TranSIESTA with external electric field

Asked by Tommaso Civitarese on 2020-10-01

Dear Nick,
I want to ask you about the usage of applied external field using TranSIESTA.
I am studying hydrogen passivated zig-zag graphene nanoribbons for spintronics applications: it is demonstrated that this kind of nanoribbons, if you consider spin degree of freedom in the calculations, are semiconductor with a band gap depending on their width. It is also demonstrated that if you apply an external electric field of a certain value, perpendicular to the direction where the nanoribbons elongate, they have half-metallic behaviour, so the spin-up manteins a semiconductor behaviour while the spin-down gets a metallic behaviour. Now, I want to use this characteristics for my studies, using these nanoribbons as electrodes, but I don't know if I am using TranSIESTA in the right way: the electrodes in TranSIESTA must to be metallic but if I perform spin polarized calculations with external field, only one spin is metallic, the other not.
So, my questions are three:
1) if I perform spin polarized TranSIESTA calculations with an applied external fiel, should I consider these electrodes metallic or not?
2) Assuming that I can consider this type of electrodes as metallic (therefore consistent with the model), so I perform tbtrans calculations with spin polarization using the TSHS matrix got out from TranSIESTA: should I eventually consider only results with spin-down (the metallic one) or I can include also the ones with spin-up?
3) I want to apply external field only in creating the TSHS matrix of the electrodes, infact the goal is to make these metallic nanoribbons so they can be used as electrodes but I don't want to apply an external electric field to the contact region: can I do this?
Thank you very much!

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Nick Papior (nickpapior) said : #1

1) They are metallic, Ef is well-defined by the spin-part that is metallic. The requirement is that the potential is well-defined, since Ef is, so is the potential.

2) both are valid

3) No, this is not viable. You want the electrodes to be extensions in the device region, so if you apply an E-field in the electrodes, it means you also need the E-field in the electrode region of the device. To make this work you need
A) to have a long screening region between the electrode and the scattering region (central non E-field part)
B) manually input an external E-field in separate regions of the device, this may be done by TS.Poisson by using a manually created file (see details in the manual).
Note that you have to have some smoothing region so you don't have abrupt changes in the potential.
C) then carefully do convergence analysis with this method. It ain't easy I think.
D) You can't trust energies by doing the above, and I would be very careful if you do bias calculations (if they are even physical in this setup)

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