Flow of S-matrix poles for elementary quantum potentials
  The poles of the quantum scattering matrix (S-matrix) in the complex momentum plane have been studied extensively. Bound states give rise to S-matrix poles, and other poles correspond to non-normalizable anti-bound, resonance and anti-resonance states. They describe important physics, but their locations can be difficult to find. In pioneering work, Nussenzveig performed the analysis for a square well/wall, and plotted the flow of the poles as the potential depth/height varied. More than fifty years later, however, little has been done in the way of direct generalization of those results. We point out that today we can find such poles easily and efficiently, using numerical techniques and widely available software. We study the poles of the scattering matrix for the simplest piece-wise flat potentials, with one and two adjacent (non-zero) pieces. For the finite well/wall the flow of the poles as a function of the depth/height recovers the results of Nussenzveig. We then analyze the flow for a potential with two independent parts that can be attractive or repulsive, the two-piece potential. These examples provide some insight into the complicated behavior of the resonance, anti-resonance and anti-bound poles. 
B. Belchev, S.G. Neale, M.A. Walton
quant-ph