[Wien] metal supercell band folding dielectric function

[Peter Blaha]

The optical matrix elements should fix this problem. They should be zero.


Am 30.03.2013 22:23, schrieb MingWenmei:
> Thank you, Professor Pblaha.
>
> What you said is true only for semiconductors, because they have finite band gap separating "valence bands" from "conduction bands" and this will not change even with band
> folding from larger supercell calculations.
>
> However, for metals, because around Fermi energy the bands are partially occupied, then some states from the SAME band in primary unit cell calculation might fold back to
> bands with DIFFERENT band index.
>
> The simplest example is say we have a metal with one single band of parabolic dispersion when do an "unit cell" calculation. When we do dielectric function calculation, we
> only have the INTRA band contribution, that is, Drude-like dielectric function, which is only determined by plasma frequency and damping parameter.
>
> This means all states are from one single band. However when we do a cell of 2 times the unit cell, then basically we will have TWO bands due to band folding. The INTRA
> band contribution won't change because the plasma frequency is the same. However in this case we will have INTER band transition contributing to the dielectric function.
> This is something extra compared to ONE band calculation. It means the dielectric function will depend on the cell you are choosing. Physically this is WRONG.
>
> Thank you for your suggestion,
>
> Wenmei
>
>
>
>
>  > Date: Sat, 30 Mar 2013 08:27:39 +0100
>  > From: pblaha at theochem.tuwien.ac.at
>  > To: wien at zeus.theochem.tuwien.ac.at
>  > Subject: Re: [Wien] metal supercell band folding dielectric function
>  >
>  > a) Do an optics calculation for the same 32 atom Au-supercell, but without dopant,
>  > just "label" one Au atom by "Au1", so that the symmetry is identical to your Cd-doped
>  > calculation. This should give identical eps2 than the small Au calculation (beware of
>  > k-mesh effects !!!)
>  > b) Use bandstructure with "fat-bands" and partial DOS for analysis of your doped system.
>  > You can also run optics with matrix-elements restricted to eg. only Cd or only Au
>  > (missing crossterms). See UG for details.
>  >
>  > Am 30.03.2013 00:27, schrieb MingWenmei:
>  > > Dear all,
>  > >
>  > > I am calculating the interband dielectric function of Au with low concentration of Cd dopant (say 1/32 atom concentration). Very strikingly in low energy (< 1eV) range,
>  > > there appears a high peak (~a few hundred). However for FCC pure gold, the interband dielectric function is very small (around zero) below 1eV.
>  > >
>  > > So my question is physically why the dielectric function changes this dramatically? I did the band structure calculation. I found within small energy range (<1eV) of Fermi
>  > > energy for the doped system, there are some extra bands. I began to realize it is this small energy difference of the bands above Fermi energy that causes the high peak of
>  > > dielectric function.
>  > >
>  > > These extra bands may be from doped Cd and also may be from the band-folding of host Au in the supercell calculation. The contribution from supercell itself to the
>  > > dielectric peak seems not to make sense physically. For if band-folding can affect the value of dielectric function, then even for pure FCC Au we may get significantly
>  > > different dielectric functions by choosing "unit cell" of different size (say 1x1x1, 4x4x4 times primary cell )
>  > >
>  > > Could anyone give some comment to understand this?
>  > >
>  > > Thank you a lot,
>  > >
>  > > Wenmei
>  > >
>  > >
>  > >
>  > >
>  > >
>  > >
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>  > >
>  >
>  > --
>  > -----------------------------------------
>  > Peter Blaha
>  > Inst. Materials Chemistry, TU Vienna
>  > Getreidemarkt 9, A-1060 Vienna, Austria
>  > Tel: +43-1-5880115671
>  > Fax: +43-1-5880115698
>  > email: pblaha at theochem.tuwien.ac.at
>  > -----------------------------------------
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-- 
-----------------------------------------
Peter Blaha
Inst. Materials Chemistry, TU Vienna
Getreidemarkt 9, A-1060 Vienna, Austria
Tel: +43-1-5880115671
Fax: +43-1-5880115698
email: pblaha at theochem.tuwien.ac.at
-----------------------------------------