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QuantumATK => General Questions and Answers => Topic started by: hol28 on July 29, 2012, 17:33

Title: Puzzled about the transmission change when bias voltage is added?
Post by: hol28 on July 29, 2012, 17:33

Hi everyone,
I am puzzled about one issue in the two-probe system. When I add voltages of -0.5V and +0.5V to the left and right electrodes, respectively, the Fermi level of the left electrode is shifted up and  the right is shifted down. When calculating the transmission, I think it is more easier for the electrons to transfer from left to right, which should lead to a larger transmission at the Fermi level. But in my calculation, the value is decreased. On the other hand, when I add a voltage of +0.5V and -0.5V to the left and right electrodes, respectively, the value is increased. I'm puzzled by the results. Could anyone help me? Thanks!

Title: Re: Puzzled about the transmission change when bias voltage is added?
Post by: mads.engelund on July 30, 2012, 10:56

There is no reason to believe that the transmission at the Fermi level should go up when bias is applied. The total current goes up because more energies contribute to the current, but the transmission at a specific energy (for instance the Fermi level) can go either up or down and will mostly not change much.

I would recommend reading the NEGF chapter in the manual to get a better feel for what the code does.
http://www.quantumwise.com/documents/manuals/latest/ReferenceManual/index.html/chap.negf.html (http://www.quantumwise.com/documents/manuals/latest/ReferenceManual/index.html/chap.negf.html)

Title: Re: Puzzled about the transmission change when bias voltage is added?
Post by: Anders Blom on July 30, 2012, 12:34

And in fact even the current can go down as the bias goes up, an effect known as negative differential resistance.

Title: Re: Puzzled about the transmission change when bias voltage is added?
Post by: hol28 on July 30, 2012, 15:48

Thanks for your advice!