Hall Conductivity for Charged Strings and Modified Spectrum-Generating   Algebra
  When open strings end with a charge q on a D2-brane, which involves constant background magnetic field B perpendicular to the brane, we calculate the Hall conductivity for these charged strings. We also construct the corresponding spectrum-generating algebra, which assures that our system is ghost-free under some conditions. 
Akira Kokado, Gaku Konisi, and Takesi Saito