Comment on "Nonlinear band structure in Bose-Einstein condensates: Nonlinear Schrodinger equation with a Kronig-Penney potential"

In their recent paper [Phys. Rev. A 71, 033622 (2005)], Seaman studied Bloch states of the condensate wave function in a Kronig-Penney potential and calculated the band structure. They argued that the effective mass is always positive when a swallowtail energy loop is present in the band structure. In this Comment, we reexamine their argument by actually calculating the effective mass. It is found that there exists a region where the effective mass is negative even when a swallowtail is present. Based on this fact, we discuss the interpretation of swallowtails in terms of superfluidity.