,
> 0.5) in 2pi/a units, with a "Fermi line" perpendicular to, and kf 
> distance along the (1,1) direction from the BZ origin.  Now, as far as 
> I can see from the ph.x documentation, nq1, etc., is only for defining 
> the Monkhorst-Pack grid for the desired phonon grid-mesh (and are 
> integers anyway).  In principle, one could set the "line of input" 
> xq's to the desired kf (fractional values are presumably allowed), but 
> which would have its origin at BZ center, whereas one would like to 
> "drive" or "excite" the phonons at -kf, +kf together.
> 
>  
> 
> I hope I've made the issue/question clear and haven't said anything 
> egregriously stupid.
> 
>  
> 
> Thanks in advance,
> 
>  
> 
> Paul Grant
> 
> IBM Research Staff Member Emeritus
> 
> Visiting Scholar, Stanford
> 


--
----------------------------------------
Dr. Miguel Martinez Canales
   Department of Physics & Astronomy
   University College London
   Gower Street, London, WC1E 6BT (UK)
Fax:  +44 207 679 0595
Tlf:  +44 207 678 3476
----------------------------------------

 "If you have an apple and I have an apple and  we exchange these apples
then you and I will  still each have one apple. But if you have an  idea and
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