  We study the existence of ground state solutions for a class of non-linear
pseudo-relativistic Schr\"odinger equations with critical two-body
interactions. Such equations are characterized by a nonlocal
pseudo-differential operator closely related to the square-root of the
Laplacian. We investigate such a problem using variational methods after
transforming the problem to an elliptic equation with a nonlinear Neumann
boundary conditions.
