Gate and Bias potentials
************************

BAND NEGF (Non-Equilibrium Green's Function) Metal wire Gate potential
Bias potential Transmission function Current

See also: DFTB-NEGF manual and BAND-NEGF manual

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

   Start up **AMSinput**
   Switch to **BAND**: [image: ADFPanel][image] **→** [image: BANDPanel][image]
   Select **Task → NEGF**
   Click on [image: MoreBtn][image] 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.** Click on **Fill** and fill the central region with 4 layers of lead material
   **3.** Set the **tips** as shown in the figure. (Select left two atoms and click the + after Left tip, similarly for the right tip.)
   **4.** Set the left and right **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 (System) → From File(s)...**

   [image]

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

   [image]

Tip:

  You can check the regions in the **Model → Regions** panel. You will
  see there the right_tip, left_tip and gate_potential regions. Use
  the checkboxes to see them individually.

We will now apply a **bias potential** of 0.1 Volt, with a **ramp-
potential** from -3.0 to 3.0 Angstrom (see picture below), do not
forget to change the unit to Angstrom, the choices shown by clicking
on the unit:

   [image]

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 0.1
   **2.** Set the bias potential range to -3.0 and 3.0 respectively

   [image]

we are now ready to run the calculation:

   Click on **File → Save** and save it as **gate_bias**
   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 around "-0.00321"

   [image]

Tip:

  You can also extract this information using amsreport: "amsreport
  gate_bias.results/transport.results/band.rkf "NEGF%current""

Warning:

  The *Current vs Bias* plot you can visualize with AMSspectra 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.
