Phys. Rev. A 68 035602 (2003)We investigate the stability of dark solitons (DSs) in an effectively one-dimensional Bose-Einstein condensate in the presence of the magnetic parabolic trap and an optical lattice (OL). The analysis is based on both the full Gross-Pitaevskii equation and its tight-binding approximation counterpart (discrete nonlinear Schrödinger equation). We find that DSs are subject to weak instabilities with an onset of instability mainly governed by the period and amplitude of the OL. The instability, if present, sets in at large times and it is characterized by quasi-periodic oscillations of the DS about the minimum of the parabolic trap. Apart from the mean-field description via the GrossPitaevskii (GP) equation [8][9][10][11][12][13], a BEC trapped in a strong OL may be described, in the tight-binding limit, by the discrete nonlinear Schrödinger (DNLS) equation [16]. This approximation is not always accurate, but its applicability can be systematically examined [17]. In cases where such a reduction is possible (e.g., when the chemical potential is much lower than the height of the potential barriers induced by the OL), the DNLS model is particularly relevant and has been successfully applied in many instances (for a recent review on DNLS see, e.g., [18] and references therein).In this paper, we study DSs in repulsive BECs (i.e., positive-scattering-length collisions) in the presence of OLs. We use both the continuous-GP and DNLS equations. In particular, we assume a cigar-shaped BEC, which can be described by the following normalized quasi-one-dimensional GP equation [1,19,20],Here, u(x, t) is the mean-field wave function, while the terms in the square brackets represent the external magnetic trap and the OL potential, respectively, with the strengths k and V 0 , while λ is the wavelength of the interference pattern created by the laser beams.To study the dynamics of a DS in the framework of Eq. (1), we consider an initial condition similar to an ansatz proposed for the description of DSs in BECs in Ref.[21]where u TF = max(0, µ − kx 2 ) is the Thomas-Fermi (TF) expression for the background wave-function distribution [1] (µ is the chemical potential) and x 0 is the initial location of the DS's center. In most cases, we set x 0 = 0, i.e., the dark soliton is placed at the bottom of the magnetic trap.In the tight-binding limit, Eq. (1) reduces to the following DNLS equation [16],where the dot denotes time derivative, n is the latticesite index, and C is the so-called coupling constant (see e.g., Refs. [16,17] for exact expressions and relevant estimates). An initial condition for a DS in the case of Eq. (3) can be given by a straightforward discretization of the continuum ansatz (2). Typically, simulations were run for a lattice with 200 sites, and free boundary conditions were used for both the continuum and discrete models. In fact, it has been verified that the results are insensitive to the choice of boundary conditions. Note that DSs in discrete lattices (in the absence of a parabolic tr...