- How is the closed loop evolution formed in solids?
This is a paragraph from Rev. Mod. Phys,82,1959(2010). From the article, I understand that Berry phase is gauge invariant only for closed loop evolution but what exactly is this evolution? Does it mean that the system initially start out with some Hamiltonian and I continuously change the Hamiltonian by somehow affecting the parameter of the Hamiltonian? How do I guarantee I go back to the initial Hamiltonian to form the close loop? How is it that by simply applying an electric field to the system I can get a close loop evolution?