Dear Artem, you already got a very exhaustive answer, but I like to add couple of remarks as I find these points of interest.
On 04/25/2013 07:14 PM, Artem Baskin wrote: > Dear SIESTA/TRANSIESTA users, > > I have recently read the paper (PRB 81, 205437) and found it very > interesting and enlightening. Since I want to reproduce these results, > I faced with some questions that I want to address here, some of them are > technical > > > whereas others are conceptual. From now on I will refer to the mentioned > above paper and the > materials that I found > http://unam.bilkent.edu.tr/mt2/transiesta/agnr8-transiesta/README.txt > > > 1) What is (are) the particular reason(-s) to choose 8AGNR as > an electrode? In this case, the scattering region is defined in a dummy > way since the only difference between scattering and electrode regions is > > > for which area the charge neutrality is preserved and how many atoms are > treated out of equilibrium. Am I right? You already got an answer on this, but I would say that choosing the same material as electrode makes easier to interpret the results and allow to focus on material properties themselves. If you choose a different contact, for example gold, things can get nasty as you could have a rectifying behavior at the interface, and the contact resistance cn dominate the transport (for example, see PHYSICAL REVIEW B 82, 161413R 2010). However the semi-inifinite ribbon minimizes these kind of effects, therefore it can be a good choice to underline some coherent effects which hold as far as the contact is "good enough". In short, I would choose the same as far as I don't want to model a particular experimental setup precisely, but this is up to the author and it's not a generale rule (also, not always a good idea). Physically, you could also think that you're assuming that the ribbon is long enough that the carriers thermalize somewhere far from the scattering region, but still in the ribbon. [cut] > 6) Due to the specific choice of the electrode for the set up (the middle > part coincides with the leads) one may interpret the transmission spectra > using the band structure of the electrode region only. From that > > > perspective it is unclear for me how one would explain the conductance > profile for the non zero bias. Moreover, what if the electrodes were of > the different material, say, gold or nikel? > As far as I understand in the mentioned paper the device region is strained and the electrode is not, therefore I think those conductance in fig. 1 are more a guide for the following. However, if we focus on the unstrained case, you can interpret the transmission spectra using the band structure of the contacts only as far as linear response holds, otherwise the charge in the device region has a non-equilibrium distribution which differs from the electrodes and the behaviour can be complicated. In this case the author specify that, as the system is uniform, the charge displacement due to non-equilibrium condition is small and you can indeed interpret the transmission using the electrode region only, at least for the first band. If you draw the conductance and valence band edges of the electrodes, you displace them accordingly to the bias and you assume unitary transmission between the two contacts, you will obtain the same plots of fig 1e, which in the paper are probably slightly smeared out because the charge distribution in the scattering region is not exactly the equilibrium one. The plateau at the fermi level should correspond to a valence to conduction band tunnelling, therefore appears for bias larger than the gap. The two small gap at the sides of the plateau are respectively the gap of the left and right contact. The transmission can be assumed unitary because there is no reflection between electrodes and scattering region (not only because they are the same, but also because the charges nearly match!), all the propagating states are transmitted with probability one. If you have a different contact the starting plot will look different and the sentence above is not true, some modes in the electrodes will be scattered. I think you should be still able to work out the transmission curve starting from the electrodes bandstructure and T(E) at equilibrium. However, the non-equilibrium distribution could matter more and if you go far from linear response, you can not work out the transmission 'by hand'. [cut] Best, Gabriele
