Collapse in the nonlocal nonlinear Schrödinger equation

We discuss spatial dynamics and collapse scenarios of localized waves governed by the nonlinear Schrödinger equation with nonlocal nonlinearity. Firstly, we prove that for arbitrary nonsingular attractive nonlocal nonlinear interaction in arbitrary dimension collapse does not occur. Then we study in detail the effect of singular nonlocal kernels in arbitrary dimension using both Lyapunoff's method and virial identities. We find that in the one-dimensional case, i.e. for n = 1, collapse cannot happen for nonlocal nonlinearity. On the other hand, for spatial dimension n >= 2 and singular kernel ~1/r^α, no collapse takes place if α < 2, whereas collapse is possible if α >= 2. Self-similar solutions allow us to find an expression for the critical distance (or time) at which collapse should occur in the particular case of ~1/r² kernels for n = 3. Moreover, different evolution scenarios for the three-dimensional physically relevant case of Bose-Einstein condensates are studied numerically for both the ground state soliton and higher order toroidal states with, and without, an additional local repulsive nonlinear interaction. In particular, we show that the presence of local repulsive nonlinearity can prevent collapse in those cases.