[Pw_forum] Phonons and charge density response

[Vasile Chis]

Dear users and developers, 

Running the ph.x one can extract the charge density response (fildrho)
due to a phonon mode for an arbitrary phonon wave vector q.
Thereafter one can presumably use pp.x to plot the induced charge density 
corresponding to mode in 1D, 2D or 3D (xcrysden). 

When this is done, the density is plotted for the super-cell no matter at
which phonon wave vector q the linear response calculation was done.

Then, I wonder if it is possible to obtain (visualizing, say with pp.x)
the charge density of the corresponding wavelength?

For example: 
One calculates a phonon mode with a wavelength which is twice the
periodicity of the undistorted lattice in a particular direction. 
Let say that a top atom (in a surface slab) vibrates in the z-direction.
Then when visualizing the charge density response for this mode in a 2D-
plane along the wave vector, the first atom moves in the positive z-
direction giving rise to a charge density pattern, then moving along
the corresponding wave vector, the next surface atom should move in the
negative z-direction giving rise to a charge density pattern with
opposite sign. 
Although when extracting the above with pp.x and xcrysden, the charge
density is just repeated (with the same sign) for the forthcoming atoms.

It looks to me that the code consider the phonon induced density in the same way 
as a ground state density and thus benefiting from the symmetry of the unperturbed lattice. 
However, when a displacement field it set up by a phonon mode with a wave vector 
in the surface Brillouin zone this symmetry is broken and the periodicity deviates 
from the periodicity of the undistorted lattice.  

The information of the induced density variation caused by a phonon mode at an
arbitrary q-vector should be possible to extract from a linear response
calculation. 
The question is HOW? 

Looking forward to any replies.

Best regards
Vasse Chis
-- 
  Vasse Chis, Ph.D Student
  Department of Physics
  Göteborgs Universitet
  Fysikgården 6B
  S-412 96 Göteborg, SWEDEN
  tel    :+46 (0) 31 7723338
  email  :Vasile.Chis at physics.gu.se