title: Dynamics of vortex quadrupoles in nonrotating trapped Bose-Einstein condensates.
abstract: Dynamics of vortex clusters is essential for understanding diverse superfluid phenomena. In this paper, we examine the dynamics of vortex quadrupoles in a trapped two-dimensional (2D) Bose-Einstein condensate. We find that the movement of these vortex-clusters fall into three distinct regimes which are fully described by the radial positions of the vortices in a 2D isotropic harmonic trap, or by the major radius (minor radius) of the elliptical equipotential lines decided by the vortex positions in a 2D anisotropic harmonic trap. In the "recombination" and "exchange" regimes the quadrupole structure maintains, while the vortices annihilate each other permanently in the "annihilation" regime. We find that the mechanism of the charge flipping in the "exchange" regime and the disappearance of the quadrupole structure in the "annihilation" regime are both through an intermediate state where two vortex dipoles connected through a soliton ring. We give the parameter ranges for these three regimes in coordinate space for a specific initial configuration and phase diagram of the vortex positions with respect to the Thomas-Fermi radius of the condensate. We show that the results are also applicable to systems with quantum fluctuations for the short-time evolution.
authors.affiliation: Institute of Modern Physics, Northwest University, Xi'an, 710069, China.|Shaanxi Key Laboratory for Theoretical Physics Frontiers, Xi'an, 710069, China.
authors.forename: Tao
authors.initials: T
authors.lastname: Yang
authors.affiliation: Institute of Modern Physics, Northwest University, Xi'an, 710069, China.
authors.forename: Zhi-Qiang
authors.initials: ZQ
authors.lastname: Hu
authors.affiliation: School of Physics, Northwest University, Xi'an, 710069, China.
authors.forename: Shan
authors.initials: S
authors.lastname: Zou
authors.affiliation: Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.
authors.forename: Wu-Ming
authors.initials: WM
authors.lastname: Liu