Messages

1

General Questions and Answers / Re: transmission spectrum and transmission coefficients

« on: December 20, 2010, 04:10 »

You could say that, provided you only pick those kz points which give the specific energy E in the band structure E(kz). However, you cannot define a transmission coefficient for a specific kz, since there is no periodicity in the Z direction and hence kz is not a good quantum number. So the picture you paint is only useful when trying to figure out which states in the electrode band structure that contribute to the transport.

   I see. Thanks a lot!

2

General Questions and Answers / Re: transmission spectrum and transmission coefficients

« on: December 19, 2010, 15:17 »

The transmission spectrum T(E) is the integrated transmission coefficients T(E,k) over the 2D Brillouin zone of incident wave vectors k.

Dear Anders Blom,

   I have just recently raised a similar question on one dimensional transport problem("http://quantumwise.com/forum/index.php?topic=896.0"). Both kstokbro and zh explained that  for me.  In my unstanding about their two explanations, the transmission spectrum T(E) is the integrated transmission coefficients T(E,k) not only over the 2D Brillouin zone, but also over the kz direction which is perpendicular to the  2D Brillouin zone.
   So in one dimensional transport problem, the electron tunnel only through the gamma point which is degenerated from the 2D Brillouin zone, but the transmission spectrum T(E) is the integrated transmission coefficients over the kz direction(from 0 to [tex]\pi[/tex]/Lz)?

3

General Questions and Answers / Re: one questions about the Graphene Nanoribbon Tutorial

« on: December 17, 2010, 08:27 »

For the transmission calculation you only specify k-points in the x and y directions,
The k-points in the transport direction are selected through the energy, i.e. T(E0, kx,ky) gives the transmission of all
states with energy, E(kx,ky,kz) = E0

Thanks very much for your and zh's every valuable word.  I get your point a little bit after several days' thinking over them.

4

General Questions and Answers / Re: a question about the conductance under finite biases

« on: December 14, 2010, 15:42 »

Under bias you should calculate the differential conductance, dI/dV.
Thus, calculate current as function of voltage and take the derivative of that curve.
In ATK10.8 there is a custom analyzer, iv curve that can do this for you.
Check out the tutorial:
http://www.quantumwise.com/documents/tutorials/ATKTutorialDevice/XHTML/chap.iv.html#sect1.iv.curve

Enjoy

Thanks for your help!

5

General Questions and Answers / Re: one questions about the Graphene Nanoribbon Tutorial

« on: December 14, 2010, 12:47 »

Regarding to the k, it also stands for the kinetic momentum of electrons. Sure, the electron states with kz[tex]\neq[/tex] 0 also have contribution to the electron transport.

Dear zh,
   Thanks for your answer. But I am still confused about the k point. For example,as Nordland mentioned in "http://quantumwise.com/forum/index.php?topic=36.0",for a 1D system the 'Quantum Numbers' should be set as  [0.0, 0.0, 0.0]. Thus  in the transmisson spectrum, all "Transmission Eigenvalues" at different energies only tunnel through gamma point [0.0, 0.0, 0.0]. I still don't know through which k-point the electrons with kz[tex]\neq[/tex] 0  tunnel? Do they also tunnel through gamma point [0.0, 0.0, 0.0] during the transport progress?

6

General Questions and Answers / a question about the conductance under finite biases

« on: December 14, 2010, 10:43 »

Dear everyone,
   In former post,"http://quantumwise.com/forum/index.php?topic=203.0",I guess the author obtained the conductance under bias in

Fig.3 by taking the value of transmission coefficient at E=0 according to each finite bias? Although,both fig.3 and fig.4 showed

the conductance variations under bias, they are far different. Which one is meanlingful?  Is it appropriate to consider the value

of transmission coefficient  at E=0 which is the average value of fermi levels of two electrodes?
    Any comment will be appreciated!!!

7

General Questions and Answers / one questions about the Graphene Nanoribbon Tutorial

« on: December 13, 2010, 10:21 »

Dear developers,

     In   Tutorial of Graphene Nanoribbon on this website "http://quantumwise.com/documents/tutorials/BasicGrapheneTutorial/XHTML/chap.zigzag_transport.html#chap.zigzag_transport.perfect_transmission", it said the enhanced transmission around the Fermi level was due to a peculiarity in the band structure of the zigzag ribbon. In my opinion, in such a one dimensional transport system, the electron would be transfered from the source electrode to the drain electrode only through the gamma(0,0,kz=0) point in the Brillouin zone and thus the peculiarity in the band structure of the zigzag ribbon far away from the gamma(0,0,kz=0) point along the kz line should be no use for electron transport and couldn't be related with the enhanced transmission around the Fermi level. But my opinion should be wrong. Do the kz points other than gamma(0,0,kz=0) point  also take part in the transport progress in such one  dimensional transport system?
   Thanks in advance!!!

8

General Questions and Answers / Re: Transmission Eigenvalues revisited

« on: November 29, 2010, 15:05 »

Dear Doc.Anders Blom,
Would you please kindly show me some references about this point?

This is a fundamental assumption, yes. You would need phonon scattering or similar to change these quantum numbers.

9

General Questions and Answers / Re: What's the relationship between the bloch state and tranmission eigenstate?

« on: July 7, 2010, 10:54 »

More or less, yes

Hi Nordland ,
I am still confused about the real part and imaginary part of tranmission eigenstates.
As mentioned in this article(PHYSICAL REVIEW B 76, 115117),when the author talked about the transmission through the OPV molecule(shown in Fig.4),he said "Since the wave is almost totally reflected, the imaginary part of the wave function is too small to be seen in the figure."
Then is my understanding about this word:It seems that it is the imaginary part of the tranmission eigenchannel represents the electron transimiting from left to right and since it can't be seen in the figure,so the wave is almost totally reflected.(I think my opinion is wrong)While the real part of the tranmission eigenchannel can be clear seen in this figure and reach the right electrode.What does the real part of the tranmission eigenchannel represents for?
I don't know the actual physical meaning of real part and imaginary part of the tranmission eigenchannel. Does the combination of real part and imaginary part of the tranmission eigenchannel represent the electron transmitting behaviour or the two can have individual physical meaning respectively?
Could please give me your opinions about this?I sincerely need your help!

10

General Questions and Answers / Re: What's the relationship between the bloch state and tranmission eigenstate?

« on: July 3, 2010, 14:06 »

They are satisfied the boundary conditions (i.e.,continuity) at the joint position between electrodes the scattering region.  The bloch state from electrode could be taken as an incident wavefunction.

Do you mean the transmission eigenstate can be considered as the decaying process of bloch state arising from the electrode in the scattering region?

11

General Questions and Answers / What's the relationship between the bloch state and tranmission eigenstate?

« on: July 2, 2010, 11:28 »

Dear all:

What's the relationship between the bloch state from the electrodes and tranmission eigenstate?
Any comments will be appreciated!

12

General Questions and Answers / Re: A problem about how to set the K-point set for optimization

« on: March 16, 2010, 09:48 »

thanks for your reply.

13

General Questions and Answers / A problem about how to set the K-point set for optimization

« on: March 16, 2010, 09:10 »

Dear everyone,

I want to relax the two-probe system like (Au wire)-(long DTB system)-(Au wire) by siesta, I don't know how to set the parameter k-sampling grid,I have a try of

 %block kgrid_Monkhorst_Pack
   1 0 0 0.0
   0 1 0 0.0
   0 0 60 0.5
%endblock kgrid_Monkhorst_Pack
but it is too slow to get the result ,so I want to know which number is enough for the k-sampling grid along z-direction?I think 60 is too big.

Thanks a lot in advance!