  Singlet-triplet spin blockade in a few-electron lateral double quantum dot is
investigated using simultaneous transport and charge-sensing measurements.
Transport from the (1,1) to the (0,2) electron occupancy states is strongly
suppressed relative to the opposite bias [(0,2) to (1,1)]. At large bias, spin
blockade ceases as the (0,2) triplet state enters the transport window, giving
a direct measure of exchange splitting as a function of magnetic field. A
simple model for current and steady-state charge distribution in spin-blockade
conditions is developed and found to be in excellent agreement with experiment.
