Condensed Matter Seminars

The fractional quantum Hall states developing in a two-dimensional electron gas in the second Landau level appear to be different from those of the lowest Landau level and continue to challenge our understanding. The even denominator state at filling factor 5/2 is currently under intense scrutiny since it is believed to arise from an exotic p-wave pairing of composite fermions described by the Pfaffian wavefunction which might support non-Abelian quasiparticles. An equally interesting and related problem is the origin of the odd denominator states of the second Landau level. While at first sight these states could be part of the composite fermion hierarchy, several recent theoretical works suggest that some might be supporting generalized Pfaffian-like correlations. Recent progress in cooling electrons allowed us to observe a new fractional quantum Hall state at the filling factor 2+6/13. We find that energy gaps of the prominent 2+1/3 and 2+2/3 states are consistent with the values predicted by the free composite fermion model. However, the weaker 2+2/5 and 2+6/13 states deviate significantly, suggesting therefore that these states are of exotic origin.