[Pw_forum] input file for transmission

[Manoj Srivastava]

Dear Gabriele, 

Thank you very much quick reply. I have  some follow up questions on this. 

On Tue, 14 Apr 2009, Gabriele Sclauzero wrote:

> 
> Manoj Srivastava wrote:
> > Dear PWSCF users and developers, 
> >  I wish to do a transmission calculation and confused about the input
> > file. I have a question on example 12 of the package, where transmission
> > of monoatomic Al wire with a H atom adsorbed on the side is done. The SCF
> > run in the device region is done with (some part of input file is given
> > below)
> 
> it is usually called the "scattering region"
> 
> 
> > 
> > &system
> >     ibrav = 6,
> >     celldm(1) =12.0,
> >     celldm(3) =1.875,
> > 
> > where the atomic postions of different atoms is 
> > Al     0.00000000     0.00000000     0.0000
> > Al     0.00000000     0.00000000     0.375
> > Al    -0.02779870     0.00000000     .75537515
> > H      0.19269012     0.00000000     .93750000
> > Al    -0.02779870     0.00000000     1.11962485
> > Al     0.00000000     0.00000000     1.5
> > 
> > So, looking at the z coordinate of above system, we notice that device
> > region is periodic with period 1.875. 
> 
> You are right, the scattering region is a periodic system, since pwscf always uses PBCs. 
> The lenght of the scattering region id 12.0*1.875 a.u. and contains all 5 Al and the H 
> impurity.

But that is exactly the trouble I am having in this. The scattering
region should not be periodic like leads, as now we have infinite
scattering region! But PWSCF always has PBC, so we should have a large 3rd
lattice vector to make the scattering region practically finite. I dont
see any super cell here. a_3 is just 1.875*a_0, where a_0 is lattice
constant. The atomic postions are all in a_0 unit, which makes me believe
that it is a practically infinite system with a_3=1.875*a_0. 
 
> 
> > So, physically we are solving for an
> > infinite device region, but in the physical setting of a transmission
> > problem leads are semi-infinite and device is finite. Shouldn't we use
> > some kind of vacum, i.e. taking 3rd lattice vector large, which
> > effectively would represent the finite device region? 
> 
> 
> > Also how much part
> > of the leads should be taken as part of device region, 
> 
> I don't understand this point. The leads are conceptually different thing than the 
> scattering region. The lead is a periodic unit of the "bulk" region (in this case an 
> infinitely long monatomic wire) and it is used to compute the generalized Bloch states, 
> which in turn are propagated in the scattering region. 

 In the above example, in principle we can have one atom H as scattering
region, and Al wire as left and righ leads, but we have taken few Al atoms
with H and treated it as scattering region. Thats what I
meant by how much part of leads should be taken as scattering region.

> 
> > Is there some kind
> > of convergence criterion? Is it like keep increasing part of lead in the
> > device reion till further increase does not substantial change device
> > behavior, e.g. Bloch's state?
> 
> There is a main convergence criterion (though I don't understand if you are actually 
> refering to this). You have to increase the scattering region, adding more Al atoms in the 
> wire, such that the complex band structure with real wave-vectors computed using the 
> leftmost periodic unit of the wire included in the supercell (the H impurity being in the 
> middle of the s.c.) converges to the band structure of an impurity-free wire, obtained for 
> instance from a pwscf calculation (or from a pwcond calculation with a 1 atom cell 
> containing an Al atom).
  
> 
> To do this you can use pwcond with
> ...
> prefixt='prefix of the scattering region'
> bdl=ratio between the lenght of the periodic unit and celldm(1)
> ikind=0
> band_file='name of file containing the CBS'
> ...
> 
> 
> then compare the real bands (contained in <band_file>.re) with those from pwscf (obtained 
> using the 1 atom cell).
> 
> Also convergence of the transmission with the lenght of the scattering region can be used, 
> but it is quite more cheap to check convergence of CBS (which can also help to understand 
> if everything is going fine), and when the CBS of your "bulk" region (leads) is correctly 
> reproduced the transmission should be converged as well.
> 
> HTH
> 
> GS
> 
> 
> 
> > 
> > Regards, 
> > Manoj Srivastava
> > Ph.D. student
> > Department of Physics
> > University of Florida, Gainesville, FL
> > 
> > 
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> > Pw_forum at pwscf.org
> > http://www.democritos.it/mailman/listinfo/pw_forum
> > 
> 
> -- 
> 
> 
> o ------------------------------------------------ o
> | Gabriele Sclauzero, PhD Student                  |
> | c/o:   SISSA & CNR-INFM Democritos,              |
> |        via Beirut 2-4, 34014 Trieste (Italy)     |
> | email: sclauzer at sissa.it                         |
> | phone: +39 040 3787 511                          |
> | skype: gurlonotturno                             |
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