Messages

1

General Questions and Answers / Re: transmission spectrum for nanotube

« on: October 20, 2016, 01:54 »

Why should it be 2? A (6,0) tube has a small band gap.

Hi, Dr. Blom.

According to the reference http://pubs.acs.org/doi/pdf/10.1021/jz100889u (Definitive Band Gaps for Single-Wall Carbon Nanotubes). They used B3LYP and found that the band gap for CNT(6,0) is 0. So I assume that the transmission spectrum would have no gap as well. Though no experimental data or LDA approximation calculation for CNT (6,0) mentioned here.

Thanks.

Jenny

2

General Questions and Answers / transmission spectrum for nanotube

« on: October 19, 2016, 17:56 »

Hello, everyone.

I found that in the tutorials (http://quantumwise.com/support/tutorials/item/515-isotope-impurity-effect-on-a-single-wall-carbon-nanotube), the electron transmission spectrum has a dip at the Fermi level for carbon nanotube (6,0). However, theoretically the transmission value should be 2, instead of 0.

what is the problem with the tutorial? And how can we get the correct result?

Jenny

3

General Questions and Answers / Re: Transmission eigenvalues for one mode larger than 1

« on: July 23, 2016, 19:18 »

Hi, Dr. Blom.

I'll try to run the tutorial and compare. Thank you for your suggestion.
But in the tutorial, it said "The contour plot of the k-dependent transmission above has a pronounced peak at (kA,kB)=(0.18,0.0), with a transmission coefficient of ~2. In the computation of the transmission spectrum earlier, we did not have a dense enough k-point sampling, and hence the transmission was underestimated, i.e. we obtained 1.45 while the 21x21 mesh gives 1.58". It indicates that T(E,k) did depend on the k-point sampling for the transmission right? Also, from my results, T shows different value for different k-point sampling at Fermi level. I'm confused why you said T does noe depend on the k-point sampling?

Jenny

4

General Questions and Answers / Re: Transmission eigenvalues for one mode larger than 1

« on: July 22, 2016, 20:58 »

Hi, Dr. Blom.

I used the default setting spin.up. So I guess it should do nothing with the spin. And I check the sum of eigenvalues is 11.218 while T=10.6307 for the transmission spectrum at [0,0]. I thought this might related to the k-point sampling for the transmission spectrum calculation. So I performed another calculation with k-point sampling 21*21 for transmission coefficient calculation at Fermi level. I got T=9.36476 which is even smaller . I tried to plot the k-dependent transmission coefficient and attached here. I found that T is largest at [0,0], which is about 21.4. I think it is with a factor 2 for spin. The code I used to plot it is attached(same as tutorial https://www.quantumwise.com/documents/tutorials/latest/MolecularDevice/index.html/chap.analysis.html). why it didn't show the factor 2 problem in the tutorial? BTW, I'm using atk-13.8.1 version.

And back to my question, I still didn't understand why the transmission eigenvalue for one channel would be larger than 1?

5

General Questions and Answers / Transmission eigenvalues for one mode larger than 1

« on: July 20, 2016, 21:44 »

Hi, everyone.

When I tried to calculate the transmission eigenvalues at Fermi level for my one-dimensional system, it came out with the result as below. The weird thing is that I got the first four channels that have eigenvalues larger than 1. I thought the range for the eigenvalues should be 0 to 1. Why would this happen? 


+------------------------------------------------------------------------------+
| Transmission Eigenvalues Report                                              |
| ---------------------------------------------------------------------------- |
| Left electrode Fermi level   = -4.316601e+00 eV                              |
| Right electrode Fermi level  = -4.316601e+00 eV                              |
| Energy zero                  = -4.316601e+00 eV                              |
| Energy                       =  0.000000e+00 eV                              |
| ---------------------------------------------------------------------------- |
| Number of transmission modes = 49                                            |
+------------------------------------------------------------------------------+
Eigenvalues (Up):
  1.376425e+00
  1.142384e+00
  1.046487e+00
  1.003812e+00
  9.181272e-01
  8.580740e-01
  8.237050e-01



The parameters I used to do the calculation is like this :
    energy=0*eV,
    k_point=[0, 0],
    energy_zero_parameter=AverageFermiLevel,

6

General Questions and Answers / Re: how to plot transmission coefficient as a function of energy and bias

« on: July 7, 2016, 22:47 »

Thank you, Dr. Blom.

That really helps a lot.

Jenny

7

General Questions and Answers / how to plot transmission coefficient as a function of energy and bias

« on: July 5, 2016, 23:34 »

Hi, everyone.

I found the chart of transmission coefficient as a function of energy and bias in the link below:
http://quantumwise.com/publications/tutorials/item/506-linear-response-current-how-to-compute-it-and-why-it-is-often-not-a-good-idea
However, the link seems doesn't work now. I tried to google the key words, but nothing came out.

I'd like to plot transmission coefficient as a function of energy and bias for my own system. Does the ATK have the plug-in function to realize it? If not, is there any available codes I can use to plot it?

Thank you very much.

Jenny

8

General Questions and Answers / DFT calculation of a perfect Cu wire

« on: April 7, 2016, 22:17 »

Hi, everyone. I tried to follow the tutorial of a perfect Al wire (https://www.quantumwise.com/documents/manuals/ATK-2008.10/chap.altranseigenchan.html)
And the same method was applied to calculate the optimum distance of a perfect Cu wire and the transmission spectrum of the wire. I've attached my script here.

I found that there are three states at Fermi level for a perfect Cu wire, which indicates that a perfect Cu chain is conductive at zero temperature. However, my professor argued that physically a perfect Cu wire should not be conductive at all because there are no electron sharing. So, was there anything wrong with my calculation? or a perfect Cu wire should be conductive as the result I got?

Thank you very much.

Jenny

9

General Questions and Answers / analysis of zero bias result of a two probe device

« on: March 14, 2016, 23:32 »

Hi, everyone.

I read the tutorial on the Au-DTB-Au device and found a question in the part of "Analysis of the zero-bias result".
https://www.quantumwise.com/documents/tutorials/latest/MolecularDevice/index.html/chap.analysis.html

Here, it mentioned that at Energy=2.4eV, T=1.45 when the calculation was performed with k=3*3 and k=21*21 gave T=1.58 at E=2.4eV. However, even with 21*21 mesh, the T value is still smaller than the sum of two dominating eigenvalues which gives a total transmission of 1.645. Does it mean that we need to increase the k-point sampling? Also, for the other two peaks at E=-3eV and E=-1.5eV, the summation of all transmission eigenvalues does not equal to the total transmission value at each energy level. Do we need to find the k mesh which is large enough to get us the summation of all transmission eigenvalues=total transmission value at each energy level?

Thank you very much.

Jenny

10

General Questions and Answers / Combining a Tersoff and a Lennard Jones potential

« on: February 10, 2016, 17:27 »

Hi, everyone.

I tried to follow as the tutorial did in http://quantumwise.com/publications/tutorials/item/848-adding-combining-and-modifying-classical-potentials
I can not locate the line where the 'C' element definition in the script, even after I set Script Detail to Show Defaults.

So, instead, I correct my script to

potentialSet = Tersoff_C_2010()
potentialSet.addParticleType(ParticleType(symbol='C',
                                          mass=12.0107*atomic_mass_unit,
                                          charge=None,
                                          # Sigma and epsilon from Ref [3].
                                          sigma=3.3611*Ang,
                                          epsilon=0.004207*eV))
potentialSet.addParticleType(ParticleType(symbol='Li',
                                          mass=6.941*atomic_mass_unit,
                                          charge=None,
                                          # Sigma and epsilon from Ref [3].
                                          sigma=0.826*Ang,
                                          epsilon=0.271115*eV))
potentialSet.addPotential(LennardJonesPotential(particleType1='C',
                                                particleType2='Li',
                                                r_cut=9.0*Ang))
potentialSet.addPotential(LennardJonesPotential(particleType1='Li',
                                                particleType2='Li',
                                                r_cut=9.0*Ang))
                                         
calculator = TersoffCalculator(parameters=potentialSet)

However, when I run the script, an error comes up, saying "Tersoff_C_2010 instance has no attribute 'addParticleType' ". So, how could I modify my script to make it work as it suggested in the tutorial?
BTW, I'm using atk-13.8.1 version.

Thank you very much.

Jenny

11

General Questions and Answers / k points setting for nanotube Blochstate

« on: January 29, 2016, 20:22 »

Hi, all.

I want to calculate the Blochstate for nanotube at G and Z?
So the k points setting should be (0,0,0) for G and (0,0,0.5) for Z?
If not, what should the k points be?

Thank you very much.

Jenny

12

General Questions and Answers / Re: questions regarding to tutorial of Isotope impurity effect on a single-wall carb

« on: September 11, 2015, 17:00 »

We have quite a lot of predefined potentials for SiC. You can simply use one of those. Send your SiC configuration to the ScriptGenerator, add a New Calculator, double-click it to open its settings, and select ATK-Classical, a list of possible potentials will be available. I'd recommend you use one of the Tersoff potentials, e.g. Tersoff_SiC_2005 or Tersoff_SiC_1989.

I got this. Thank you.
What if I'd like to study about C and metal(eg: C+Al)?  I found there were no available potentials can be used under ATK-Classical. Btw, I'm using atk-13.8.1 version.

Thanks again in advance.

Jenny

13

General Questions and Answers / Re: questions regarding to tutorial of Isotope impurity effect on a single-wall carb

« on: September 10, 2015, 22:34 »

Dear Jenny.

The tutorial does not say, but you need to manually add a few lines in the script if using ATK 13.8.
You can see below how I have defined parameters for the dynamical matrix calculation. Those parameters should be passed on to the calculator:

# -------------------------------------------------------------
# Calculator
# -------------------------------------------------------------

dynamical_matrix_parameters = DynamicalMatrixParameters(
    atomic_displacement=0.01*Angstrom,
    repeats=Automatic,
    acoustic_sum_rule=False,
    symmetrize=True,
    )

calculator = TersoffCalculator(parameters=Tersoff_C_2010(),
                                                              dynamical_matrix_parameters=dynamical_matrix_parameters,
                                                              )
########################

Then your calculation should work.

Thank you, Jess Wellendorff.

It seems work well now. I have another question is that, if I'd like to use this method to calculate my own device, which might consist of two elements(eg: C and Si). So, I need to build the specific potential for that by myself or to refer to others, right? If I'd like to build it on my own. Where can I find the parameters needed as suggested in the tutorial below? Is there something like a library? Thanks again.
https://www.quantumwise.com/publications/tutorials/item/848-adding-combining-and-modifying-classical-potentials

Jenny

14

General Questions and Answers / Re: questions regarding to tutorial of Isotope impurity effect on a single-wall carb

« on: September 9, 2015, 17:46 »

And the transmission spectrum for the device with isotope C14 ring is attached here.

15

General Questions and Answers / questions regarding to tutorial of Isotope impurity effect on a single-wall carb

« on: September 9, 2015, 17:45 »

Hello, everyone.

I've done as the tutorial suggested. However, I'got the different results from the tutorial.
I noticed that the default electrode length here I got is different from the tutorial. And I've tried to build a device with cnt(6,0) without optimization. The electrode length seems match the numbers in the tutorial. However, the resultant transmission spectrum is still different from that in the tutorial. I don't know why. Can somebody help me with it?
And I've attached the script and its result here.


Thank you.