Hi Jonathan,

In the documentation it is written thjat the tight binding approximation is good for all quantum states with a wave length considerably larger than the lattice constant a. Now, if you are dealing with a system made out of a single material, then you can just choose a to be such that this is true, and make sure that the system is still solvable in a reasonable amount of time. But what if I am dealing with a heterostructure, in which one system has a much smaller wavelength? Say I put a metal on top of a semiconductor. Here you would need a much finer grid for the metal than for the semiconductor, but using that same grid for the semiconductor makes no sense computationally. Is this something that Kwant can deal with? Can I make a device that has a1 inside region 1 and a2 inside region 2? I would asssume the boundary layer is problematic, as you have different lattice constants, effective masses, and so on.

This is not something that Kwant will deal with for you; you will have to decide what the best way is to model your system. The choice will depend on a number of factors that will probably be pretty specific to exactly what behaviour you are trying to model. This is why stuff like this is not implemented directly in Kwant, and probably never will be. That being said, there are probably members of the Kwant community who have addressed similar problems, and who could tell us what they did. I would imagine that the way to go would be to treat the leads as a self-energy, rather than directly constructing them as tight-binding models; this would mean that you would not have to finely "mesh" the lead. All of the information about what exactly happens at the interface is then in the self energy, so the problem is then just shifted to coming up with the right self energy. I would imagine that there is a reasonable literature on constructing such things, but I'm not an expert on this topic. Happy Kwanting, Joe