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Messages - Petr Khomyakov

Pages: 1 ... 79 80 [81] 82 83 ... 86
1201
General Questions and Answers / Re: spin-dependent Seeback coefficients
« on: November 22, 2016, 14:35 »
The spin-dependent Seebeck coefficient for a given spin channel is defined in terms of K0 and K1 calculated for this spin channel, which corresponds to either spin_up or spin_down transmission spectrum. This is now corrected in the 'seebeck_coefficient.py' script enclosed.

The total Seebeck coefficient is not defined correctly in the PRB 2016 paper cited in a previous post. It is neither (S_up + S_down) nor (S_up + S_down)/2 as defined in the PRB paper. The total thermopower is given as  (conductance_up S_up + conductance_down S_down)/(conductance_up + conductance_down), see two slides (in the enclosed zip file) with a formal derivation of this formula. These slides also contain a complete set of equations for calculating S, S_up, S_down, conductance_up, conductance_down as well as useful references used to derive these physical quantities.

Enclosed you can find an updated python script that calculates the spin-dependent and total Seebeck coefficient as well as the conductance for spin up and down channels. An important update is that the definition of the Seebeck coefficient sign has been corrected; it is now S = -K1/(K0*e*T), where the elemental charge e>0 (positive). This definition of the thermopower is now consistent with the definition given in a textbook on Solid State Physics by Ashcroft & Mermin, see Chapter 13.

To test the script, one can use an nc-file (Device_New_Configuration.nc in the zip-file) with the spin-dependent transmission spectrum calculated for a 1D carbon chain with parallel magnetization configuration of the electrodes. The description of how to run the script is given in the script header.

NOTE: we are working on updating the thermoelectrics plugin in the VNL to fix the sign problem in the definition of the Seebeck coefficient. 



 

 

1202
General Questions and Answers / Re: Plotting Band structure in Origin
« on: November 20, 2016, 15:51 »
To add to the post, you may import or export atomic geometry of your system from or into CASTEP's format, respectively, but not really importing the band structure calculated in CASTEP to VNL, at least to the best of my knowledge.

1203
General Questions and Answers / Re: How to build phosphorene nanoribbons?
« on: November 20, 2016, 15:39 »
Please have a look at the previous post of mine.

1204
General Questions and Answers / Re: Problem with Bandstructure
« on: November 20, 2016, 15:37 »
This error comes from the Quantum Espresso routine dos.x that calculates DOS, so it has nothing to do with DOS plotting in the VNL as such. I see nothing obviously wrong in the QE input file for DOS calculation.  You may try to adjust the Al dos input file (which works as you say) for halite. Maybe your text editor has added something hidden in the halite_dos.in file, i.e., there might be something wrong with text formatting, and the dos.x is reading something it is not supposed to be in the dos input file. This is just my guess.

1205
General Questions and Answers / Re: Vasp scrip Problem
« on: November 19, 2016, 18:02 »
Do you mean that there is no 'VASP scripter' listed in Scripters in the Custom Scripter? First, restart VNL, and then check if the plugin is really installed in the AddOn Manager (if not, install it), and if the 'VASP scripter' has appeared in the list of scripters. Are you using the very recent ATK2016.3?

1206
General Questions and Answers / Re: Plotting Band structure in Origin
« on: November 19, 2016, 17:28 »
You may have a look at this one at http://docs.quantumwise.com/tutorials/crystal_bandstructure/crystal_bandstructure.html.

1207
General Questions and Answers / Re: Problem with Bandstructure
« on: November 19, 2016, 17:15 »
Please go through the tutorial at http://docs.quantumwise.com/tutorials/espresso_bs_dos/espresso_bs_dos.html on how to calculate and plot the band structure, using VNL and Quantum Espresso,

1208
General Questions and Answers / Re: Plotting Band structure in Origin
« on: November 19, 2016, 17:10 »
You can plot the band structure in the VNL. Did you have any problems with the band structure calculation and/or plotting of the band structure in the VNL-ATK?  For any Origin software-related issues, please contact their helpdesk. 

1209
General Questions and Answers / Re: New TB set for black Phosphorous
« on: November 17, 2016, 14:11 »
You may try implementing it for black phosphorus, following the procedure described at http://quantumwise.com/documents/tutorials/ATK-2015/SlaterKosterModel/index.html/chap.carbonpi.html for carbon atom treated with a single-orbital tight-binding model, which is also adopted for phosphorus in the PRB paper you have mentioned.

1210
General Questions and Answers / Re: DFT for Magnetism
« on: November 17, 2016, 13:20 »
You have to do spin-polarized calculations to study ferromagnetic and antiferromagnetic states of your system of interest. For bulk SnMnCrO2, you should do bulk calculations, using a spin-polarized density functional such as SGGA and LSDA.

For the self-consistent DFT calculation to converge to ferromagnetic or antiferromagnetic state, it would be a good idea to set the InitialSpin. This can be done by setting an initial spin configuration for each atom in the Initial State in the Scripter, see more details on InitialSpin at http://docs.quantumwise.com/manuals/Types/InitialSpin/InitialSpin.html#initialspin-c.

As an example of setting the InitialSpin, you may have a look at http://docs.quantumwise.com/tutorials/fe_mgo_fe/fe_mgo_fe.html.

1211
General Questions and Answers / Re: gate geometry
« on: November 17, 2016, 11:48 »
You can only make Box, Tube and Sphere shaped spacial regions for dielectric spacers and metal gate electrodes. Of course, you may try making a triangle spacial region by filling it up with smaller box-shaped regions.  Just for me to understand, why do you actually need a triangle-shaped gate electrode? Are they really used in real device setups?


1212
General Questions and Answers / Re: Shift to Gamma
« on: November 16, 2016, 21:01 »
'Shift to Gamma' means that the k-point grid chosen will include the Gamma point. This is done by shifting the entire grid in 'checked directions', which are just reciprocal lattice vector directions (reciprocal to direct lattice vectors A, B and C). One may want to shift the k-grid in all three directions, or any two directions, or one of the three directions.

1213
General Questions and Answers / Re: How to build phosphorene nanoribbons?
« on: November 13, 2016, 20:08 »
You have to choose the vacuum thickness such that the physical properties of your interest do not virtually depend on the actual value adopted. So, you have to do a couple of calculations for different thicknesses to figure out what is the thickness that suits your requirement for the accuracy of the calculation.

Regarding the dispersion direction, I guess this is your choice of making a nanoribbon in the direction of your research interest as well as setting  all the other nanoribbon parameters.

1214
General Questions and Answers / Re: TMR
« on: November 13, 2016, 19:55 »
Yes, it does give the TMR value at zero bias in terms of conductance calculated for parallel and anti-parallel magnetization configurations of the ferromagnetic electrodes.

1215
General Questions and Answers / Re: Green-Kubo or autocorrelation function of velocity
« on: November 7, 2016, 20:57 »
To calculate the heat capacity from the MD simulations, you may adopt, for example, the procedure discussed in PHYSICAL REVIEW B 72, 132204 2005. It might also be useful to take a look at the following post http://quantumwise.com/forum/index.php?topic=3359.msg15378#msg15378.

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