The quantum Hall effect is manifested in a two dimensional electron gas under strong perpendicular magnetic field, with current flowing along the edges of the samples (chiral edge modes).  In the fractional quantum Hall regime, for some fractional states (the so called ‘hole-conjugate’ states), chiral neutral modes (not carrying current) are expected to flow along the edge - either in the opposite direction to current flow (up-stream) or in the same direction (down-stream). Recently Bid et. al. [1] demonstrated a method for detection of up-stream neutral modes. Bid et. al. showed that when an up-stream neutral mode impinges upon a quantum point contact constriction, the neutral quasi-particles may be stochastically fragmented into charge carriers, which in turn can be detected using shot noise measurement techniques despite the fact they carry zero average charge current.

We apply this method to investigate neutral modes in fractional quantum Hall states at the first excited Landau level, and focus on the ν=5/2 and ν=8/3 fractional states.  Our measurements reveal that both the ν=5/2 and the ν=8/3 state have a chiral up-stream neutral mode.  Moreover, we investigate the details of the noise that is being generated when the neutral mode impinges on a quantum point contact constriction, and in particular study its dependence on the transmission probability of current impinging on the constriction. In the case of the ν=5/2 state, these results may indicate that the wave function describing this state is one of the proposed non-abelian wave functions and not an abelian one. In the case of the ν=8/3 these results indicate that this state is an abelian state similar to the ν=2/3 or ν=5/3 states, and not the non abelian Read-Rezayi k=4 state [2].  In order to strengthen this conclusion we performed also charge measurements by partitioning the current with the same constriction - resulting with backscattered charges e/3 - as expected in the abelian case.  We performed also similar measurements at the abelian ν=5/3 state – with similar results.

1. Aveek Bid et. al, Observation of neutral modes in the fractional quantum Hall regime, Nature 466, 585 (2010)

2. N. Read, E. Rezayi, Beyond paired quantum Hall states: parafermions and incompressible states in the first excited Landau level, Phys. Rev. B. 59, 8084 (1999)