1
General Questions and Answers / Re: Negative zero bias conductance
« on: September 24, 2013, 17:44 »
thanks for this clarification
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2
General Questions and Answers / Negative zero bias conductance
« on: September 24, 2013, 13:32 »
I am calculating zero bias conductances (ZBC) for narrow width twisted armchair graphene nanoribbons. For 3-AGNR and 5-AGNR (width) i am getting low but positive values..something like 10-18 and 10-9 Siemens respectively.... but for 4-AGNR (width) these values are low and NEGATIVE.
Why i am getting negative zero bias conductance?? Is it possible? I have kept same conditions for calculations of all three 'width' cases. Do i need to increase the (+E,-E) energy window of transmission. Right now it is (+2eV -2eV) See attached screenshot of transmission and 'negative' value of ZBC..
Why i am getting negative zero bias conductance?? Is it possible? I have kept same conditions for calculations of all three 'width' cases. Do i need to increase the (+E,-E) energy window of transmission. Right now it is (+2eV -2eV) See attached screenshot of transmission and 'negative' value of ZBC..
3
Installation and License Questions / Re: PC configuration
« on: April 12, 2012, 13:20 »
Thanks..Will come here again....if more info is needed 
4
Installation and License Questions / PC configuration
« on: April 11, 2012, 09:12 »
Considering a single node license of ATK-VNL. which pc configuration is good enough for simulation of materials and devices.
If i put a recent quad ,six or 8-core CPU with 8GB ram with decent GPU in windows will it be ok??
(say for eg, something like 4-core Intel 3.5 GHz LGA 1155 Core i7-2700K Processor
OR 8-core AMD 3.6 GHz AM3+ FX 8150 Processor
OR intel xeon server processor on single machine??)
Does the extra number of cores offer practical advantages on a single machine or more Ghz cycles are useful?
I know paralleling has lots of advantages but i am talking about a single machine only.
Thanks
If i put a recent quad ,six or 8-core CPU with 8GB ram with decent GPU in windows will it be ok??
(say for eg, something like 4-core Intel 3.5 GHz LGA 1155 Core i7-2700K Processor
OR 8-core AMD 3.6 GHz AM3+ FX 8150 Processor
OR intel xeon server processor on single machine??)
Does the extra number of cores offer practical advantages on a single machine or more Ghz cycles are useful?
I know paralleling has lots of advantages but i am talking about a single machine only.
Thanks
5
General Questions and Answers / Exchange correlation,mesh cut off and k points
« on: April 9, 2012, 11:53 »
Although there are no of posts related to the same topic i just want to know briefly:
1. I want to calculate very accurate values of band gap of the given structure and the effect of electric field on the gap.As DFT generally underestimates the gap ... which exchange correlation is considered the best (LDA,LDA+u,GGA+u etc..) considering the accuracy of gap.
2.the mesh cut off can be tested for convergence by calculating total energy of the structure at different cut off points and selecting the one at which energy values start to saturate.Is there a similar test method available for deciding k-point or it is strictly dependent on geometry of the structure i.e, for a 2D graphene sheet 9 X9 X1 or 21 X 21 X 1 etc...can be selected.How the correct decision is made in k point selection.We have to re-run at various Kkpoints and then look for the changes??
Thanks
1. I want to calculate very accurate values of band gap of the given structure and the effect of electric field on the gap.As DFT generally underestimates the gap ... which exchange correlation is considered the best (LDA,LDA+u,GGA+u etc..) considering the accuracy of gap.
2.the mesh cut off can be tested for convergence by calculating total energy of the structure at different cut off points and selecting the one at which energy values start to saturate.Is there a similar test method available for deciding k-point or it is strictly dependent on geometry of the structure i.e, for a 2D graphene sheet 9 X9 X1 or 21 X 21 X 1 etc...can be selected.How the correct decision is made in k point selection.We have to re-run at various Kkpoints and then look for the changes??
Thanks
6
General Questions and Answers / Re: Avg spin polarization density
« on: April 8, 2012, 18:06 »
Thanks 'zh' and Dr. Blom for the help 
7
General Questions and Answers / Re: Avg spin polarization density
« on: April 8, 2012, 09:56 »
I ran it for zigzag Si based nano ribbon.I am attaching the graph for both the old and new spin polarization.The difference is visible
Old graph is for:
av = numpy.array(n[:,:,:].sum(axis=0))
New one is for:
av = numpy.average(numpy.array(n[:,:,:]),axis=0)
For the new graph, i am getting values at the edges like 0.128, 0.096 without any % sign.
Does it mean these values are 12.8% and 9.6% as a relative measure of spin polarization??.
of course for Armchair ribbon i am getting much lesser values like....0.000020, 0.000015 etc
Old graph is for:
av = numpy.array(n[:,:,:].sum(axis=0))
New one is for:
av = numpy.average(numpy.array(n[:,:,:]),axis=0)
For the new graph, i am getting values at the edges like 0.128, 0.096 without any % sign.
Does it mean these values are 12.8% and 9.6% as a relative measure of spin polarization??.
of course for Armchair ribbon i am getting much lesser values like....0.000020, 0.000015 etc
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General Questions and Answers / Avg spin polarization density
« on: April 6, 2012, 08:52 »
The following script in the Graphene-bloch state tutorial calculates avg spin pol density by calculating electron densities of both up and down-spin electrons:
http://quantumwise.com/documents/tutorials/latest/GrapheneBloch/index.html/examples/density_average.py
I was able to get colorful graphs for zigzag and armchair ribbons with an indication that spin pol density for zigzag ribbons is much higher at the edges in comparison to armchair.
(see dig at the bottom of the page : http://quantumwise.com/documents/tutorials/latest/GrapheneBloch/index.html/electrondensity.spin.html)
The 'y' and 'z' axis display the size of ribbon in angstrom.There is a vertical colored scale (from -8 to +8) at the right side of graph
I am wondering what is the unit of this scale.Is it dipole moment per unit vol?? or is it just a number showing relative ratio?What i should write down in my manuscript if i use such graph?
Also spin polarization is calculated by using up and down spin electron densities through formula:
r = (n_u - n_d)/(n_u + n_d)
but in avg spin pol density calculation a factor + 1e-19 has been added in denominator why?
# Calculate the polarization density from the up/down densities
(n_u, n_d) = nlread('density.nc', ElectronDensity)
n_up = n_u[:,:,:].inUnitsOf(n_u.unit())
n_dn = n_d[:,:,:].inUnitsOf(n_d.unit())
n = (n_up - n_dn)/(n_up + n_dn + 1e-19)
av = numpy.array(n[:,:,:].sum(axis=0))
http://quantumwise.com/documents/tutorials/latest/GrapheneBloch/index.html/examples/density_average.py
I was able to get colorful graphs for zigzag and armchair ribbons with an indication that spin pol density for zigzag ribbons is much higher at the edges in comparison to armchair.
(see dig at the bottom of the page : http://quantumwise.com/documents/tutorials/latest/GrapheneBloch/index.html/electrondensity.spin.html)
The 'y' and 'z' axis display the size of ribbon in angstrom.There is a vertical colored scale (from -8 to +8) at the right side of graph
I am wondering what is the unit of this scale.Is it dipole moment per unit vol?? or is it just a number showing relative ratio?What i should write down in my manuscript if i use such graph?
Also spin polarization is calculated by using up and down spin electron densities through formula:
r = (n_u - n_d)/(n_u + n_d)
but in avg spin pol density calculation a factor + 1e-19 has been added in denominator why?
# Calculate the polarization density from the up/down densities
(n_u, n_d) = nlread('density.nc', ElectronDensity)
n_up = n_u[:,:,:].inUnitsOf(n_u.unit())
n_dn = n_d[:,:,:].inUnitsOf(n_d.unit())
n = (n_up - n_dn)/(n_up + n_dn + 1e-19)
av = numpy.array(n[:,:,:].sum(axis=0))
9
General Questions and Answers / Re: Calculate fermi velocity
« on: February 25, 2012, 07:29 »
i used eps = 0.0001. the 'c' parameter of the hex crystal was kept very large 42.32 Angstroms to avoid inter layer periodic interactions.
10
General Questions and Answers / Re: Calculate fermi velocity
« on: February 24, 2012, 13:48 »
Repetaed with GGA-PBE, 41X41X1 k points and DZDP basis
Graphene: 1.785 X 106 m/sec
Germanene :8.7786 X 105 m/sec
Silicene : 4.1896 X 105 m/sec
slowest in silicene?
Graphene: 1.785 X 106 m/sec
Germanene :8.7786 X 105 m/sec
Silicene : 4.1896 X 105 m/sec
slowest in silicene?
11
General Questions and Answers / Re: Calculate fermi velocity
« on: February 24, 2012, 10:33 »
Lines from A review on silicene—New candidate for electronics
The Fermi velocity calculated by Lew Yan Voon et al. in graphene, silicene and
germacene are 6.3×105 5.1×105 and 3.8×105 m/s,respectively
Another paper Two- and one-dimensional honeycomb structures of silicon and germanium S. Cahangirov et. al,
says that Low buckled Si and Ge structures all have Vf of the order of 106 m/sec very close to graphene!!!
The Fermi velocity calculated by Lew Yan Voon et al. in graphene, silicene and
germacene are 6.3×105 5.1×105 and 3.8×105 m/s,respectively
Another paper Two- and one-dimensional honeycomb structures of silicon and germanium S. Cahangirov et. al,
says that Low buckled Si and Ge structures all have Vf of the order of 106 m/sec very close to graphene!!!
12
General Questions and Answers / Re: Calculate fermi velocity
« on: February 24, 2012, 10:07 »
Can i write these values in my abstract/paper. Is that ok? or i again need to run calculations by changing the K point or making a selection between GGA/LDA or choosing double zeta double polarised basis sets?Will it improve the accuracy of result?
I have taken 21X21X1 k points for silicene,graphene and germanene.Also i have taken double zeta single polarized basis set.
Can you suggest me something here 
I have taken 21X21X1 k points for silicene,graphene and germanene.Also i have taken double zeta single polarized basis set.
Can you suggest me something here
13
General Questions and Answers / Re: Calculate fermi velocity
« on: February 23, 2012, 14:30 »
I used your formula and got these results:
Graphene:2.41 X 106 m/sec
Germanene: 1.373 X 106 m/sec
Silicene: 1.14 X 106 m/sec
What i should do to get more accurate results.In some technical papers velocity in silicene is 10 times slower than that of graphene . I am not getting that difference.
Graphene:2.41 X 106 m/sec
Germanene: 1.373 X 106 m/sec
Silicene: 1.14 X 106 m/sec
What i should do to get more accurate results.In some technical papers velocity in silicene is 10 times slower than that of graphene . I am not getting that difference.
14
General Questions and Answers / Re: Calculate fermi velocity
« on: February 17, 2012, 17:21 »
Mr Blom, You are getting gr8 result!..i am still trying to calculate 'dK' value correctly.I am out for one week..then i will again try to replicate the same.
Thanks again i have learnt a lot from you
Thanks again i have learnt a lot from you
15
General Questions and Answers / Re: Calculate fermi velocity
« on: February 17, 2012, 06:40 »
thanks for the explanation
Here is what i am getting:
h* = 6.5821 X 10-16
eps = 0.0001
E (Energy at K-eps) = 0.002874eV
coordinates = (0.333233,0.333233,0)
converted into reciprocal coordinates for hexagonal by taking 4(pi)
------
a*(sqroot(3))
and got (21.7721,21.7721,0)
vector length between (0,0,0) and (21.7721,21.7721,0) = 30.786m-1
now vf = 0.002874
-------------- gives something of the order of 1011 which is very high!!
30.786 X (h*)
am i doing this correctly???
Here is what i am getting:
h* = 6.5821 X 10-16
eps = 0.0001
E (Energy at K-eps) = 0.002874eV
coordinates = (0.333233,0.333233,0)
converted into reciprocal coordinates for hexagonal by taking 4(pi)
------
a*(sqroot(3))
and got (21.7721,21.7721,0)
vector length between (0,0,0) and (21.7721,21.7721,0) = 30.786m-1
now vf = 0.002874
-------------- gives something of the order of 1011 which is very high!!
30.786 X (h*)
am i doing this correctly???