[Pw_forum] Gamma point frequencies are not unique in the dispersion curve

[stefano de gironcoli]

in polar materials the long range nature of Coulomb interaction makes 
the dynamical matrix non analytical in the q->0 limit. This originates 
the so called LO-TO splitting.
In cubic semiconductors this splitting is such that the LO frequency is 
direction independent but in systems of low enough symmetry the 
dielectric response and the effective charges tensors are not simply 
multiples of the identity. Hence the LO (and TO) frequencies may depend 
on the
direction of approach to the gamma point.

see for instance the phonon dispersion in wurtzite semiconductors

Bungaro and de Gironcoli, "Ab initio study of phonons in wurtzite 
AlxGa1-xN alloys", Applied Physics Letters 76, 2101-2103 (2000),DOI: 
10.1063/1.126268

stefano


On 27/04/2016 14:29, Atanu Paul wrote:
> Hello all,
> I am new to use PWscf code. I have been trying to calculate the phonon 
> dispersion of a 2d structure using Pwscf code. Dynamical matrices have 
> been calculated with 4x4x1 mesh. Then I have used 'q2r.x' and 
> 'matdyn.x' to get the phonon dispersion curves. The structure of the 
> unit cell is orthorhombic and the high symmectric kpoint path is taken 
> as G-Y-S-X-G. Now If I look at the dispersion curve, the frequencies 
> at Gamma(G) point ( approaching from G-Y, i.e along y-axis in 
> reciprocal space) is different from the frequencies at Gamma(G) point 
> (approaching from X-G, i.e along x-axis in reciprocal space). How can 
> frequency be different at Gamma point? Though I have got positive 
> frequency at any k-points after execution of 'matdyn.x'. Am I doing 
> anything wrong?
>
> Thanking you in advance.
>
> atanu
>
>
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