[Pw_forum] Charged impurity

[Giuseppe Mattioli]

Dear M.V.Kondrin
An impurity or a dopant in a host matrix cannot be considered as separated from the electronic structure of the matrix. Let us suppose that you have a 
substitutional Mn atom which replace a Ga atom in a GaAs supercell, a very well investigated example. If the cell is neutral then you must have a Mn 
3+ with 4 unpaired electrons. A fifth Mn 3d majority orbital, mixed with some contribution with the neighboring As orbitals, will be unoccupied: an 
acceptor level slightly above the GaAs valence band. If you put an excess electron in the system, then such an electron fills the Mn orbital which is 
very happy to restore a Mn 2+ state with five unpaired majority electrons. Local changes in the Mn electronic environment are in this case quite 
significant. On the left hand, if you subtract one electron from the same system, you necessarily subtract it from the GaAs valence band maximum, which 
is basically spread across all the As 2p orbitals, so that changes in the local electronic structure of all the atoms of this charged supercell are 
negligible. Differences such as those sketched above generally reflect the interesting Physics of doped semiconductors, and often depend also on the 
level of theory used to describe their electronic properties (GGA, DFT+U, EXX, ...). As I do not know your system, I can only say that your impurity 
seems to be "passivated", that is, it does not react to changes of the Fermi level of the system.
HTH
Giuseppe

On Thursday, July 09, 2015 01:56:15 PM mkondrin wrote:
> Dear QE developers and users!
> 
> I don't quite get how I can instruct QE to calculate band structure of
> material with charged impurities. As I gather from the previous
> discussion in this mailing list it just requires setting tot_charge
> parameter to non-zero value. I tried this but difference between the
> charged and non-charged impurity to me seems negligible. It is also
> unclear to me  -- tot_charge parameter corresponds to the whole cell (or
> supercell) which can be very large so the charge is smeared between
> hundreds or so atoms among which only one or two ones are impurities.
> Can I just set the charge of just these few atoms to non-zero value?
> 
> Thank you in advance for your advice!
> 
> Sincerely yours,
> 
> M.V.Kondrin,
> 
> High Pressure Physics Institute RAS , Troitsk, Russia
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   Giuseppe Mattioli                            
   CNR - ISTITUTO DI STRUTTURA DELLA MATERIA   
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