converge problem all again.

[rose]

Hi,
I got into the problem in converging my system under finite bias. The zero voltage was converged properly, hence, the finite bias voltage is not converged after 250 iterations. I check all including increasing history step, reduce damping factor, etc.
Any idea how can I solve it? I attached the device,py for further detail.
thanks
Rose

[Petr Khomyakov]

Did you use the self-consistent solution of your zero-bias device calculation to start the finite-bias device calculation, see the tutorial http://docs.quantumwise.com/tutorials/atk_transport_calculations/atk_transport_calculations.html? You may also try increasing the bias in smaller steps, reusing a previous calculation for the next one. 

I notice that the script enclosed seems to be incomplete, as there is information about the device geometry only, no any other potentially useful information that might give more clues on what might have gone wrong with your device calculation.

[rose]

Thanks for the response. Sorry for wrong upload. I did what had been mentioned in the tutorial. The scf calculation is OK in zero bias but as I increase the voltage even in small orders such as 0.02v or sth, it did not converged. I try to use the previous calculations for the run but no convergence achieved. Please find the total setup in the attachment.
thanks
Rose.

[Petr Khomyakov]

It seems that your density mesh cut-off is rather small; default is 75 H, whereas you have adopted 36 H. I also notice that your system has no scatters in the central region, so I am not sure that doing finite bias calculations makes much sense in this case of a perfect system. The linear response calculation (when V_bias = 0 V) of conductance is what describes the electron transport through a periodic structure of a perfect, defect-free material like yours.

[rose]

Thanks for the response, I try to use higher mesh cut off and let you know the results.
thanks
Rose

[rose]

Hi,
I tried to use higher mesh density but I can not still got the converged.
I did not got the comment on the defect. I want to find the pristine current-voltage characteristic and then find the defect one. Is there any other way so I can find the IV curve for the pristine, then?
Any idea how can i got the convergence for finite bias in pristine one?
thanks
Rose

[Petr Khomyakov]

To understand the issue related to finite bias calculation for perfect systems, you may want to consult a reference book on Electron Transport by S. Datta, http://www.cambridge.org/catalogue/catalogue.asp?isbn=0521599431, e.g., Chapter 2.1.

In brief, you make an assumption in your calculations that the potential drop takes place entirely across a perfect conductor. But this assumption is only valid if the number of conducting channels in the electrodes is infinite or at least much larger than the number of conducting channels in the conductor. That would be true if you would have connected your perfect conductor to metallic leads, but in your case the leads and conductor are made of the same perfect material. It means that the number of channels is the same in the leads and conductor, in contradiction with your assumption that the potential drop occurs essentially across the conductor, which is given by the central region in your ATK transport calculations.

That is why I said that your finite-bias calculations do not make sense for the device setup assumed.