Messages

1

General Questions and Answers / Re: About the unit of external potential

« on: December 27, 2012, 11:45 »

If you consider the definition of the unit "electronvolt", the conversion factor is obvious

All quantities which are expressed on a 3D grid Q(x,y,z) support arithmetic operations. Transmission pathways is thus not included immediately, since it's a discrete function of the pairs of all atoms. But, since ATK uses Python, you can always extract the numbers and perform any operations you want.

The proper quantity to compute for the position-dependence of the current is the current density, although the pathways can in a sense be used to approximate it.

Thank you, Dr Anders Blom. Well then potential in in unit of eV has its value unchanged if in unit of V, right? What I feel strange is when I calculate the external potential of a device with a gate, the potential at the metallic region (which is about 18eV) doesn't equal what I set in the script (10V) if Neumann boundary condition is employed. There is no such problem with Dirichlet boundary condition. I'm using ATK 12.2. Is it a bug?

Also, the electric field derivated from the external potential is a little parabolic inside the dielectric region. Shouldn't it be linear?

2

General Questions and Answers / Re: About the unit of external potential

« on: December 26, 2012, 06:17 »

Voltage drop (in unit of eV) =  |e| * Voltage drop (in unit of volt).  The electric field induced by the applied bias is calculated by the gradient of voltage drop in the unit of volt: E = -dV/dx.

"Do all properties' objects support arithmetic operators? "
Yes.

Many thanks to both of you. So it seems that one can easily divide any potential (external potential, electrostatic difference potential and effective potential) in eV by |e| to get potential in V, regardless of the electron density of the device. Is |e|=1.6E-19 here or |e|=1? Actually what I want to know is the electric field induced by the gates, so is it OK to subtract the external potential of Vg=0 from that of finite Vg?

BTW, what does it mean by real space properties? Is "Transmission Pathways" included? How can I calculate the net current flows into an atom, when there are arrows pointing at it and pointing out from it with similar cylindrical radius? Or at least I want to know the sign of the net current at a certain place to determine the direction of current.

3

General Questions and Answers / Re: About the unit of external potential

« on: December 25, 2012, 06:05 »

Thank you very much, but the "voltage drop" is still in unit of eV (so why is it called voltage drop?). How could I get the electric field in the device?

BTW, I didn't know the object ElectrostaticDifferencePotential could directly be subtracted from each other. Do all properties' objects support arithmetic operators?

4

General Questions and Answers / Re: About the unit of external potential

« on: December 24, 2012, 15:05 »

So it is potential energy rather than electric potential? Then is there any way to get the electric potential in voltage induced by the gates? Actually I just want to view the electric field created by gates in my device. May I just divide the external potential (or maybe the difference between the potential of Vg=0 and that of infinite Vg ?) by the electron density?

5

General Questions and Answers / About the unit of external potential

« on: December 24, 2012, 08:57 »

Dear Quantumwise Staff:
    Recently I calculate the external potential and get confused by its unit. Why is it in the unit of eV? Shouldn't it be V?
   
    PS: The manual says "ExternalPotential, returns the electrostatic potential due to the electrodes and gates in the system". How to tell the external potential from the internal one? Is the screening effect included in the external potential calculation?

    Thank you!

6

General Questions and Answers / Re: Question about annealing and current calculation

« on: November 23, 2012, 06:49 »

Thank you for your advice! I'll rerun them with 12.8 and see if the results will change.

7

General Questions and Answers / Re: Question about annealing and current calculation

« on: November 22, 2012, 12:01 »

Yes I do have gates in my device. What does this bug affect? Does it mean that even if the SCF is converged, the final .nc file is unreliable with several warnings like that?

8

General Questions and Answers / Re: Question about annealing and current calculation

« on: November 19, 2012, 08:21 »

I think I know where I made the mistake. In my old script I changed only the T of central region without modifying those of the electrodes. Those .nc files obtained by the old script have the problem of discrepant current. Later I fixed it and obtained some new .nc files (one of which is sent to you) and the new .nc files don't have such problem. However, I stored them together with similar filenames and thus I can't tell the old ones from the new ones... It seems that I've sent a new one to you.

Thank you for your answer!

Another problem here. Often the annealing is very hard to converge, accompanied by such warnings:

Code

################################################################################
# WARNING                                                                      #
#                                                                              #
# The computed multigrid residual is greater than the required accuracy.       #
#                                                                              #
# Computed residual :   6.68609e-10                                            #
# Required accuracy :   1.00000e-12                                            #
#                                                                              #
################################################################################

Do they lead to hard convergence? How to eliminate them?

9

General Questions and Answers / Re: Question about annealing and current calculation

« on: November 17, 2012, 16:41 »

I can only speak in general terms without the NC file (which you can send using some service for large files, if you want, for instance http://sprend.com/).

When you do transmission_spectrum.current(), the current is computed assuming the same electrode temperatures as in the calculation. In the analyzer you can specify a different value, and the default is 300 K, so if that is not the same as in your calculation, then yes the current will be different. It may also be helpful to know that the current is not saved inside the transmission spectrum object, it's computed on the fly every time (it's fast).

As for the PS, there is no well-defined temperature in the central region - it's in non-equilibrium, which is the "NE" in NEGF, the method used for computing the transmission

Your comment is indeed correct, but the calculator of central region (as well as those of the electrodes) has a parameter called "electron temperature" in numerical_accuracy_parameters. Is it useless to modify the temperature in the central region? What's the relationship among the three temperatures? I see only the electrodes' temperature included in the current integration.

.nc file is sent to your email. Any advice is appreciated.

10

General Questions and Answers / Re: Question about annealing and current calculation

« on: November 15, 2012, 07:52 »

1. Yes
2. Both are correct, but most likely they are referring to different transmission spectra, perhaps if you have the high-T and room-T transmission spectra in the same NC file?

There is only one spectrum in my .nc file (which is of room temperature), so it couldn't be mistaken. Would you mind examining my .nc file? Although I think it's too large to be attached...

Anyway, thank you!

PS: What if I lower only the T of central region while keep the T of electrodes at 1000 K during the annealing? I see no difference in the procedure of SCF...

11

General Questions and Answers / Question about annealing and current calculation

« on: November 14, 2012, 06:31 »

Dear Quantumwise Staff:

I have two questions and they may be related or not.

First, when I find it hard to converge for a certain device, I raise the temperature in both electrode and central region to a higher one, say 1000 K. Then the obtained .nc file is used as the initial state for the room temperature calculation by manually reading and adjusting the former "electron_temperature" in electrodes' and central region's calculators. Is such annealing procedure correct?

Second, when I calculate the current of room temperature .nc file obtained by annealing, a great discrepancy occurs between the current of "transmission_spectrum.current()" (I added it to the script) and the one shown by the analyzer. Such mismatch can be up to 2 orders of magnitude. Which current is reliable? Or neither?

Thank you!

12

General Questions and Answers / Re: Question about hydrogenation in builder 12.2.1

« on: July 3, 2012, 09:34 »

Thank you very much for your answer! I have some more questions about some other aspects:

1. I'm dealing with a system which contains Si and B, and I want to try the new Slater Koster module. However, it doesn't work. I inspect the reason and find that there is no B-Si interaction file (or something like that, I'm not quite familiar with SK method). Where can I download it?

2. Is it reasonable (or is there any precedent) to optimize the structure with semi-experimental method and calculate properties with DFT? Since SE method is usually much faster and optimization requires lower accuracy. I should do some test, but DFT optimization is too time-consuming for my system. Thus I have no way to compare it with the SE.

Any help will be appreciated!

13

General Questions and Answers / Question about hydrogenation in builder 12.2.1

« on: June 29, 2012, 17:31 »

Dear Quantumwise staff:

    Recently I'm trying to learn the new builder, it looks cool but does have some defect. I have scanned the tutorials with no answers, or maybe I just missed them...
    First, where can I choose the form of hydrogenation? I'm curious about silicene, in which the hydrogenation is probably sp2 like and only edge atoms get hydrogenated, but the hydrogenate button always gives an sp3 form and make every Si atom connected with at least an H.
    Second, edge Boron in h-BN can't be hydrogenated, but Nitride is no problem.
    Finally, how can I add a single atom in the builder without directly modifying the python script? I hope there is some way to add atoms and make their distance to edge reasonable. Imagine the edge of GNR is functionalized by atoms with a variety of bond lengths. Such system is hard to build using current builder.
    Last but not least, is there anyway to reset camera to xy, yz and xz plane? And may I save current work place as a whole project and reload all needed parts in the stash at a time?

    The new builder is really cool in the selection function and many other things. Tags are convenient. I have used Materials Studio and it has a really strong builder system, you can learn something from it. I think everything will be better in ATK:)

14

General Questions and Answers / Re: Convergence Problem

« on: April 27, 2012, 03:16 »

Yes, do that. Sorry about the initial typo; it should be Neumann.

Strangely, changing the boundary condition to Neumann leads to larger residual error of 1E-6 and larger dH of 1E+3 in first 10 steps. It seems unable to converge with Neumann condition...

I'm curious about the role played by Iteration parameters in SCF. I think they must have influence on convergence but have no idea how to modify them. Manual is consulted but I'm still not sure about whether to set them larger or smaller. Can you give me some advice? Thank you!

15

General Questions and Answers / Re: Convergence Problem

« on: April 26, 2012, 04:36 »

PS, the new structure still has the warning of residual but the computed one is about 1E-10, much less than the old (unconverged) one (1E+2), while the required accuracy is 1E-12. I'm going to recalculate it with Neumann boundary condition to see whether this matters.