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Messages - Petr Khomyakov

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Which version of QuantumATK are you using? Could you post your script, log-file, and transmission spectrum image related to this calculation?

The band structure should not look the same for a repeated unit cell, since there is band folding for the repeated unit cell, because the Brillouin zone for the repeated unit cell is smaller than that for the original unit cell, meaning that all the bands in the original Brillouin zone has to fold accordingly.  You may take a look at any textbook on Solid State Physics (e.g., by Ashcroft & Mermin) to understand how the Brillouin zone and related band structure folds when going to a repeated unit cell.

Questions and Answers / Re: Spin transport
« on: August 31, 2020, 12:24 »
I am not sure I understand what zero dimensional magnetism is, as magnetic order occurs due to exchange interaction among atoms. If you mean whether one can calculate a single Fe atom in QuantumATK, then the answer is yes, just place an Fe atom in a large-enough unit cell. One may use periodic boundary conditions for that, or Dirichlet ones to avoid spurious interaction with periodic images of the Fe atoms.

Questions and Answers / Re: Spin transport
« on: August 24, 2020, 23:31 »
In QuantumATK, U parameter is a free parameter. U-parameter(s) is set for a given element and its orbital(s), so I think you may use U parameter value for a known material containing the corresponding element.

One thing that I have noticed that the device is virtually undoped because compensation charge per atom is too small. For 2D material, one should set doping in units of charge per atom, not charge per volume.

The best way is to decide on doping concentration in units of charge per area as it would be normally defined in experimental work, and then recalculate it to charge per atom, and set it in the GUI or in the script directly. In the GUI, there is an option to charge units from charge per volume to charge per atom.

Regarding work function, one does not need to set it explicitly, as it is implicitly defined/computed for given potentials of source, drain and gate electrodes. If one uses the IVCharacteristics study object, then source potential is set to zero by default, and bias voltage V_ds is essentially drain potential. The gate potentials are then defined with respect to zero potential of the source electrode. For example, if gate potential is zero and V_ds=0 too, then work function of the metal gate = -E_Fermi, i.e., given by the Fermi energy of the system, which is computed and outputted into the log file after QuantumATK device calculation is finished.

If one uses IVCurve  analysis object, then the source and drain potentials, as well as the gate potentials, can be set separately with respect to the common zero-energy reference level of the system. 


Questions and Answers / Re: Spin transport
« on: August 19, 2020, 21:51 »
Fe|MgO|Fe tunnel junction is one of the most studied TMJ system in Spintronics. You may take a look at the original study by Butler et al cited in the tutorial, as well as the papers on this TMJ that cite the Butler's work. You may find them, e.g., through Google Scholar or Web of Science, or directly on the Publisher website.

Questions and Answers / Re: Spin transport
« on: August 19, 2020, 00:33 »
What would be the reason to use it for Fe in this electron transport study?

Questions and Answers / Re: Calculation of doping concentration
« on: August 18, 2020, 10:25 »
I am not sure I fully understood your question. One may give doping concentration in charge per carbon atom. In this definition, there is no ambiguity. If one wants charge per area, then just compute the total charge in a 2D unit cell and divide by its area. 

Questions and Answers / Re: Periodic Boundary Conditions
« on: August 10, 2020, 14:11 »
To avoid using periodic boundary conditions, you may adopt Dirichlet-Dirichlet (or Neumann-Neumann) boundary conditions in the out-of-plane z-direction, or mixed Dirichlet-Neumann boundary conditions if the 2D system of study is asymmetric (no mirror symmetry) in z-direction. In this case, one may avoid an artificial dipole formation due to this asymmetry.

Questions and Answers / Re: Calculation of doping concentration
« on: August 10, 2020, 13:58 »
In principle, it is up to you. I would probably avoid doping hydrogen atoms used for passivation of nanoribbon edges, as it looks a bit artificial.

This feature for using Grimme's correction in the PW calculator will become available in the coming release of QuantumATK.

In the Calculator, search for Numerical Accuracy -> Broadening. You should then set Broadening to 300 K. This is the electronic temperature mentioned in the tutorial.

Questions and Answers / Re: Electron Density
« on: July 13, 2020, 10:32 »
It means numerical noise around zero; 10^-22 is a pretty small number, below machine precision.

You may make visible or invisible selected bands directly in the Band structure analyzer; just go to Edit, i.e., click on the Edit button to start Plot Editor, and then go to Band structure -> Bands. In the Bands, you may particular bands invisible by deselecting Visible in Parameters.

Note that all bands are set to Visible by default, and then you can make invisible those bands that you do not want to see in the band structure plot. Also, you can also save your plot in a hdf5-file to view and edit it in the Plot viewer plugin later, if needed. There are many things you can change in your plot using the Plot Editor.

It is a bug related to the use of Neumann-Neumann boundary conditions in the transport (C)-direction. That will be fixed in the coming release. Could you try using Dirichlet-Dirichlet boundary conditions? That should not create this issue.

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