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

[Pavel Ondračka]

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.