Messages

1

General Questions and Answers / scf failed

« on: October 20, 2011, 14:44 »

When I calculated a two probe system, the scf process failed.
 
At the beginning of the calculation, there was a warning:   The provided initial state parameter has no pre-calculated data.   Proceeding without setting an initial state.
And then, the density matrix calculation of the two electrode converged in 15 steps. But further calculation failed with a message "This application has requested the Runtime to terminate it in an unusual way.Please contact the application's support team for more information."

Can anybody meet with similar problems and how can you solve it?

2

General Questions and Answers / Re: the I-V characteristics of graphene: zigzag vs armchair?

« on: September 27, 2011, 05:26 »

yes, the k-point for the current and transmission is different. So I changed the k-point to (51,1,9) for scf and (51,1) for transmission then. The current keep the same as before : graphene with zigzag unit cell has a much higher current compared to graphene with armchair unit cell. But the transmisson spectra changed a lot, see below.

The k-point of transmission spectra and current need to be equivalent,is that right?

Your transmission spectrum with 10 kpoints is just not correct, that's why. It has a gap around the Fermi level which is not physical, we know for a fact that graphene is metallic, hence immediately you can see something is wrong there, and it's because your k-point sampling doesn't capture the K point where all the low-energy transmission occurs.

I'm not sure how you got the linear I-V curve for the Z "edge", if the current is evaluated from the transmission spectrum you showed in the later post you should have zero current at least up to 0.7 V bias or so. You must have used different k-points for the current and transmission?

3

General Questions and Answers / Re: the I-V characteristics of graphene: zigzag vs armchair?

« on: September 26, 2011, 11:12 »

Thank you! I will try to recalculate the transmission.
But if the scf calculation is right, how can I get such different results of I-V characteristics of graphene for just different unit cells? I can't understand it from physics. And I notice many other people in the forum are trying to calculate the transport properties of graphene, interestingly most of them set up graphene models using the zigzag unit cell. Should the I-V characteristics dependent on the unit cell?

27 k-points should indeed be enough for the scf loop, however you need perhaps 200 points for the transmission. Note that you don't need 200x200, there is no dispersion in Y, you should do (200x1).

4

General Questions and Answers / Re: the I-V characteristics of graphene: zigzag vs armchair?

« on: September 26, 2011, 10:41 »

The k point is different in the scf process and transmission calculation. In the process of self consistence, the k point is set to be (27, 1, 9).The higher point (27,1,15) was also tested to get the same results of I-V characteristics in both units. So I can't understand why the I-V characteristics are so different for the two kinds of unit cell even the k-point has been proved to be enough.
For transmission calculation, the kpoint is set to be (10,10), which usually is not enough, but the  transmission  calculation of higher k-point was not convergence. However, the the transmission spectra ( (10,10) k point) show similar step-like spectra, like the picture.

There is not enough information in your post to answer the question. For instance, what is the k-point sampling for transmission and the self-consistent calculation? If the transmission spectra are the same then the current will be the same.

5

General Questions and Answers / Re: the I-V characteristics of graphene: zigzag vs armchair?

« on: September 26, 2011, 08:57 »

yes, the transmission spectrum is similar, but why are the I-V characteristics are so different? The current of unit cell along zigzag edges is nineteen orders higher than that of unit cell along armchair edges. It's incredible.

If you use a lot of k-points in the direction perpendicular to the transport direction, the transmission spectrum at zero bias of the two structures should be similar.
see also other post on the forum.
Note that ATK2008.10 is no longer supported, and we recommend upgrading.

6

General Questions and Answers / the I-V characteristics of graphene: zigzag vs armchair?

« on: September 26, 2011, 04:22 »

I wanna calculate the transport properties of graphene using atk 2008.10. Two kinds of unit cell were used to set up the two-probe system, which the z-direction of one was along with zigzag edges and that of the other one along with armchair edges. The structure of both two were infinite graphene.However, the I-V characteristics were totally different. I don't know why. Can anybody tell me the reason? Thank you.

7

General Questions and Answers / Re: transmission eigenvalue and eigenstate

« on: February 24, 2011, 13:53 »

We cannot really offer much support on this. One potential answer could be that you have not chosen the correct quantum numbers, but without pictures and script I cannot say anything.

We know the implementation of the transmission eigenstates was not perfect in that version of ATK, but it has since been improved and we are happy to announce that it is being re-released in ATK 11.2, which is already available in a beta-version, with final release expected in a week or so.

I followed the tutorials to calculate zgnrs, and I can show you the py file and the result in the email if you would like. But i want to know whether i can conclude the transmission eigenstate must be blocked  when the eigenvalue reduced greatly(for example, from 1.0 to 0.0).

And another question: How can i choose 'the absolute isovalue' to plot eigenstate.
Thank you very much!

8

General Questions and Answers / Re: transmission eigenvalue and eigenstate

« on: February 24, 2011, 13:23 »

Which version of ATK is this computed with?

ATK 2008.10

9

General Questions and Answers / transmission eigenvalue and eigenstate

« on: February 24, 2011, 11:17 »

hello!
I have calculated the transmission eigenvalue and eigenstate of graphene ribbons recently. To my surprise,when one result showed the eigenvalue=0, the eigenstate distributed in the whole two-probe system without obvious break; in the contrast, when one result showed the eigenvalue =0.9, the eigenstate are blocked in the interface between the central scattering region and the left electrode. So i am confused what's the relationship between transmission eigenstate and eigenvalue.
by the way, the eigenstate was shown by the VNL nanoscope, i chose isosurface plot. And in my VNL, 'the absolute isovalue' could not be ticked. Would you like to  tell how can i choose this option? Does it need some other code.
Thanks very much!