  Using an isospin-dependent transport model, we study isospin effects on
two-nucleon correlation functions in heavy-ion collisions induced by
neutron-rich nuclei at intermediate energies. We find that these correlation
functions are sensitive to the density dependence of nuclear symmetry energy,
but not to the incompressibility of symmetric nuclear matter and the medium
dependence of nucleon-nucleon cross sections. This sensitivity is mainly due to
effects of nuclear symmetry energy on the emission times of neutrons and
protons as well as their relative emission sequence. We also study the
variations of the symmetry energy effects on nucleon-nucleon correlations with
respect to the impact parameter, incident energy, and mass number of heavy ion
collisions.
