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Messages - nori

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31
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However, the sharp PDOS at 1.0 eV separate away from the delocalized PDOS at 1.3 eV.
Indeed it's subtle but I interpreted that the delocalized PDOS at 1.3 eV seems to have a long tail and would interact with the sharp PDOS at 1.0 eV.
Of course my interpretation might be wrong, so you should investigate the physical properties of your system more carefully.

32
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But the transmission at 0.3 and 1.0 eV are 0.74 and 0.73 respectively, which are noticeable.
These look like a Fano resonance which is derived from hybrid between continuum states (= delocalized states) and bound states (= localized states).
So I guess your calculation result is reasonable physically.

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To my knowledge, a localized PDOS should has a very small transmission.
Bound states have no transmission coefficient.
So if a very localized state is just a bound state, your thought is right.

33
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what can I do with this situation?
You can obtain numerically precise values with the method illustrated in the manual:
http://www.quantumwise.com/documents/manuals/ATK-2008.10/ref.calculatetransmissionspectrum.html

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Should I change some parameters?
I think k-point sampling is enough.
Firstly, you should check the converged mulliken population in order to confirm that the SCF converged at the appropriate state.
And if the mulliken population look strange, it may be due to too short electrodes or too short central region.
In addition, of course you should check again whether or not the bias is 0.01V.

 

34
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When the bias is 1.0 V
is this a typo and is the actual bias 0.1V ?

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the simultaneously calculated transmission coefficient for spin up and spin down is zero whthin bias window.
More precisely the transmission coefficients aren't zero but very small finite value numerically.
The output just represents that the transmission coefficients within the bias window are smaller than 5.e-5.

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The band gap is 0.1 eV. Theoretically, the threshold voltage should be 0.2 V.
I think  the threshold voltage should be 0.1 V in this case.

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However, when the bias is set to 0.01 V, the current still can reach 1.19036166221e-07 A.
I'm afraid I can't offer the appropriate answer without the script you used.
But it would be derived from inaccuracy of self energies or that the fermi level does not converge enough.

35
The gap around Fermi level is due to a forbidden tunneling derived from the symmetry of the wave function.
So the Bloch states are still present around Fermi level but are reflected almost perfectly.

If you analyze transmission eigenstates around Fermi level, you will realize the above thing clearly ;)

36
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However, the transmission spectrum obtained under bias has an energy gap,
The reason why such a result is obtained is written in Phys. Rev. Lett. 100, 206802(2008).

37
General Questions and Answers / Re: is this possible?
« on: May 30, 2011, 14:17 »
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What is the difference of KrylovSelfEnergy and the two you mentioned?

Briefly some (fast) decaying modes are ignored in KrylovSelfEnergy and in some cases this causes inaccuracy.
Please see the manual and the following literature for more information:
  • KrylovSelfEnergy: Phys. Rev. B, 77, 155301, 2008
  • RecursionSelfEnerghy: J. Phys. F., 15, 851, 1985
  • DirectSelfEnerghy: Phys. Rev. B, 59, 11936, 1999

38
I could get the SCF converged with graphene2.py and graphene2_scf2.py.
Although I've changed several parameters in order to reduce calculation cost, changing twoProbeAlgorithmParameters() would be the most important for better SCF convergence.

By the way, Can you access the latest version(11.2.3)?
If so, I strongly recommend that you use ATK 11.2.3 because a SCF convergence is more stable and an accuracy is improved.

39
General Questions and Answers / Re: is this possible?
« on: May 28, 2011, 14:17 »
I think your calculation is something wrong because current shouldn't be positive in negative bias voltage.
First of all, you need to check the transmission spectrum carefully at about -1.8V.
And then it is expected that you can see the large negative transmission coefficients within bias window.

In most of my experience, large negative value of transmission coefficients comes from inaccuracy of KrylovSelfEnergy.
So if my speculation is right, you would be able to solve this issue by using RecursionSelfEnergy or DirectSelfEnergy.

40
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   More importantly, the reluts are very different from ATK-2008.10? why?
Perhaps it comes from the difference of the algorithm.
Finite bias calculation of ATK 11.2 is more reliable than that of ATK 2008.10.
http://www.quantumwise.com/documents/tutorials/latest/UpgradeGuide/index.html/chap.upgrade.html#sect1.upgrade.twoprobe

About convergence, It seems that more fine tuning for SCF parameters is needed.
But I agree with Anders that we can't give you further advice without the geometry information.

41
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Error: the total width of the left and right electrodes may not exceed the width of the central region.

Do you use VNL 11.2.0?
If so, I recommend that you use VNL 11.2.2 because Device From Bulk feature in Builder is improved.
I've confirmed that optimized bulk Fe/MgO/Fe structure can be converted to 2probe normally with VNL 11.2.2;)
 

42
I'm afraid that I don't have an answer for the second question because I don't know what algorithm is used for Nanowire Wulff constructor.

43
If you really do only the step as "custom->nanowire->wire direction(100)->radius 10A-> (100) surface energy 0.1", you should obtain the Si nanowire in which Si-Si bond length is 2.35152 angstrom and only the unit cell size along z direction is wrong.
But in your structure Si-Si bond length shrink to 2.19488 angstrom, that means you did some additional step to get the structure.

I'm not sure what you did, but this is because you obtained the wrong result.

44
Nanowire Wulff constructor has a bug now, that's why you have to modify a unit cell length along z direction before sp3 passivation.

For instance of Si nanowire(100), you should change the length of C vector from "4.70304 angstrom" to "5.4306 angstrom".

45
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Compare to DoubleContour, whether the SingleContour have large effect on result
Yes, it could be especially when SingleContour DOES NOT include bound states within bias window.
Let me show you one example of the influence of Bound states with attached.
You will clearly understand including bound states would make the result difference so much.

I guess that the density matrix includes bound states is more accurate.
But please note that bound states are added to density matrix as an equilibrium state even if the states are within bias window when you use SingleContour integration.
So I also feel that SingleContour does not treat bound states within bias window perfectly.

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and whether the bound state unconsidered in all the older versions (such as ATK-2008).
Yes.
More precisely, old ATK only includes bound states in density matrix blow the lower electrode fermi level.

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