Gate and Bias potentials

In this tutorial we will show how to include a Gate and Bias potentials in NEGF calculations.

Start up ADFInput
Switch to BAND: ADFPanel BANDPanel
Select Relativity → Scalar (important for heavy elements)
Select Task → NEGF
Click on ... to go to the NEGF panel (or click on Model → NEGF)

Download the lead and molecules .xyz files:

Click here to download the lead file Li_lead.xyz
Click here to download the molecule CuAg.xyz

Note

The system studied in this tutorial is just a toy system.

Set up the system:

1. Select the file Li_lead.xyz as Lead
2. Fill the central region with 4 layers of lead material
3. Set the tips as shown in the figure
4. Set the right and left lead offsets to -8.5 and 8.5 respectively
5. Select Method → self-consistent
Import the file CuAg.xyz by clicking on File → Import Coordinates...
../_images/negf_t5_system_set_up.png

To apply a gate potential of 0.1 Volts for the CuAg molecule:

1. Select the CuAg molecule
2. Click on the + next to Potential region
3. Set the gate potential to 0.1 V
../_images/negf_t5_gate.png

We will now apply a bias potential of 1 Volt, with a ramp-potential from -3.0 to 3.0 Bohr (see picture below):

Note

You can only apply a bias potential for the self-consistent and self-consistent + align NEGF methods

1. Set the bias potential Voltage to 1.0
2. Set the bias potential XMin and XMax to -3.0 and 3.0 respectively
../_images/negf_t5_bias.png

we are now ready to run the calculation:

Click on File → Save
Run the calculation with File → Run
Wait for the calculation to finish

The current (in atomic units) is saved on the binary .rkf file.

Click on SCM → KFBrowser
Activate Expert mode by clicking on File → Expert mode
Click on File → Related Files → band.rkf
Look for the NEGF section
Click on the small arrow next to the NEGF section
The current should be “-0.004158”
../_images/negf_t5_kfbrowser.png

Tip

You can also extract this information using adfreport: adfreport band.rkf "NEGF%current"

Warning

The Current vs Bias plot you can visualize with ADFSpectra is computed from a the transmission function at fixed bias, and it is therefore just an approximation of the real Current vs Bias characteristic. If you want to compute the actual Current vs Bias characteristic you have to run multiple calculations using different bias potentials.