Messages

1

General Questions and Answers / Re: Negative differential conductance in perfect zigzag GNR?

« on: July 2, 2013, 01:53 »

>The I-V characteristics of perfect zigzag graphene ribbons should exhibit linear response.
This is the case only when N is odd in the literature and your structure is not odd but even.

I think what is importance is not negative differential conductance but that conductance is strongly reduced when N is even.

2

General Questions and Answers / Re: Negative differential conductance in perfect zigzag GNR?

« on: July 1, 2013, 16:02 »

>if there is anything miscalculated or any problem with the device set up?
I think there is no problem in your calculation.

I recommend that you calculate the transmission eigenstate at fermi energy under finite bias both from left to right and  from right to left.

3

General Questions and Answers / Re: Simple GNR transmission question

« on: June 13, 2013, 15:03 »

when Vbias=0.1V, the transmission at Ef is from the valence band of left electrode to the conduction band of right electrode.
Maybe this transmission is prohibited due to symmetry of wave function.

4

General Questions and Answers / Re: Transmission of graphene ribbon is a pulse instead of spike around Fermi level

« on: June 13, 2013, 14:55 »

I guess that the band structure at E = 0 eV is not completely flat but a bit distorted numerically.
That is the reason why T(E=-0.02) = 2.01.

But I think your calculation is OK because this artifact brings no influence.

5

General Questions and Answers / Re: About contact resistance

« on: April 4, 2013, 14:56 »

I think your calculation is reasonable.
Because (7,0)CNT is a semiconductor, there is no (quasi) propagating mode of (7,0)CNT within a bias window at least when a bias voltage is low, so that electron transport occurs via only decaying modes.
In other words, all scattering states within a bias window are decaying modes of (7,0)CNT coupled with propagating modes of Au electrods.

6

General Questions and Answers / Re: Somthing wrong with my test mesh-cut-off.

« on: December 16, 2012, 10:40 »

Your calculation seems right.
It is often difficult to get total energy numerically converged for mesh cutoff if GGA is used.
According to my experience, 500 to 1500 Ry is needed.

If you switch XC functional to LDA, you will get a good convergence with small  mesh cutoff

7

General Questions and Answers / Re: how to achieve self-consistent

« on: September 17, 2012, 16:50 »

So is it right that your system is 1-dimensional, spin-polarized, and has semi-conducting electrodes?
I can give you further advice although I'm not sure it works well...

First, you should add surface layers more in central region.
In general, 1-dimensional electrodes need more screening layers than 3d.
And it's also true for semi-conducting electrodes.

Second, the cause may be bound states in bias windows.
If so, you would improve convergence using SingleContour instead of DoubleContour.

8

General Questions and Answers / Re: how to achieve self-consistent

« on: September 17, 2012, 15:07 »

Is it sure that your electrodes are 1-dimensional?
If not, you should increase k-point sampling both for SCF and physical quantities.

In addition, I strongly recommend for you to use the previous SCF result as an initial state for SCF.
(For instance 0.6V SCF result for 0.8V SCF calculation)

9

General Questions and Answers / Re: The calculation of Indium Arsenide

« on: August 7, 2012, 18:27 »

If my memory is right, this problem only occuers when GGA is used for 'In'.
So if you use LDA instead, it's expected that the band gap is under-estimated.

this problem was also seen in the calculation of InP.
I agree with zh that the pseudopotential of Indium would be the key to this issue.

10

General Questions and Answers / Re: a problem about the result of MTJ

« on: July 28, 2012, 08:19 »

About SCF, you can reduce calculation time by setting initial_state obtained in previous calculation.
(For instance, using 0.8V SCF result as initial_state for 1.0V SCF calculation)

About transmission, energy range and points can be reduced like linspace(-1.5, 1.5, 31).
If you find sharp and large peaks in the spectrum, you should add points around the peaks.

11

General Questions and Answers / Re: a problem about the result of MTJ

« on: July 27, 2012, 18:41 »

First of all, the k-point sampling for transmission should be increased.
Gamma point approximation is too crude for MTJ.
You should check how large k-point sampling is needed to obtain converged transmission spectrum.

12

General Questions and Answers / Re: Not physically reasonable in mulliken population

« on: July 23, 2012, 14:57 »

How about changing exchange_correlation from GGA.PW91 to GGA.PBE?

13

General Questions and Answers / Re: Transmission spectrum with trench

« on: July 11, 2012, 18:10 »

I was wondering about the trench in the right part cause it's not nicely looking.

If the central region of your structure is the same as the electrodes and gamma point approximation is used, the transmission coefficient at each energies is coincident with the number of Bloch state at each energies.
So the trench can be possible.

What should I use if LDA and GGA would underestimate the bandgap of a semiconductor?

DFT+U method and meta-GGA are sorts of a prescription.

14

General Questions and Answers / Re: Charge-missing problem, plz help(structure pre-optimized with VASP)

« on: July 10, 2012, 17:32 »

First of all, you should not use 'NeutralAtom' but 'EquivalentBulk' if the electrodes are homogeneous.
In addition, I feel mixing parameters could be improved as mixing = 0.01 and history step = 20.

I've not looked into your structure carefully but it may be better that a few unrelaxed electrode layers are inserted in the central region.

15

General Questions and Answers / Re: Question about calcutaing the current by transmission spectrum?

« on: May 27, 2012, 17:52 »

Temperature  is fixed to 300K,

If you change both left and right temperatures to 0K, it's expected that  (-2,2,201) and (-0.05,0.05,6) are similar.

if it just belongs to numerical error, how can I correct this error?

Sorry that my previous post is confusing.
I meant that small value of transmission like 1e-9 is actually 0 and has no physical meaning.
And the reason why it's not 0 but some very tiny value like 1e-9 comes from numerical error.