  We study sharp peaks in the conductance-voltage characteristics of a double
quantum dot and a quantum dot spin-valve that are located around zero bias. The
peaks share similarities with a Kondo peak but can be clearly distinguished, in
particular as they occur at high temperatures. The underlying physical
mechanism is a strong current suppression that is quenched in bias-voltage
dependent ways by exchange interactions. Our theoretical results are based on
the quantum master equation methodology, including the Born-Markov
approximation and a numerically exact, hierarchical scheme, which we extend
here to the spin-valve case. The comparison of exact and approximate results
allows us to reveal the underlying physical mechanisms, the role of first-,
second- and beyond-second-order processes and the robustness of the effect.
