Spectrum-Generating Algebra for Charged Strings
  When an open string ends with charges on a D2-brane, which involves constant background magnetic field perpendicular to the brane, we construct the spectrum-generating algebra for this charged string, which assures that our system is ghost-free under some conditions. The application to the Hall effect for charged strings is also shortly remarked. 
Akira Kokado, Gaku Konisi and Takesi Saito