Messages

991

General Questions and Answers / Re: units in 1d projection of electron density

« on: July 12, 2017, 20:34 »

I do not have currently access to the 2014.2 version, but everything works as described in my previous post in the ATK-2014.3. The units are 1/Angs^3, i.e., this is the electron density that is visualized in the VNL. If I right click, I get an option to export data.

- You may try making the viewer window bigger to better see the units.

- A second option is to upgrade to ATK-2014.3 (I guess it should work with your 2014.2 license), http://www.quantumwise.com/products/version-archive, or to the newest ATK-2017.0 version, http://www.quantumwise.com/products/download#download.

992

General Questions and Answers / Re: Two question about InAS p-i-n junction metal gate workfunction

« on: July 12, 2017, 19:52 »

The definition and actual calculation of metal gate work function is discussed in this tutorial http://docs.quantumwise.com/tutorials/inas_p-i-n_junction/inas_p-i-n_junction.html.

A gate electrode is treated as a continuum (nonatomistic) material  having dielectric properties of metal, i.e., assuming that the dielectric constant is infinite for this continuum material, and the electrostatic potential is constant across the entire metal gate region.

In the ATK, the default value for the gate electrostatic potential defined with respect to the Fermi energy is set to zero, and in the tutorial, the Fermi energy is considered as a reference (zero) energy . However, the Fermi energy and gate potential value defined with respect to the vacuum level are not zero, and the Fermi energy defined with respect to the vacuum level (in the gate region) is called work function (of metal gate). The metal gate treated within the continuum approximation can, in principle, be related to a real atomisic metal with similar work function.

The work function of the metal gate electrode can be tuned by applying a gate voltage that means that we set the gate electrostatic potential (defined with respect to the Fermi level) of the metal gate to a non-zero value, changing the energy difference (=work function) between the vacuum level in the gate region and the Fermi level. This also gives rise to a shift of the semiconductor bands  in the semiconductor channel (under or above the top or bottom gate, respectively) with respect to the Fermi level. This can be seen as a local electrostatic doping of the channel material because either conduction or valence band of the channel semiconductor material gets closer to the Fermi level due to the band structure shift, becoming more occupied by electrons or holes, respectively. 

Note that the Fermi level is fixed by the electrodes and does not shift upon gating the channel region.

993

General Questions and Answers / Re: Wigner-Seitz

« on: July 12, 2017, 18:14 »

The scheme is not directly related to phonon transport calculations. It is based on an approximation that allows for computationally-efficient calculation of dynamical matrix (phonon band structure) for bulk materials. 

994

General Questions and Answers / Re: units in 1d projection of electron density

« on: July 12, 2017, 11:25 »

- Units for electron density is 1/Angs^3, not e/Angs^3. The latter would be true for charge density.

- It must be possible to export an electron density plot. Try doing right-click on the plot viewer to export the plot to a graphical file.

995

General Questions and Answers / Re: Questions about Geometry Optimization of Defective Graphene with Al atoms

« on: July 11, 2017, 11:11 »

I would suggest having a look at http://docs.quantumwise.com/tutorials/builder_manual/builder_manual.html, especially, reading the section on 'Move Tool'.

As a general guideline:

1. Add PtO2 crystal from the Database to the Stash.
2. Convert it from bulk to molecular configuration, using the corresponding plugin in the Builder.
3. Select and delete all the atoms except the O-Pt-O atoms aligned in a row. This is how you get your O-Pt-O molecule.
4. Rotate the row of atoms such that it is aligned along the z-axis, using the Rotate plugin in the Coordinate Tools.
5. Click on the actual graphene structure in the Stash.
6. Drag-and-drop the O-Pt-O molecule from the Stash to this structure to merge the molecule and graphene structures.
7. Select the O-Pt-O atoms.
8. Use Move Tool to position the molecule where you want on the graphene sheet.

The detail information on the Builder plugins is given in the tutorial I have mentioned. 
 

996

General Questions and Answers / Re: Is the IV curve correct?

« on: July 11, 2017, 01:12 »

This depends on the voltage and temperature as both enter the Fermi-Dirac distribution in the equation for the current, see http://docs.quantumwise.com/manuals/NEGF.html.

For example, the energy range is to be exactly given by the chemical potential difference at T=0K, but it needs to be extended upon increasing the temperature to account for the tails of the Fermi-Dirac distribution, which decays exponentially.  So, the energy range does not need to be too huge, but I cannot tell you exact criterion because it also depends on the transmission spectrum.

I can guess that for your particular problem the energy range chosen looks good enough, assuming that the temperature is of 300 K -> 0.025 eV.

997

General Questions and Answers / Re: Is the IV curve correct?

« on: July 10, 2017, 18:44 »

The statement made in the tutorial is correct, in principle. As a rule of thumb, the energy range chosen for calculating the IV curve must be much larger than the chemical potential difference.  If you are in doubt, you need to do a convergence test by increasing the energy range to see how the IV curve changes. Otherwise, the conclusion on the IV curve correctness will be based on a belief, rather than on a calculation.

998

General Questions and Answers / Re: Energy per atom of metal

« on: July 10, 2017, 11:51 »

In the case of Na for example, I add the BCC Na structure from database (i.e. Na(beta)), use 16x16x16 k-points to simulate bulk and as the cell contains just one atom, that should be the total energy/atom (Em) of BCC Na?

Yes. Otherwise, the total energy of an ideal crystal is infinite because it is comprised of an infinite number of atoms.

Also how I get the cohesive energy of Na? Simulate a single na atom and subtract from previous energy/atom(Em)?

I guess so, see https://en.wiktionary.org/wiki/cohesive_energy.

999

General Questions and Answers / Re: Is the IV curve correct?

« on: July 10, 2017, 09:40 »

There is no problem. The blue-colored area just shows you the difference between the chemical potentials in the left and right electrodes for a set of applied voltages, e.g., E_Fermi_left = E_Fermi_right (blue line becomes a point) for V_bias=0 Volt, but the spectral current is defined in the energy range from -infinity to +infinity, in principle. In practice, one always sets upper and lower limits for this energy range. You may take a look at our tutorials and manual on the transport calculations, see http://docs.quantumwise.com/tutorials/atk_transport_calculations/atk_transport_calculations.html,
http://docs.quantumwise.com/manuals/NEGF.html and many other examples at http://docs.quantumwise.com/.

1000

General Questions and Answers / Re: Are there any questions in this py files?

« on: July 10, 2017, 09:00 »

It is not clear what are the calculator settings in the first script. Also, the bias voltage values specified in the two scripts are different.

1001

General Questions and Answers / Re: Energy per atom of metal

« on: July 10, 2017, 08:54 »

You can calculate the total energy using the analysis object 'Total energy' and divide it by the number of atoms to get the total energy per atom.

1002

General Questions and Answers / Re: Questions about Geometry Optimization of Defective Graphene with Al atoms

« on: July 10, 2017, 08:51 »

What is your actual question?

1003

General Questions and Answers / Re: The problem of device NEGF DFT convergence?

« on: July 9, 2017, 22:03 »

Which py file? And why do you need to use ATK-2012 if you have got the version of 2016?

1004

General Questions and Answers / Re: Are there any questions in this py files?

« on: July 8, 2017, 23:31 »

You may use the Initial State in the VNL to start the calculation from a converged solution (old calculation), see http://docs.quantumwise.com/tutorials/initialize_from_a_converged_state/initialize_from_a_converged_state.html.

1005

General Questions and Answers / Re: The problem of device NEGF DFT convergence?

« on: July 8, 2017, 15:05 »

The posted script for the 0.9 V case seems to suggest that you have not used your 0.8 V converged solution as an initial state for the higher bias voltage calculation.  You may try changing that.

As I said previously, you might need to use a more robust Poisson solver for this particular problem, e.g.,  [Parallel] Conjugate gradient available in ATK-2017. What is the ATK version you are using?