Upstream neutral modes in the fractional Quantum Hall regime

Counter propagating (upstream) chiral neutral edge modes, which were predicted to be present in hole-conjugate states, were observed recently in a variety of fractional Quantum Hall States, by measuring charge noise that resulted after partitioning the neutral mode by a constriction. The observation of such modes raised new questions regarding their microscopical nature and the interplay between the charge and neutral edge modes.  Moreover, it opened a new way to probe the wave function of the fractional Quantum Hall states at the first excited Landau level, where some states are predicted to be of non-abelian nature.

In this talk, I will present this emerging new field. First I will focus on measurements performed at the ubiquitous n=2/3, establishing our measurement techniques and revealing new details on the properties of the neutral modes.  Secondly, I will present detailed investigation of the neutral modes at the first excited Landau level, demonstrating the existence of an upstream neutral mode in the n=8/3 and n=5/2 states and excluding its existence at the n=7/3 state.  These results supports the non-abelian, anti-pfaffian to be the n=5/2 wave function.