Zero Bias Transmission

[sumitn60]

While calculating transmission at zero bias in a molecular junction device, is it required to consider the whole DFT-NEGF cycle? As per my knowledge, the non-equilibria is the consequence of bias voltage (non-linear regime). At 0 bias (equilibrium), the transmission coefficient can be calculated from the equilibrium part of the density matrices; the non-equilibrium part of the density matrices will be zero. Is it possible to consider only the DFT cycle without NEGF formalism to calculate transmission at 0 bias? However, the DFT-NEGF considers integration over both equilibrium and non-equilibrium density matrices.

Looking forward to your response/comments...

Thanks

Sumit

[Dongzhe]

Hi Sumit,

To calculate the zero-bias T(E), you can either use the wavefunction-matching approach (e.g., PWCOND code) or Green's function-based method (e.g., QuantumATK, TranSIESTA...).

If u use the latter approach, once you have the Hamiltonian and overlap matrices from DFT, you can, in principle, calculate the T(E) without iterating the Green's function (i.e., equilibrium density matrix). However, in most NEGF codes, we do the Green's function iteration anyway for zero-bias because the periodic DFT solution might differ from the NEGF one.

I hope this answers your question.
Best regards,
Dongzhe

[Anders Blom]

There are many examples in the literature showing that just extrapolating the zero-bias transmission spectrum into a I-V curve can be completely different from the real solution, where you converge the SCF at each bias point. The most obvious effect which you cannot capture with the zero-bias approach is negative differential resistance.

See e.g. https://journals.aps.org/prb/abstract/10.1103/PhysRevB.85.184426 and note how different the transmission is as a function of bias. The band edges will often even "follow" the bias window.