I am trying to calculate some berry curvature (BC) in a 2D lattice and I have some things I am getting lost with.
In the 2D lattice, we set up the eigenvalue problem $H|u_1\rangle = \epsilon_i|u_i\rangle$. Numerically, I can find $\epsilon,u$ by diagonalizing some matrix. This gives my energy bands E(qx,qy).
When I want to calculate the BC, the formula is $$\Omega_i(q)=i\left(\langle \partial_{qx}u_i|\partial_{qy}u_i \rangle-\langle \partial_{qy}u_i|\partial_{qx}u_i \rangle\right)$$
I am kind of confused as to what $u_i$ I should be using. I had just been using $u_i=E_i$, so taking the derivatives of the energy. Should I be using the eigenvectors. If I assume bloch waves, then I know there is the $e^{iqr}$ factor, but I want the derivatives in k space, so I am just not even sure what some pseudo code would look like.
As an aside, I am following this thesis which does the Harper model for a flux of 1/5.
So the prodedure is:
for each qx,qy:
- Find E(q), $\vec{u(q)}$ from here I just dont know what to use for u.