For a Hamiltonian with degeneracies due to a conservation law, the scattering states will in general not have a definite value of the conservation law. In your case, Kwant returns scattering states that are arbitrary linear combinations of spin up and down, so it is not possible to label the amplitudes in the scattering matrix by spin.
However, in Kwant 1.3 a feature will be added that allows for the construction of scattering states with definite values of a conservation law. See here https://firstname.lastname@example.org/msg01025.html for an explanation of the basic idea behind the algorithm.
We're currently working on implementing this feature in Kwant itself. The good news is that we're practically done - here https://gitlab.kwant-project.org/anton-akhmerov/kwant is a link to a git repo with a functioning implementation. After you clone the repo, check out the branch cons_laws_combined, which contains a version of Kwant with conservation laws implemented. This http://nbviewer.jupyter.org/urls/dl.dropbox.com/s/9fr8pq65ujmtr20/symm_modes.ipynb notebook contains a simple example to illustrate how to work with conservation laws and the scattering matrix.
I invite you and anyone else who is interested to give it a try. We'd appreciate any feedback!
In your case specifically, there would be two projectors in the new implementation - P0 which projects out the spin up block, and P1 that projects out the spin down block. If they are specified in this order, then the spin up and down blocks in the Hamiltonian have block indices 0 and 1, respectively. In the new implementation, it is possible to ask for subblocks of the scattering matrix relating not only any two leads, but also any two conservation law blocks in any leads. To get the reflection amplitude of an incident spin up electron from lead 0 into an outgoing spin down electron in lead 0, you could simply do smat.submatrix((0, 1), (0, 0)). Here, the arguments are tuples of indices (lead index, block index).
Best regards, Tómas
On Fri, Jan 6, 2017 at 3:46 PM, Camilla Espedal email@example.com wrote:
This question is basically the same as this: https://www.mail-archive.com/ firstname.lastname@example.org/msg00076.html
I want to calculate some things using the scattering matrix. I started out with a very simple system, most basic two-terminal system. For some energy there is one propagating mode. I now add matrix structure to the mix (just multiply by s_0 everywhere) and there are now 2 propagating modes (which makes sense).
Now, if I look at the reflection coefficients for lead 0 by using submatrix(0,0), it is now a 2x2 matrix after I introduced the matrices. How are the elements ordered? Is it
[[r_upup, r_updown],[r_downup, r_downdown]]
I know that I could make two lattices, but since I do not plan to use the other functions such as transmission. I just want the smatrix.
Hope you can help me, and thanks in advance.