Topics

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 / 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))
*******************************************************************************

3

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?

4

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?





 

5

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?

6

General Questions and Answers / questions about surface layers

« on: April 1, 2009, 06:49 »

Hello, I've just start to learn ATK recently and I found the manipulation of "surface layers" confusing. In the "atomic manipulator" module of the VNL program, one can set the number of either left or right surface layers. One could then drag the two-probe configuration to the "nanolanguage scripter" module to get a .py file. Actually, the .py file is the only need file for the atk program. But I found that there is only scattering elements which are the combination of surface layers and scattering region such as a molecular. Does it means we don't treat "surface layers" explicitly and separately, we treat the central region which contains the "surface layers" as a whole?

BTW:I have learned in some articles that the surface layers act as the screening layers to protect the electrode from influenced by the scattering region. Then how can I test whether the screening is enough? Do I need to inscrease the number of surface layer until my interest current or transmission spectrum converged?

Many thanks