A note on the spectrum of a two-particle Rashba Hamiltonian

In a series of recent papers it was shown that, when the attractive s-wave interaction is dominant, the spin-orbit coupled fermions form a bound state. Attributed to a convenient momentum representation, it became a common condition of agreement to express the bound state as a function of the center-of-mass momentum Q. In this letter we prove that the bound state of Rashba fermions does not depend on the chosen representation. That is, all the states characterized by nonzero Q fail to obey the translation symmetry.