Two-dimensional loosely and tightly bound solitons in optical lattices and inverted traps

TL;DR

This paper investigates 2D Bose-Einstein condensate solitons in optical lattices, revealing negative effective mass states confined by inverted traps, their stability, dynamics, and a transition to tightly bound states as atom number increases.

Contribution

It introduces the concept of negative-mass loosely bound solitons in 2D BECs with optical lattices and describes their stability, dynamics, and transition to tightly bound states.

Findings

Negative effective mass solitons are confined by inverted traps.

Loosely bound solitons are stable and can move freely on optical lattices.

Transition from loosely bound to tightly bound solitons occurs at specific atom numbers.

Abstract

We study the dynamics of nonlinear localized excitations (solitons) in two-dimensional (2D) Bose-Einstein condensates (BECs) with repulsive interactions, loaded into an optical lattice (OL), which is combined with an external parabolic potential. First, we demonstrate analytically that a broad (loosely bound, LB) soliton state, based on a 2D Bloch function near the edge of the Brillouin zone (BZ), has a negative effective mass (while the mass of a localized state is positive near the BZ center). The negative-mass soliton cannot be held by the usual trap, but it is safely confined by an inverted parabolic potential (anti-trap). Direct simulations demonstrate that the LB solitons (including the ones with intrinsic vorticity) are stable and can freely move on top of the OL. The frequency of elliptic motion of the LB-soliton's center in the anti-trapping potential is very close to the analytical prediction which treats the solition as a quasi-particle. In addition, the LB soliton of the vortex type features real rotation around its center. We also find an abrupt transition, which occurs with the increase of the number of atoms, from the negative-mass LB states to tightly bound (TB) solitons. An estimate demonstrates that, for the zero-vorticity states, the transition occurs when the number of atoms attains a critical number N=10^3, while for the vortex the transition takes place at N=5x10^3 atoms. The positive-mass LB states constructed near the BZ center (including vortices) can move freely too. The effects predicted for BECs also apply to optical spatial solitons in bulk photonic crystals.