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Messages - mads.engelund

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1
General Questions and Answers / Re: the question about the gate voltage in ATK
« on: April 18, 2013, 10:37 »
This also means that the position and geometry of the gate is just as important as the gate voltage. The closer the gate is to the atoms, the bigger effect it has.

2
General Questions and Answers / Re: the question about the gate voltage in ATK
« on: April 18, 2013, 09:46 »
The gate voltages are not directly applied to the orbitals. That would indeed be unrealistic. The gate voltage is applied to a region of space away from the molecule and the electrostatic equation is solved as an integrated part of the NEGF-DFT(Slater-Koster and Huckel too) method.

Take a look at this article.
http://prb.aps.org/abstract/PRB/v65/i16/e165401

3
General Questions and Answers / Re: About contact resistance
« on: April 3, 2013, 11:42 »
Could you please provide a script?

4
General Questions and Answers / Re: carbon nano tube
« on: October 9, 2012, 16:23 »
As zh said, the two electrodes have to be aligned. This is a constraint of ATK that will remain for the foreseeable future.

A way around this constraint could be to make two crossings.
I have made a sketch of what I mean in the attached *.py file.

But do not use this input file, it has not been constructed carefully, it is only an example of what I mean.

5
General Questions and Answers / Re: help！！The problem about calculation of TMR in ATK
« on: October 5, 2012, 10:55 »
Well 'G=I/V' is just an approximative rule-of-thumb for simple systems. And I can tell you, if it always held, then our work would be a lot easier.

In reality conductance can be a complicated function of voltage, especially on the atomic scale. As an example, engineers are often searching for systems with negative differential conductance, meaning systems where the conductance goes down when the bias increases.
Reality is more complicated than 'G=I/V'.

6
General Questions and Answers / Re: Nanomesh Design
« on: September 21, 2012, 10:55 »
Gates are added under Miscellaneous/Spatial Regions.
Regions are added by right clicking the list and the potential can by adjusted by editing directly.

7
General Questions and Answers / Re: Bias loop question
« on: September 20, 2012, 09:25 »
....unless there is a gate potential. Only the potential differences matter, but if a gate potential is set, then it matters how the source/drain bias are set.

8
General Questions and Answers / Re: how to achieve self-consistent
« on: September 18, 2012, 13:36 »
No, nori is talking about Changing the Contour Integral parameters (under 'New Calculator/Calculator Settings'), not the Basis set.
Change the NEQ contour. We have changed the wording, 'DoubleContour' has become 'Weigthed' and 'SingleContour' has become 'Left' and 'Right'.
So nori's suggestion is to use either 'Left' or 'Right'.

9
General Questions and Answers / Re: Nanomesh Design
« on: September 18, 2012, 09:19 »
Graphene nanomesh right?
I would start by making a perfect graphene structure of the correct size. Find graphen in the database and use Repeat or SuperCell plugin to give it the correct size. Then use the circular selection tool to select all the atoms in the hole and then delete them by pressing delete.

10
General Questions and Answers / Re: problem in Hydrogen passivation
« on: September 10, 2012, 10:16 »
Unfortunately I do not think we have an easy workflow for this case.

I think you will have to put them in one by one.

11
General Questions and Answers / Re: how can I make a device periodical in one direction
« on: September 10, 2012, 10:11 »
You are correct esp,
Basically, if you set some unit cell ATK will assume that all directions are periodic (or A and B for device). Then if you don't want to actually model a periodic system you makes sure that the periodicity is large enough so that they don't actually interact.

If you are not actually modelling a periodic system then there is no reason to have more than 1 k-point in that direction.

12
General Questions and Answers / Re: vnl crashes when changing lattice dimensions in bulk mode
« on: August 10, 2012, 09:29 »
I can't reproduce the error you report with the latest released version (12.2.2) on the OS you report. Perhaps it has been fixed in that version, I know there were some bugfixes between those releases. 

Thanks for taking the time to report bugs, it is greatly appreciated.

13
General Questions and Answers / Re: Can someone give me ideas on how to make a basic FDSOI device?
« on: August 10, 2012, 09:16 »
You are right, the number of atoms is no problem in that case.
But unfortunately ATK can't do 3-terminal devices yet. And the two terminals must be straight ahead of each other.

It is, however, possible to add gates to a two-terminal device. You can add continuum regions at a fixed potential. These can be added in using the Builder > Michellaneous > Spatial Regions.

14
General Questions and Answers / Re: Calculator settings for transport properties of 2d Graphene and 1D nanoribbon
« on: August 9, 2012, 09:58 »
No, the correct pattern for graphene transport is (1x Ny x Nz), assuming that y is the periodic direction.

K-points are used when a direction is periodic (and also in the transport direction). Exactly how big Ny should be depends on how delicate a property you are trying to model. But think that at least Ly/Ny > 25 Angstrom, where Ly is the size in the y-direction. This may not be enough, however, think of it as a convergence parameter.

Also, for graphene transport, use an odd number for Ny, since for some transport directions, this will make sure that the Dirac point is included in the calculation. This will improve the results.

15
General Questions and Answers / Re: Can someone give me ideas on how to make a basic FDSOI device?
« on: August 9, 2012, 09:38 »
The builder tutorials gives a good impression of what can be built in our software.
http://www.quantumwise.com/publications/tutorials/builder/

Full device simulations like the one you are talking about, cannot realisticly be calculated in ATK due to the large number of atoms.
Interface resistance, tunneling across layers, small model devices, are things we can do :)

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