Comment on "Fock-Darwin States of Dirac Electrons in Graphene-Based   Artificial Atoms"
  Chen, Apalkov, and Chakraborty (Phys. Rev. Lett. 98, 186803 (2007)) have computed Fock-Darwin levels of a graphene dot by including only basis states with energies larger than or equal to zero. We show that their results violate the Hellman-Feynman theorem. A correct treatment must include both positive and negative energy basis states. Additional basis states lead to new energy levels in the optical spectrum and anticrossings between optical transition lines. 
P. S. Park, S. C. Kim, and S.-R. Eric Yang
cond-mat.mes-hall