Messages

1

General Questions and Answers / Re: Spintronics and MRAM in ATK

« on: June 2, 2011, 03:30 »

Hi Anders Blom,

You mentioned: "ATK actually computes the Gibbs free energy for device configurations".
Does it mean that we should use a device configuration to calculate the intralayer exchange coupling, not a bulk configuration?
For example, in the Fe/MgO/Fe system, we need calculate the total energy with P and AP configuration of both electrode Fe and surface Fe.
Can we use a Fe/MgO/Fe bulk to calculate the intralayer exchange coupling by setting P and AP configuration of Fe at two sides?

If we don't consider the bias, these two methods look the same.

thanks.

Both of these quantities are, at least in their simplest version, rather trivial to compute once you have the currents and total energies for the parallel and anti-parallel electrode configurations.

Following Phys. Rev. Lett. 97, 237205 (2006), in the collinear approximation you can compute the spin torque parallel to the interface planes as

T||(theta) = hbar/2e*sin(theta)*(I_AP-I_P)/2

where I_AP/I_P is the anti-parallel/parallel SPIN current (i.e. spin up minus spin down current).

The interlayer exchange coupling is the difference in total energy between the two configurations; see http://cnst.nist.gov/epg/Pubs/pdf/epg734.pdf, Eq. (3). Now, ATK actually computes the Gibbs free energy for device configurations, rather than the total energy, and I don't know for sure how that influences the results, esp. given the notes in the references article that it's often difficult - and crucial - to determine the energy difference accurately.

I'm sure the general computed trends wouldn't be much affected, but it is clear one must take care to converge the calculations to a low tolerance, and make sure to have proper k-point sampling etc (as we know this influences at the transmission very much, and hence the current, and the difference between the two configurations).

2

General Questions and Answers / Re: Spintronics and MRAM in ATK

« on: June 1, 2011, 03:58 »

Hi Anders Blom,

Thanks for your prompt response.
I found the script in the tutorial of MTJ to calculate the k-point depencence of the transmission coefficients.
Do you have the script (it is better a analysis file which can be dragged and dropped directly into the analyzer window, like I-V curve and PDOS  )  to calculate the spin torque and intralayer exchange coupling from the .nc file?
Many thanks!!!

3

General Questions and Answers / Spintronics and MRAM in ATK

« on: May 30, 2011, 03:20 »

In the introduction part of ATK (http://www.quantumwise.com/products/12-products/27-spintronics-with-atk), it says that "calculate (collinear) spin-torque transfer (STT) and intralayer exchange coupling;
investigate the details of the spin transport mechanisms (such as barrier tunneling vs. resonant tunneling), e.g. by analyzing the k-point dependence of the transmission coefficients or scattering eigenchannels. (How?)"

May I know the details how to calculate STT and intralayer exchagne coupling? How to identify barrier or resonate tunneling?
Could you please help to give more detail tutorials or reference papers? Many thanks!

4

General Questions and Answers / Re: boron doped graphene

« on: April 5, 2011, 03:56 »

a tip to build asymmetry ZGNRs:

remove one line of carbon atoms next to the edge. Then, select all edged carbon and hydrogen atoms -> translate (y-2.131A; z+1.2305A) ->central  (done)

5

General Questions and Answers / Re: transmission pathways, electron or current?

« on: April 4, 2011, 04:39 »

Thanks for your confirmation of my guess.
May I have one more question?
Based on band structures, the edged state of H-terminated GNR is flat pi near the Fermi level. The C-C sp2-sigma is far away from the Fermi level. (Son et al. PRL 97, 216803 (2006)) Therefore, the electron transport should be at the edged pi bonds. The calculated transmission pathways of H-terminated GNR (http://quantumwise.com/forum/index.php?topic=1068.msg5305#msg5305) prove the current flows along the edge. However, it shows the current mainly flows between C-C pair without bond and partially flows between C-C sp2-sigma bonds. How to explain the transmission pathway is not along the pi bonds? Thx

6

General Questions and Answers / transmission pathways, electron or current?

« on: April 3, 2011, 16:45 »

From the video on homepage of quantumwise website, it says that "the arrow of transmission pathways inself obviously designates the direction of the electron flow". However, in the update-ATK11.2.pdf, it says "These can then be plotted as arrows, which gives a very pictorial representation of how the current flows"
As we know the current flow direction is the opposite direction of electron flow direction. Therefore, one of them must be wrong.
In my opinion, the arrows indicate the current flow since the arrow direction is from high voltage electrode to low voltage electrode if a small bias is applied (Note that it is not zero bias in all tutorials about transmission pathways).  Any physical explanation on this? Thanks.

7

General Questions and Answers / local current in graphene nanoribbon

« on: March 14, 2011, 17:29 »

As we know, the current flows at the edge of graphene nanoribbon. And the admin once gave a script to calculate the local current of graphene nanoribbon for 2008 version (sorry, I cannot find it now).
May I know why the local current flows both at the edge and in the central area in the VNL tutorial of 2011.2 (attached)? thanks.

8

General Questions and Answers / Re: optimization under bias (10.8.0)

« on: August 27, 2010, 04:43 »

Anders Blom, Thanks for your help.
it is not the end of the log file since it does not converge after 100 steps (the voltage is 4.0V). I have figured out how to optimize structure under high bias using 10.8 just like what we do to calculate I-V curve in 2008.10 version.
Firstly, we need optimize the structure under 0.0 V (constrain the electrode and several surface layers)
Next, we restore the .nc file of 0.0 V and change the voltage to 0.5 V. Do it again and again till to the high bias.
Note that we should increase the optimization steps.
However, I don't know how to realize the above calculation in the python script as that of the I-V calculaiton in 2008.10 version. Wish someone could provide the script.
Moreover, I don't understand the I-V curve calculation in the new version. Based on the tutorial, it just setup 1.0V and then plot the I-V curve based on 0.0V and 1.0V two points?! Does anyone could provid the script to calculate I-V curve script as well as 2008.10 version?
Many thanks!
Ray.

9

General Questions and Answers / optimization under bias (10.8.0)

« on: August 18, 2010, 04:42 »

I have two questions:
Q1, how to import the structure file "Au-DTB-Au.VNL" from the example folder of VNL2008.10 to VNL10.8.0, since the VNL format file is invalid in 10.8.0?
    (I try to open the Au-DTB-Au.VNL file in 2008.10 and save as to Au-DTB-Au.py file, and then import it into 10.8. However, an error shows "An error occurred
    The following error message was generated when running the script:
    The last 27 atoms of the central region must match the last 27 atoms of the right electrode region. Atom 75 of the central region was Au at (4.32537207 Ang, 2.497254786 Ang, 23.85660639 Ang), it should have been Au at (0.0 Ang, 0.0 Ang, 23.85660639 Ang).
Typically this is caused by a syntax error or a spelling mistake; the message above should be helpful in correcting the issue in such cases.
If you believe the error is caused by VNL itself, please report the issue to QuantumWise, and make sure to include the traceback (click "Show Details...") and the script")

Q2, I want to optimize the above structure under a bias (4 V) in the VNL10.8.0, since it argues that the new version (10. can do the real optimizaion under bias. The optimization calculaion is fine (
| Optimization Information                     Fx       Fy       Fz                    |
+------------------------------------------------------------------------------+
|   0 Au  [   7.930 ,   0.416 ,   1.177 ]   0.00000  0.00000  0.00000          |
...
|  26 Au  [   2.163 ,   7.076 ,   5.886 ]   0.00000  0.00000  0.00000          |
|  27 C   [   2.163 ,   3.746 ,   9.340 ]   0.00002 -0.37070 -0.47289          |
|  28 C   [   2.163 ,   2.537 ,  10.038 ]   0.00012  0.15417  1.02052          |
|  29 C   [   2.163 ,   2.537 ,  11.434 ]  -0.00002  0.58899 -2.46883          |
|  30 C   [   2.163 ,   3.746 ,  12.132 ]  -0.00014  0.45219  0.79718          |
|  31 C   [   2.163 ,   4.955 ,  11.434 ]  -0.00001 -1.06966 -2.09744          |
|  32 C   [   2.163 ,   4.955 ,  10.038 ]   0.00019  0.22143  0.79862          |
|  33 S   [   2.163 ,   3.746 ,   7.586 ]   0.00144 -0.06909  0.41823          |
|  34 H   [   2.163 ,   1.599 ,   9.496 ]  -0.00001 -3.08222 -1.47391          |
|  35 H   [   2.163 ,   1.599 ,  11.976 ]   0.00003 -3.46500  2.21446          |
|  36 S   [   2.163 ,   3.746 ,  13.886 ]  -0.00004  0.09841 -3.48203          |
|  37 H   [   2.163 ,   5.893 ,  11.976 ]  -0.00008  3.48056  2.25191          |
|  38 H   [   2.163 ,   5.893 ,   9.496 ]  -0.00010  3.06288 -1.45637          |
|  39 Au  [   7.930 ,   0.416 ,  15.586 ]   0.00000  0.00000  0.00000          |
...
|  65 Au  [   2.163 ,   7.076 ,  20.296 ]   0.00000  0.00000  0.00000          |
+------------------------------------------------------------------------------+
| Optimization step =  0 E = -7.1426e+04 eV Maximum force =  3.4806e+00 eV/Ang |
+------------------------------------------------------------------------------+
+------------------------------------------------------------------------------+
| Device Calculation  [Started Tue Aug 17 15:50:42 2010])

My question is why there is only one step of optimization before the device property calculation? What is the meaning of values of Forcexyz? The force difference before and after optimization? How to view the optimized structure? Why the structure of analysis.nc file is the same as the initial structure?
Sorry for too many questions. Many thanks.
BTW: I have 3 year experience in using ATK, but I still cannot manipulate the new version. Therefore, a detail manual/tutorial of the new version is necessory and important!                     

10

General Questions and Answers / Re: I_V curve (restore calculation)

« on: June 11, 2009, 04:15 »

Dear Bolm, Many many thanks for your help.
May I know whether we should use the ivcurve.py (original version) for calculations of gate voltage sweep?
If yes, I think I can modify the script to do restore initial claculation of gate voltage sweep.

11

General Questions and Answers / Re: I_V curve (restore calculation)

« on: June 9, 2009, 17:58 »

Dear Blom,
May I know do you have the script for calculate the GATE voltage sweep?  many thanks.

12

General Questions and Answers / Re: Calculation in high magnetic field ..

« on: June 9, 2009, 08:51 »

So, how about the electronic field?

13

General Questions and Answers / Re: I_V curve (restore calculation)

« on: June 9, 2009, 07:47 »

That's good. here is the link of I_Vcurve script (lih2hi_iv.py) wroten by Anders Blom. http://quantumwise.com/forum/index.php?topic=19.0
In his script, we calculate the I_V curve of lih2li from 0.0V to 0.3V and get four .nc file and one .vnl file.
My question is how can I continue this calculation to a high voltage (such as calculate 0.4V from restoring the 0.3V)
It is very useful since I have got lots of transmission calculations, ie, I have alreay gotten the initial charge density of 0.0 V. If I want to continue to calcuate I-V properties, I guess I'm no need to calculate the 0.0eV case again as wroten in Blom's script.
One more question, the calculation is converged in transmission spec calculations (0.0V). However, it is not converged in I-V curve calculations using the above script at 0.0V. may I know it is reasonble?

many thanks.

raymond

14

General Questions and Answers / I_V curve (restore calculation)

« on: June 7, 2009, 08:11 »

Hi,
based on the script of I-V curve calculations, I get the current from 0.0 to 0.3V. My question is how can I modify the script to continue the I-V curve calculation from 0.3V. That is to say, how to restore the lih2li_iv-0.3.nc as the initial input file to calculate the 0.4V and above since we do not want to recalcuate the cases of 0.0-0.3V.

many thanks

rgds,

raymond

15

General Questions and Answers / Re: how to choose the eigenchannel index

« on: May 25, 2009, 07:56 »

First of all - since it is a 1D system you can remove the k-points [0.5, 0.5, 0.5]. This you can do by right-clicking on it in the box 'Quantum Numbers'
You will only need [0.0, 0.0, 0.0] for your study.

May I know how to set the K-points of eigenvalues in 2D system? Since the high symmtry points are [0,0,0];[0.5,0,0]; [0.5,0.5,0]; and [0,0.5,0] in 2D system, I should set four rows as [0,0]; [0.5,0]; [0.5,0.5]; [0,0.5]? Or,  I only need set [0,0] in all two-probe system (1D, 2D, and 3D).

May I know what is the difference of eigenvalues (channel) at different high symmtry points? I guess the number of channels will be the same at all high symmtry points.

Many thanks.