The Quantum Hall Effect of Interacting Electrons in a Periodic Potential
  We consider the influence of an external periodic potential on the fractional quantum Hall effect of two-dimensional interacting electron systems. For many electrons on a torus, we find that the splitting of incompressible ground state degeneracies by a weak external potential diminishes as $\exp ( - L/ \xi)$ at large system size $L$. We present numerical results consistent with a scenario in which $\xi$ diverges at continuous phase transitions from fractional to integer quantum Hall states which occur with increasing external potential strength. 
Daniela Pfannkuche (a,b) and Allan H. MacDonald (a) ((a) Department of   Physics, Indiana University, Bloomington, USA, (b) ...