Dear David, Currently a Kwant symmetry stores the extra directions that describe how it handle various lattices. If you don't do anything, the choice of the extra directions happens automatically, and a Bravais lattice vector is chosen. However you can manually override this by using the add_site_family method of the TranslationalSymmetry (see http://kwant-project.org/doc/1.0/reference/generated/kwant.lattice.TranslationalSymmetry#kwant.lattice.TranslationalSymmetry.add_site_family for details). I know several people have used this to solve exactly the problem that you currently encounter.
Best, Anton On Thu, Oct 23, 2014 at 4:40 PM, David Abergel <david.aber...@nordita.org> wrote: > Dear all, > > I am having a problem defining a system which matches what I want. > > I start by defining a rectangular graphene lattice of size 0<=x<L in the x > direction and 0<=x<W in the y direction. (By "rectangular" I mean in the > real space coordinates, not the crystallographic coordinates.) > > I want to attach a lead to the left-had end of this rectangle, going to > minus infinity. Therefore, I define a lead with translational symmetry > (-1,0) and the appropriate hopping. I attach the lead and plot the system. > > When I plot the system, I find that the lead has been attached along the > (0,1) crystallographic direction (so, that is along the (1/2, sqrt(3)/3) > real space vector). A triangle of extra sites have been added for x<0 (real > space) so that the total shape of the scattering region is now not > rectangular. > > If I attach another lead with lead.reversed(), a similar thing happens on > the right of the sample so that my scattering region is now a parallelogram. > > As I understand it, this should not affect the physics in any way, since the > lead is semi-infinite. But, if I want to draw pictures, plot functions over > the scattering region, and gain physical understanding, it is a bit of a > pain. So, my question is whether there is any way to make the lead attach > along the (0,1) real space direction (which is the same as the (-1,2) > crystallographic direction) and yet maintain the (-1,0) translational > symmetry? > > If you require a sample program which reproduces this behavior then I can > easily provide that, but I thought I should not extend an already long post > unnecessarily. > > Thanks in advance. > > David >