Frequency doubling of Bloch oscillations for interacting electrons in a static electric field

We study numerically the effect of the on-site Hubbard interaction U on the dynamics of two electrons subjected to an external electric field and restricted to move in a linear chain with open boundaries. We solve the time-dependent Schrodinger equation to follow the time evolution of an initially localized two-electron state. For electrons initially far apart, the wave packet develops Bloch oscillations whose characteristic frequency is in agreement with a semiclassical calculation. For initially close electrons in a singlet state, a frequency doubling sets up, which is more pronounced for intermediate couplings. We discuss this effect by revealing the opposite trends the electron-electron coupling produces on the wave-packet components corresponding to bounded and unbounded states.