Vortex-antivortex pair in a Bose-Einstein condensate - Type-II quantum lattice gas as a nonlinear φ4 theory of a complex fieldJ. Yepez, G. Vahala and L. VahalaEur. Phys. J. Special Topics, 171 (2009) 9-14DOI: https://doi.org/10.1140/epjst/e2009-01005-y