  We investigate thermoelectric efficiency of systems with broken time reversal
symmetry under a three-terminal transport. Using a model of Aharonov-Bohm
interferometer formed with three noninteracting quantum dots, we show that
Carnot efficiency can be achieved when the thermopower is a symmetric function
of the applied magnetic field. On the other hand, the maximal value of the
efficiency at maximum power is obtained for asymmetric thermopower. Indeed, we
show that Curzon-Ahlborn limit is exceeded within the linear response regime in
our model. Moreover, we investigate thermoelectric efficiency for random
Hamiltonians drawn from the Gaussian Unitary Ensemble and for a more abstract
transmission model. In this latter model we find that the efficiency is
improved using sharp energy-dependent transmission functions.
