Lieb's soliton-like excitations in harmonic traps

We study the solitonic Lieb II branch of excitations in the one-dimensional Bose gas in homogeneous and trapped geometry. Using Bethe-ansatz Lieb's equations we calculate the effective number of atoms and the effective mass of the excitation. The equations of motion of the excitation are defined by the ratio of these quantities. The frequency of oscillations of the excitation in a harmonic trap is calculated. It changes continuously from its soliton-like value omega(h) / root 2 in the high-density mean-field regime to omega(h) in the low-density Tonks-Girardeau regime with omega(h) the frequency of the harmonic trapping. Particular attention is paid to the effective mass of a soliton with velocity near the speed of sound. Copyright (C) EPLA, 2013