Calculating energy shifts in terms of phase shifts

Details Calculating energy shifts in terms of phase shifts Calculating energy shifts in terms of phase shifts

2009-11-15

arxiv.org/abs/0911.2853v2

J. Phys. B: At. Mol. Opt. Phys. 44 (2011) 195207

Physics

Condensed Matter

Quantum Gases

7 pages, 2 figures, NORDITA-2009-72, small changes in language at several places

Scientific paper

10.1088/0953-4075/44/19/195207

To clarify the relation of energy shifts to scattering phase shifts in one-body and many-body problems, we examine their relation in a number of different situations. We derive, for a particle in a container of arbitrary shape with a short-range scattering center, a general result for the energy eigenvalues in terms of the s-wave scattering phase shift and the eigenstates in the absence of the scatterer. We show that, while the energy shifts for a spherical container are proportional to the phase shift over large ranges, those for a cubic container have a more complicated behavior. We connect our result to the description of energy shifts in terms of the scattering T-matrix. The general relation is extended to problems of particles in traps with smoothly varying potentials, including, e.g., the interaction of a small neutral atom with a Rydberg atom. We then consider the many-body problem for particles with a two-body interaction and show that the free energy change due to the interaction is proportional to an average of a generalized phase shift that includes the effects of the medium. Finally, we discuss why, even though individual energy levels are very sensitive to boundary conditions, the energy of a many-body system is not.

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