[Wien] Plasma frequency in semi-metal case and effect of band folding on optical properties

[Peter Blaha]

If you have a "metal", i.e. some bands cross EF, you should have a 
finite (maybe small) plasma frequency (like in transparent conducting 
oxides)

Am 13.07.2013 18:37, schrieb venkatesh chandragiri:
>
> Dear sir,
>
> *I thought your material has a gap, and the question was only direct or
> indirect .. ??*
>
> If i did the calculations using the structure stimulated from the space
> group (225), it shows a negative indirect band gap in the band structure
> (negative indirect band gap means, the valance band maximum falls above
> the Fermi level at gamma point and conduction band minimum falls below
> the Fermi level at X-point ). Then, can i expect an intra-band
> contribution to my optical property. Does it give any finite value of
> plasma frequency in the indirect band gap case...?
>
> At present, i did optical calculations on the primitive cell of same
> structure where i could find the band folding across the Fermi level.
> Here,  At gamma point, both the valance band maximum falls above the
> Fermi level and conduction band minimum falls below the Fermi level,
> showing a direct overlap between them due to band folding. As suggested
> by your reply, the back folding of bands will not effect the system
> optical properties. Then, this case is also similar to be a negative
> indirect band gap like.
>
> So, what is importance of direct, indirect and negative indirect band
> gaps while calculating the optical properties....may be i should need
> some examples of experimental data to understand this. Can you suggest
> me some references.
>
> Looking forward to your answers and comments.
>
> thanks for your contributions....
>
> regards,
>
> venkatesh
>
>
>
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-- 
Peter Blaha
Inst.Materials Chemistry
TU Vienna
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A-1060 Vienna
Austria
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