[Pw_forum] Computing Second order response

[Stefano Baroni]

well, you probably know more than I do, but the cross section for (off-resonant) Raman scattering is basically the derivative of the dielectric constant with respect to the amplitude of the vibrational distortion. I do not know what exactly is measured (you probably do), but this is what is calculated, and the dielectric tensor of a metal is infinite ... in any case, it is always a good idea to know what is measured (which I don't, in this case), and what is calculated (which you do not seem to do, in this case ...).

graphene may be another story, because the density of states at the fermi energy is zero ...

Stefano B

On Jun 15, 2010, at 2:56 PM, loc duong ding wrote:

> I don't understand why Raman activity in this case is zero. In practical, metallic system also provide Raman peak (ex. Carbon nannotube).  I try to calculate graphene (no gap) and I get the proper result. Can you give some more explanation or suggest me some reference about this topic?
> 
> I appreciate to your instructions.  
> 
>> So, the the Raman activity should be 
>> zero ...
>> SB
> 
> On Jun 15, 2010, at 3:50 AM, loc duong ding 
>> wrote:
> 
>> I calculate zigag nano ribbon with hydrogen terminated at the 
>> edge. I checked DOS that showed no gap. 
>> 
>>> yes - SB
>> 
>> 
>> On Jun 14, 2010, at 
>>>> 11:03 AM, loc duong ding 
>> wrote:
>> 
>>>> what's the gap of the 
>>> unperturbed 
>> 
>>>> system?
>>>> SB
>>> 
>>> Actually, 
>> this 
>>> is the firs time I calculate the Raman spectrum. I don't clear 
>> about the gap of 
>>> the unperturbed system? It means the electronic 
>> band gap?
> 
> 
> -----------------------------------------------
> Loc Duong Dinh
> Ms-Ph.D Student
> Sungkyunkwan Advanced Institute of Nanotechnology,
> Sungkyunkwan University,
> Suwon, 440-746, Korea 
> Email: mambom1902 at yahoo.com, ddloc at skku.edu
> 
> 
> 
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

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