Messages

1

General Questions and Answers / Electrode temperature independent transmission spectrum?

« on: March 5, 2010, 04:21 »

Recently, I come across some problems about the temperature dependency of current. In my opinion, although the electrode temperature has some physical meaning that it can be interpreted as the electron temperature in the electrode region, we are actually calculating the transport properties at zero temperature. Thus the transmission spectrum is independent of the electrode temperature. If I want to study current at different temperatures, I just need to calculate the transmission spectrum of certain temperature T₀ once, then I change the temperature in f_L-f_R and get the current from the Landauer-Büttiker formula.

I wonder if there is something wrong in my opinion? We cann't have the actually temperature dependent current from ab initio studys now?

2

General Questions and Answers / Re: Disagree of the transmission spectrum and the current

« on: October 24, 2009, 05:35 »

Oh, I had a test to restrict the integration of transmission only in the interval that spans the bias window plus/minus 5*kB*T, which is  (-0.27376,0.27376) for the bias 0.289 V. And I got 9.74e-5 miuA which agrees well the current calculation 9.7e-5 miuA.

  Thus I think the peak at  E=-0.2875 eV had been omitted which causes the disagreement. As for my case, I guess I must write
a script to get the current from the transmission data rather than directly perform the current calculation.

3

General Questions and Answers / Re: Disagree of the transmission spectrum and the current

« on: October 23, 2009, 15:56 »

Sorry I didn't write the formula carefully.
In that formula, [tex]f_L[/tex] means [tex]f(\frac{E-V/2}{k_bT_L})[/tex].
The chemical potentials of the electrodes are supposed to relative to the average fermi level. I think the formula are similiar to that one.

ps: when increasing the number of points, the converged current is the reasonable value. Thus could tell the proper choose of the number of points. But how to determine the infinitesimal. When should I change it from the default value.

4

General Questions and Answers / Disagree of the transmission spectrum and the current

« on: October 23, 2009, 04:35 »

I calculated the spin-resolved current and transmission curves of a graphene nanoribbon recently. The I-V curve have a discontinuous point at bias=0.289 V. I went on to study the transmission curve and didn't found discontinuity in the transmission curve. I doubt that I didn't get the correct current. Then I integrating the transmission curve according to [tex]e/h*\int_{-\infty}^{\infty}T(f_L-f_R)dE[/tex]. I found the alpha spin current from transmission agrees well with the current alculation. The former is 2.004e-5 miuA, and later is 1.993e-5 miuA. But the beta spin current from transmission is completely different from the current calculation. The former is 2.47e-3 miuA, and later is 3.61e-5 miuA!
The electrode temperature is 300K. The script of calculating current is as below. Furthermore, when I increase the points from 100 to 5000,10000. The beta spin current increase from 3.61e-5 miuA to 9.7e-5 miuA,9.69e-5 miuA. When integrating the transmission curve, I found the transmission at E=-0.2875 eV contributes very much. Thus I guess the calculation energy window is just taken not too larger than [-bias/2,bias/2], which is not enought for my case. ???I also want some help on how to choose a proper number of points and green_function_infinitesimal in current calculation.
*******************************************************************************
bz_int_param = brillouinZoneIntegrationParameters( (1,1) )
current_up = calculateCurrent(
    self_consistent_calculation = scf,
    brillouin_zone_integration_parameters =bz_int_param,
    spin = Spin.Up,
    green_function_infinitesimal = 1.0e-5*electronVolt,
    number_of_points = 100
    )
current_down = calculateCurrent(
    self_consistent_calculation = scf,
    brillouin_zone_integration_parameters =bz_int_param,
    spin = Spin.Down,
    green_function_infinitesimal = 1.0e-5*electronVolt,
    number_of_points = 100
    )

print >>tcfile, "%s\t%.2e\t%.2e" %(voltage,current_up.inUnitsOf(Ampere)current_down.inUnitsOf(Ampere))
*******************************************************************************

5

General Questions and Answers / Re: non-integer transmission possibility in zero-bias

« on: October 22, 2009, 04:03 »

I enlarge the length of the electrode and that solve the problem.
Thank U all!

6

General Questions and Answers / Re: question about the zero energy point in the transmission spectrum

« on: October 21, 2009, 17:09 »

It is using ATK-DFT.
The parameter part of the script which may have some problems is posted.

7

General Questions and Answers / Re: question about the zero energy point in the transmission spectrum

« on: October 21, 2009, 16:19 »

Thank U, I will check them carefully.
I found that although the transmission spectrum seems strange, the current
seems not influenced by the unphysical shift of transmission curve. The I-V curves
go on well under finite bias.
Thus I guess the fermi energy determined in the current calculation is different from
that in the transmission calculation? 

8

General Questions and Answers / non-integer transmission possibility in zero-bias

« on: October 21, 2009, 05:06 »

I calculated a 1-D homogeneous system under zero-bias and found
that some points in the transmission curves are not integer, while other
points are all nearly integer (1.999 etc) and match the band structure well.

The nearest distance of atoms in the near cells is larger than 15A. I then
enlarge the cell, the transmission possibility change from 3.39 to 3.5.
But it suppose to be 4. The unit cell I used is very large and my computer
's memory is nearly all used. Could anyone give some suggestions on how
to get the correct result?

9

General Questions and Answers / Re: question about the zero energy point in the transmission spectrum

« on: October 21, 2009, 04:35 »

Thank you, zh! I have read the post. I think you mean the relative electrostatic potential  can be shifted by
a random constant number and it's different for the bulk system and two-probe system?
But the system I calculated is homogeneous, and it's under zero bias.
And I have calculated other homogeneous system by using ATK before. The band and the transmission curve
match well....

10

General Questions and Answers / question about the zero energy point in the transmission spectrum

« on: October 20, 2009, 11:53 »

Hello, everyone! I have met a problem about the position of zero energy point  in the transmission spectrum.
I tested a 1-D nanoribbon device. The nanoribbon is devided into two electrode regions and a scattering region.
Thus I think the transmission spectrum under zero bias can be totally determined by the band structure of the
nanoribbon. But the result turned out not to be what I thought.... (see figure 1)
However the shape of the transmission curve seems reasonable if I shift it upward. (see figure 2)
I guess the zero energy point in the transmission curve is a relative value.
But what's the physical meaning of the zero energy point in the transmission curve?
Is it means the fermi energy of the device ? And what's the difference between the fermi energy from the band
structure and the two probe device?





 

11

General Questions and Answers / Re: MPSH in quasi-1D homogeneous systems?

« on: April 7, 2009, 05:42 »

Thank you, all!
I just use it as a model system to see the effect of wavefunction mismatching. 

Oh, I didn't know the DOS in two-probe systems means "surface density of states".
What does "surface" here means? Because it includes the DOS of the electrode?

12

General Questions and Answers / Re: MPSH in quasi-1D homogeneous systems?

« on: April 6, 2009, 15:55 »

Thanks. Excuse me, I don't know how to calculate surface density of states. Would there be some tutorial?
In the first post, I just take vacancy for example. The system in my post is a perfect nanoribbon in which central region have different spin from the electrode. The peaks in the transmission spectrum should be due to the mismatch of the wavefunction between the central region and electrode. The MPSH is calculated from the whole central region. When I increase the bias voltage, the peak in the transmission spetrum moves, but the MPSH level seems don't have corresponding changes . So I think there is no obvious relation between the transmission and MPSH. And I wonder if there is some other method could descrbe the move of the transmisson peak if MPSH is not applicable?
below are results of higher bias voltage of the same system:

13

General Questions and Answers / Re: MPSH in quasi-1D homogeneous systems?

« on: April 6, 2009, 14:10 »

Thank you, Nordland. Do you mean that I may calculate MPSH of the whole central region to see if it obeys the assumption that the algorithm needs? For example, if it introduces an artifical scattering state, then we can't use MPSH for analysis?
If the coupling is too strong, MPSH can't be applied in such a system. The peaks in the transmission bias windows doesn't have a corresponding MPSH state, How to descrbe the the physical meaning of the peak?  I am not quite sure of the meaning of the eigenstates of the transmission eigenchannels, how to relate it to the central region?
I have some results below. And the peaks in the MPSH and the transmission spectrum doesn't have obvious relation. Does it means the peaks in the MPSH are just fake, and means nothing?

14

General Questions and Answers / MPSH in quasi-1D homogeneous systems?

« on: April 4, 2009, 09:55 »

MPSH in a molecular device model has clear physically meanings. One can use it to see how the molecular orbitals changes due to the applied bias voltage.
But in a  homogeneous system, such as quasi-1D nanotubes and graphene ribbons, I don't know how to distinguish the surface layers and the real middle part, because they are the same!
Also I wonder what is the physical meaning of MPSH in such a system? Can we still use MPSH to analysis the middle part?
For example, if I introduce a vacancy in the central region, can I still calculate MPSH? How does it means? If so, how to choose the related layers? All central region?

15

General Questions and Answers / Re: questions about surface layers

« on: April 3, 2009, 13:25 »

Yes. In my last post, I just want to say that due to technique considersation, we will get a spin unpolarized result if we initial it as spin unploarized.