central region width

[jdgayles16]

Hi all,
I get some strange problems with my voltage drop, and i think it may come from my atomic configuration.

In the atomic viewer I noticed that my central region width is 49.94 angs when it should be 49.22 angs
picture attached.

 However and I saved my two-probe system as a equivalent bulk and subtracted the width of two unit cells from the z vector and i get 49.22 angs which is correct.

My question is where is the extra .72 angs (49.94-49.22) coming from. It seems as if the two probe system is incorrect in size and the equivalent bulk is.

Thanks,
Jacob

[youngjfly]

could you please attach your script,i want to know what gives rise to this situation too.
Thank you

[jdgayles16]

Here are is a similar situation, the central region width according to vnl is 15.49 angs when it should be14.77 angs according to the subtracting two unit cells from the equivalent bulk

**this is a toy system.

[zh]

The distances between the center region and the electrodes were missed in your estimation.

[youngjfly]

The problem may exist in the contacts between the electrode and the central region.
look at the contact between the left electrode and the scattering region,it is different from the right contact between the right electrode and scattering region.
The structure of your supercell may lead to this difference,because the left border and right border of the supercell is not symmetrical 

This is just my view.

[jdgayles16]

Yeah I've seen this, its seems impossible to get the spacing I want in the central region  (CR) because of the equivalent bulk atoms are always shifting the CR.

I'm sorry, I'm still confused on how my equivalent bulk system is the correct system, with the correct coordinates and vectors but is created from my incorrect two-probe system. All I do is click "Save as Equivalent Bulk".

Sorry to ask questions that are fixed in the new version, but I really want to use GGA,
Thanks for the help
Jacob

[Anders Blom]

There is no problem. The number stated in VNL is the distance between the two outermost layers of the two electrodes, not the distance between the two unit cells. You can easily check this by setting the central region with to 0 in the Atomic Manipulator.

Confusing? Yes. Not a good idea? Agree. And that's why it's changed in the new version! Which, however, sadly, will not have GGA for a little while yet...

[jdgayles16]

Hmmm, so my configuration is okay? I looked at the distance between the atoms in the left unit cell and the central region and they are about the same as on the right lead. however with any geometry similar to this one I see a small resistance at the  junction between the right lead and central region when applying a voltage, and I believe it may be causing a resonant tunneling state in my systems with a gap.

*** I have included a voltage drop of my gap geometry and a voltage drop of the toy geometry posted above.

Thanks again
Jacob

[Anders Blom]

I don't see any special voltage drop in your toy system, it seems well distributed over the structure.

[Anders Blom]

Keep in mind that presenting ATK with a perfect conducting system is actually putting a lot of "stress" on the algorithms, which are really designed for the case when there is a natural place for the voltage to drop. Normally the transmission etc come out good, but the voltage drop will in some sense be artificial no matter what you do, because

a) it's a perfect conductor, so the voltage has nowhere to drop
b) you have imposed a boundary condition that it must drop by a certain value

So, if you expect perfect conductivity, check the transmission and make sure it's integer. If not, you have artificial scattering, either as a result of a mismatch between the atoms, or because of a too short electrode.