Topics

1

General Questions and Answers / Strange artefact/bug(?) since last update of QuantumATK

« on: March 17, 2020, 18:16 »

Dear QuantumATK developers,

we recently observed an artefact when switching from QuantumATK version 19.03 to 19.12. The transmission spectra, which we have calculated for two overlapping graphene sheets, shows a transmission of zero in a certain interval around the Fermi energy when using the newer version. You can find the comparison in the attachment including also the input scripts. There is no physical explanation for this artefact and we therefore assume a bug since the recent QuantumATK update (the results using QuantumATK 19.03 make perfect sense).

Thank you in advance.

2

General Questions and Answers / Fermi level not midgap in semiconductor nanowire

« on: August 9, 2017, 13:10 »

Dear ATK-experts,

I calculated the band structure of a couple of silicon nanowires (SiNWs) using DFT and the meta-GGA potential by Tran and Blaha with ATK 15.1.
For one single SiNW I observed that the Fermi energy E_F is not located in the center of the band gap. I tried to change some settings and finally realized that this depends on the k-point sampling.
Attached you can find the band structures for 51, 52, and 53 k-points in the direction of the wire (1 k-point in the other directions). For 52 k-points, E_F is located in the center. For 51 k-points it is close to the valence band and for 53 k-points it is close to the conduction band. I tried 31 k-points and 61 k-points and for those values, E_F is also in the middle of the band gap. (*)
I did not observe this behavior for the GGA functional (51 k-points). But when using the meta-GGA potential, E_F is also not centered for different tolerance values in the IterationControlParameters or different c-parameters (I varied these values for 51 k-points). The non-centered E_F also appeared before and after the relaxation of the SiNW. For all other SiNW under study, E_F was centered in the band gap.

Any idea what could be the reason of this? It is probably not very critical, because the band structure itself does not change at all for 50+ k-points. But it is still very puzzling.

The corresponding python-script is attached.

Thank you very much.

(*) I similar problem was discussed in http://quantumwise.com/forum/index.php?topic=3006.0 (even though they used ATK 14) and the problem could be solved with an odd number of k-points. In my case however, the problem seems to appear mainly for odd k-points.

3

General Questions and Answers / Implementation change of the TB09 functional

« on: April 28, 2016, 16:29 »

Hello modelling experts,

I made a couple of DFT calculations using meta-GGA functional by Tran and Blaha. I used ATK 12.8 before and now switched to ATK 15. I realized that the results changed. Two example observations:
1) In order to achieve the same band gap in bulk silicon, I now need to use a different value for the c-parameter.
2) I did some two-probe simulations using version 12.8 and looked at the local density of states. Near the electrodes I observed some deviation from bulk properties in the electrodes (it was already bulk-like further away from the electrodes, so it was not a convergence problem with respect to the device length) . This problem however,  disappeared when using ATK 15 (which is very nice!).

My question is: What changed between ATK 12.8 and ATK 15 related to the Tran and Blaha functional? In the release of ATK 14 I read "New MGGA library and implementation - now also with TPSS functional" and I suppose that this causes the deviations. I would like to know some more details. How meaningful are the results calculated with the older implementation in ATK 12.8?

Thank you!

4

General Questions and Answers / Bug report: Bonds overlap atoms

« on: April 11, 2016, 14:18 »

Dear VNL developers,

When exporting a .png or .jpeg from the VNL viewer, bonds are drawn in front of the atoms. This seems useless. An image in favor of better illustration is appended.

Thank you for fixing this!

Details:
Build: 2015.1.5b36b25
Platform: Linux
Python 2.7.2
Qt: 4.8.5
PyQt: 4.10.3

5

General Questions and Answers / About the charge option

« on: April 8, 2014, 16:01 »

Hello everyone,

I want to clarify myself about the meaning of the charge option for electrodes (ATK version 12.8.2).
It is clear that the Fermi level gets shifted when additional charge is added.

To estimate the influence of the electrode charge in a device configuration, I looked at one single (7,0) CNT (bulk configuration) and varied the charge (periodic boundary conditions along the CNT, Neumann boundary conditions perpendicular to the CNT (as this is needed for the device configuration)).
Afterwards, I subtracted the Fermi level from the resulting band structures (to see the influence on the band structure and the Fermi level separately).
 
The result is attached.
You can see the energy values at the Gamma point versus different amounts of additional charge. The red curve is the Fermi level, the black curves correspond to the band structure. The inset shows the band structure of the (7,0) CNT for 0.0 additional charge.

The movement of the Fermi level relative to the bands is clear. However, there is also a shift of the whole band structure in addition, which I don't understand yet (shouldn't the black curves be constant?).

Can someone explain this behavior?
I would also be interested in further information on how exactly the additional charge is added to the system.

Thanks in advance!

6

General Questions and Answers / CNTFET using doped electrodes

« on: February 24, 2014, 14:26 »

Hello everyone,

I'm trying to simulate a cylindrical gated CNTFET using ATK 12.8.2. The system is based on a (7,0) CNT, where I dope the electrodes using the "charge" keyword (each electrode has two unit cells of (7,0) CNT - 56 atoms - and the charge was set to "-0.5").
The gate is a spatial region and no dielectric region is inserted.
Extended Hueckel is the chosen method.

I have a question about the most suitable choice for the boundary conditions:
According to the tutorials and previous discussion (http://quantumwise.com/forum/index.php?topic=1597.0#.UwtBlduVvRY), Neumann boundaries are most suitable for studying the gate influence.
However, multipole boundaries are suggested for charged molecules.
I'm now wondering if I could use multipole boundaries for the transistor as well (as I have some kind of charged molecules as electrodes).

When I create a script using VNL to simulate the CNTFET, I cannot access the Multipole option. But when I directly edit the python script and change "NeumannBoundaryCondition" to "MultipoleBoundaryCondition", the job starts and ends successfully. Is there a reason why Multipole boundaries should not be used for a device calculation (e.g. a CNTFET)?