Shift current is a second-order nonlinear optical effect that occurs only in materials lacking inversion symmetry. It refers to a direct current generated by photons exciting electrons from a lower to a higher energy band.
In this master's thesis, we study the shift current in a one-dimensional crystal. First, we describe the crystal within Rice-Mele model and analytically calculate the shift current. We extend the previously known analytical solution for the shift current to account for arbitrary atomic positions within the unit cell. The electric field is described using the Peierls substitution. We also compute the shift current numerically. The numerical results show good agreement with the analytical solution. Next, we investigate the influence of atomic orbital shapes on the shift current. To achieve this, we describe the crystal using the Kronig-Penney model, which we analyze within the tight-binding approximation. We ensure the orthogonality of the basis functions using Löwdin’s orthogonalization. The current is computed both directly and through mapping onto the Rice-Mele model. The two approaches yield consistent results, indicating that the influence of atomic orbital shapes on the shift current is negligible.
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