[Wien] How to calculate optical absorption edge ?

[Rocquefelte]

I am also using origin to determine the absorption edge based on epsilon2.
You must simply draw a tangent line along the edge and then read the 
intersection between this line and "Eps2 = 0" axis.




Le 7/9/2013 8:44 AM, Masood Yousaf a écrit :
> I would be grateful if you guide me how to apply this tangent method 
> to the data of imaginary part of the dielectric function. can I use 
> Origin to apply this ?
>
> Best wishes
> Masood
>
>     ------------------------------------------------------------------------
>     *From:* Rocquefelte <xavier.rocquefelte at cnrs-imn.fr>
>     *To:* A Mailing list for WIEN2k users
>     <wien at zeus.theochem.tuwien.ac.at>
>     *Sent:* Tuesday, July 9, 2013 2:30 PM
>     *Subject:* Re: [Wien] How to calculate optical absorption edge ?
>
>     If you plot epsilon2 (the imaginary part of the dielectric
>     function) and use the tangent method, you can estimate the
>     absorption edge from WIEN2K.
>     However, it requires to use a proper functional, allowing to
>     overcome the inherent problems of DFT, which is a ground state
>     theory, i.e. LDA+U for correlated systems or hybrid functional...
>
>     Regards
>
>     Xavier
>
>     Le 7/9/2013 7:00 AM, Masood Yousaf a écrit :
>>     Dear Sir, thanks for your elaboration. Need some further
>>     explanation to overcome the confusion. Is it possible to
>>     determine accurately absorption edge from WIEN2k code only i.e.,
>>     from case.epsilon ?
>>
>>     Best wishes
>>     Masood
>>
>>         ------------------------------------------------------------------------
>>         *From:* Rocquefelte <xavier.rocquefelte at cnrs-imn.fr>
>>         <mailto:xavier.rocquefelte at cnrs-imn.fr>
>>         *To:* A Mailing list for WIEN2k users
>>         <wien at zeus.theochem.tuwien.ac.at>
>>         <mailto:wien at zeus.theochem.tuwien.ac.at>
>>         *Sent:* Tuesday, July 9, 2013 12:40 PM
>>         *Subject:* Re: [Wien] How to calculate optical absorption edge ?
>>
>>         Experimentally an absorption edge (optical gap) is estimated
>>         using the tangent method.
>>         Thus you should do the same with your calculated data to
>>         compare optical gap values, which differ from a fundamental
>>         band gap.
>>
>>         Indeed, a fundamental band gap is the smallest band gap
>>         allowing to excite an electron in unoccupied states (deduced
>>         from electrical measurements - thus no transition rules).
>>         In contrast, the optical gap is related to an excitation
>>         using photons, thus it respects the dipolar transitions rules
>>         (this gap is usually larger than the fundamental band gap).
>>
>>         Regards
>>
>>         Xavier
>>
>>
>>         Le 7/9/2013 3:07 AM, Masood Yousaf a écrit :
>>>         Dear wien2k users
>>>
>>>         Kindly suggest how to calculate optical absortion edge in
>>>         wien2k ? How it differs from band gap of material ? I read
>>>         that first critical point in imaginary part of the
>>>         dielectric function represents optical absorption edge. But
>>>         how to ensure that which is the first critical point. Kindly
>>>         guide
>>>
>>>         Best wishes
>>>         Masood
>>>         Universiti Tecknologi Malaysia
>>>
>>>
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>>
>>
>>
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