Topics

1

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..

2

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

3

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

4

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))

5

General Questions and Answers / Calculate fermi velocity

« on: February 14, 2012, 10:44 »

Graphene ,silicene and germanene show linear dispersion at the K point in band dig.How can i get the slope dE/dk from bandstructure???  ???Is it possible to use some kind of nanolanguage script to calculate fermi velocity of electrons by taking slope of E-K graph.Please help.

6

General Questions and Answers / Warning msg : Graphene junction

« on: February 10, 2012, 11:15 »

I was running some transmission spectra calculations for Graphene junction device. I used multi grid solver keeping Neumann boundary condition for electrodes and Dirichlet for central regions .K points were kept as 1X1X51 using extended huckel device calculator.The calculation is running fine but i am getting warning messages in the log as shown below.What i should do to correct this?

################################################################################
# WARNING                                                                 
#                                                                             
# The computed multigrid residual is greater than the required accuracy.     
#                                                                             
# Computed residual :   1.29180e-010                                           
# Required accuracy :   1.00000e-012 

7

General Questions and Answers / Phonon dispersion curves

« on: February 9, 2012, 06:10 »

In atk 11.8.2 is there a way to draw phonon dispersion curves (for nanoribbon or some other configuration) to check stability??

8

General Questions and Answers / Silicene nanoribbon configuration

« on: January 7, 2012, 11:33 »

Hi,

I have used the out of plane geometry of silicon atoms and created a silicene nanoribbon.(see image attached).I want to know what appropriate k-points and mesh cut off i need to use for this arrangement??...generally i have seen something like (1X1X100).As this  arrangement is periodic in Z direction and i want to simulate isolated ribbons what value should i select  for 'x' in primitive vectors of unit cell??.Do i have to keep it large say(10A to 20A) so that there is minimum interaction between them.

Now in order to get a minima point in energy-vs-vol curve i want to set 'x'  primitive vector to a large value and select all atoms in fractional coordinates mode and stretch them to a proper value of 'y' so that Si-Si bond lengths are approximately matched with my previous 'bulk silicene crystal' calculation .Once this is done i will vary 'z' primitive vector and calculate energy at each of those z-points(or volume) to get correct structure of unit cell lattice having minimum energy.Then i will go ahead and perform force optimization (to a low value 0.005eV/A or something..) keeping x,y,z all "ticked". This should give me the stable structure of ribbon.Is this method ok?? OR should i do force optimization first and then play with lattice?

Please comment and advice on k-points,mesh cut off and above methodology of obtaining energetically favorable configuration of ribbons.

9

General Questions and Answers / Silicene bandstructure calculation

« on: December 22, 2011, 11:48 »

Hello,
1)I am new to ATK VNL and i am am trying to calculate the bandstructure of silicene.Read some articles on the same and found that Si-Si bond distance is approx 2.12 to 2.21A in a honeycomb puckered structure with lattice of 3.860A.To draw the geometry I first selected the graphene crystal from database and then replaced the C atoms with Si atoms.Now since we know that Si atoms in silicene are not in same plane i translated the two Si atoms in Z direction by + and -0.8A to get the out of plane structure thereby leading to a bond distance of about 2.2A between the two translated atoms.I did ATK dft calculations with optimized geometry and got the bandstructure showing some sort of dirac cone arrangement.I have attached the images for geometry arrangements of graphene and translation of atoms leading to "silicene crystal" and also the final bandstructure of silicene.Can somebody please look at the crystal builder image of silicene and advice me if iam doing it the correct way or i need to do somethin else?? ie have i made the geometry correctly using builder???

2)someone..Advice me on how to use and interpret the "total energy" calculation as it gives something negative -355.46ev

If i understand the geometry of silicene later i will move on and try to play with silicene nanoribbons and may be devices