Isospin effects on rotational flow in intermediate energy heavy ion collisions

Within the framework of an isospin-dependent quantum molecular dynamics model, the rotational flow in reactions of $^{58}$Fe + $^{58}$Fe and $^{58}$% Ni + $^{58}$Ni at 40 MeV/nucleon for different impact parameters is investigated by analyzing the mid-rapidity azimuthal distribution. The rotational observables are also calculated semiquantitatively. For the first time, it is found that the more neutron-rich system ($^{58}$Fe + $^{58}$Fe) exhibits stronger rotational collective flow. This isospin dependence of rotational collective flow is more appreciable in semi-peripheral collisions and it is shown to mainly result from the isospin dependence of nucleon-nucleon cross section rather than the symmetry energy. Meanwhile, it is indicated that the rotational flow depends strongly on the impact parameter.