[Wien] non-zero value of epsilon 2 below the energy band gap

[Peter Blaha]

You can plot  case.joint (unbroadened eps-2) or case.epsilon (broadened 
eps-1 and eps-2 according to your case.inkram).

Another possibility:   Did you use an increased k-mesh and executed
  x lapw1 -up/dn WITHOUT -orb ???


On 12/17/18 8:57 AM, Pavel Ondračka wrote:
> Dear Anup Pradhan Sakhya,
> 
> this probably just due to the broadening procedure. If you set
> broadening to zero in kram, than you show have no absorption below the
> band gap (with plain optic and no scissor). Or just take a look at the
> output from joint which should contain the unbroadened imaginary part
> of the dielectric tensor (epsilon 2).
> 
> In general if you want to compare your spectra against experiment some
> broadening is needed for a good agreement (under the assumption that
> your DOS is already good and the approximations in optic package works
> for your material). There are processes which cause absorption below
> the gap also in the experiment (the Urbach tail, instrumental
> broadening, etc.) hence this is nothing to worry about.
> 
> The Lorentzian broadening in the kram is not really a good model
> though, especially regarding the absorption bellow the band gap, since
> it decays quite slowly and hence can produce nonzero absorption way too
> low below the gap. I have much better experience with Gaussian
> broadening (and it should be theoretically a better match for the
> processes which happen in experiment). Unfortunately, kram only
> supports Lorentzian broadening.
> 
> Best regards
> Pavel
> 
> 
> On Sat, 2018-12-15 at 01:19 +0530, Anup Shakya wrote:
>> Dear All,
>>
>> I have performed calculations for two double perovskite oxide
>> materials and the band gap of the material is found to be more than
>> 1. 3 eV for both materials. The calculations have been performed
>> using GGA+U, since it contains rare earth materials. The value of U
>> have been used from the literature. The energy convergence was
>> performed till 0.0001 Ry and the optical properties were calculated
>> using optic. However, the imaginary part of the dielectric constant
>> (epsilon 2) shows non-negligible value below the energy band gap. PBE
>> was used as the exchange correlation functional and if I am not wrong
>> then for the calculation of epsilon 2 the contribution of inter-band
>> transitions are taken into consideration and the intra-bands are
>> neglected. Then what is the reason for the non-zero value of epsilon
>> 2 below the energy band gap?
>>   If anyone could suggest some views, I would be very grateful. Please
>> let me know, if anyone needs more information.
>>
>> Sincerely,
>> Anup Pradhan Sakhya,
>> TIFR.
> 
> 
> _______________________________________________
> Wien mailing list
> Wien at zeus.theochem.tuwien.ac.at
> http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
> SEARCH the MAILING-LIST at:  http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/index.html
> 

-- 

                                       P.Blaha
--------------------------------------------------------------------------
Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-1-58801-165300             FAX: +43-1-58801-165982
Email: blaha at theochem.tuwien.ac.at    WIEN2k: http://www.wien2k.at
WWW:   http://www.imc.tuwien.ac.at/TC_Blaha
--------------------------------------------------------------------------