Hi Patrik,
I was trying to get up/down conductance through the system from one end to the other, similar to the conductance from lead 0 to lead 1.
What would be the equivalent of conductance from lead 0 to lead 1 (in the 'if trans'), but for either up or down spin?
Ah, this is a new feature introduced in Kwant 1.3! The way to do this is to first tell Kwant that your leads have a conservation law associated with them, and to tell Kwant the basis that you want to use for the modes in the lead. Without this information the modes Kwant calculates in the leads will be an arbitrary superposition of spin up and spin down, and you will not easily be able to separate these. You provide this information with the 'conservation_law' parameter when creating the Builder for your leads. This parameter should be a Hermitian matrix with integer eigenvalues, who's eigenvectors you want to take as the basis for the modes. The modes will be ordered by their eigenvalues. This is explained in the docs[1]; the example given is for superconductivity, but the principle is the same. For your case a reasonable choice would be sigma_z: sym = kwant.TranslationalSymmetry([0, 0, -a]) dn_lead = kwant.Builder(sym, conservation_law=-sigma_z) … There is a minus sign in front of the sigma_z so that the "up" modes come first, and then the "down" modes. Then, when computing the transmission, instead of saying smatrix.transmission(1, 0) which will give you the total transmission from lead 0 to lead 1, you can say smatrix.transmission((1, 0), (0, 0)) to get the transmission from the spin up block (block 0) of lead 0 to the spin up block of lead 1. The first number in each pair is the lead number, and the second is the block index with respect to the conservation law you defined when creating the Builder (0 being spin up, and 1 being spin down in this case). Similarly you can use smatrix.transmission((1, 1), (0, 1)) for transmission from spin down to spin down, or any other combination to calculate the transmissions between different spin blocks. Hope that helps, Joe [1]: https://kwant-project.org/doc/1/tutorial/superconductors