  We study Coulomb interactions in the finite bias response of Mach-Zehnder
interferometers, which exploit copropagating edge states in the integer quantum
Hall effect. Here, interactions are particularly important since the coherent
coupling of edge channels is due to a resonant mechanism that is spoiled by
inelastic processes. We find that interactions yield a saturation, as a
function of bias voltage, of the period-averaged interferometer current, which
gives rise to unusual features, such as negative differential conductance,
enhancement of the visibility of the current, and nonbounded or even diverging
visibility of the differential conductance.
