[Pw_forum] Should I scrap the nonanalytic part of DM in electroded thin films?

[Nicola Marzari]

Dear Serge,


fascinating question. Not sure about the answer - I'll
ramble a bit here.

Let's see - if we go from the bulk to a thin film (let's
forget the shortcircuiting for a moment) what should happen
is that the 2 TO phonons that were degenerate should now split
in TOpara and TOperp, depending if the modes are perpendicular
or parallel to the thin film.

Would the LO and the TOpara be degenerate ? I think not, for
a suspended thin film, since in the limit of q going to 0 (q of
course now lives in 2d, not in 3d), for a LO mode, you would
still create a dipole per unit cell, i.e. an additional energy
density.

Would contacting this with a metal change what's happening ?
Well, I guess it there were  screening, probably, with the
LO mode dropping down - in case of perfect screening,
becoming degenerate with TOpara. In that case, I presume dropping
the nonalaytic term would do the trick, as you suggest.

Still, I cannot see perfect screening happening, in general -
it will depend on the metal/insulator contact, the thickness, etc...
So you might actually try some calculations in which you keep 
everything, and do a sandwich of real metal and real ionic material ?

I'd be curious to know what people think about Serge's question.

				nicola



Serge Nakhmanson wrote:
> Dear All,
> 
> A small question to those of you with expertise in
> phonon calculations of ionic crystals.
> 
> I understand that in bulk calculations the dynamical
> matrices at small q are split into analytic and
> nonanalytic parts, with the latter accounting for the
> effects of macroscopic electric field. However, if
> we want to study phonons in a model representing an
> electroded thin film instead of a bulk crystal (i.e.,
> the electrodes are not explicitly included in such a
> calculation), we should assume that such an electric
> field will be screened out by the free charges and
> its influence onto ionic vibrations should not be
> considered.
> 
> I am under an impression that to achieve this in pwscf
> I need to block the calls to nonanal(...) subroutine
> in dynmat and matdyn, while leaving everything else
> intact. Would this work out or could it possibly be not
> as trivial as I imagine it?
> 
> Would appreciate your comments,
> 
> Serge
> 


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