  We study spin correlation in a double quantum dot containing a few electrons
in each dot (around 10). Clear current rectification with negative differential
conductance is observed in the cotunneling regime, which is well explained by a
Pauli spin blockade. The current feature is discussed in connection with
exchange singlet-triplet splitting based on two simple models, one of which
takes coherent interdot coupling and the other incoherent interdot coupling
into account.
