Kohn condition and exotic Newton-Hooke symmetry in the non-commutative   Landau problem
  $N$ "exotic" [alias non-commutative] particles with masses $m_a$, charges $e_a$ and non-commutative parameters $\theta_a$, moving in a uniform magnetic field $B$, separate into center-of-mass and internal motions if Kohn's condition $e_a/m_a=\const$ is supplemented with $e_a\theta_a=\const.$ Then the center-of-mass behaves as a single exotic particle carrying the total mass and charge of the system, $M$ and $e$, and a suitably defined non-commutative parameter $\Theta$. For vanishing electric field off the critical case $e\Theta B\neq1$, the particles perform the usual cyclotronic motion with modified but equal frequency. The system is symmetric under suitable time-dependent translations which span a (4+2)- parameter centrally extended subgroup of the "exotic" [i.e., two-parameter centrally extended] Newton-Hooke group. In the critical case $B=B_c=(e\Theta)^{-1}$ the system is frozen into a static "crystal" configuration. Adding a constant electric field, all particles perform, collectively, a cyclotronic motion combined with a drift perpendicular to the electric field when $e\Theta B\neq1$. For $B=B_c$ the cyclotronic motion is eliminated and all particles move, collectively, following the Hall law. Our time-dependent symmetries are reduced to the (2+1)-parameter Heisenberg group of centrally-extended translations. 
P-M. Zhang and P. A. Horvathy
hep-th cond-mat.str-el math-ph math.MP