Observation of a Transition in the $ν=5/2$ Fractional Quantum Hall State at Large Landau Level Mixing

We have studied the $\nu=5/2$ fractional quantum Hall state in the regime of extremely low densities, for which the Landau level mixing parameter $\kappa$ spans the so far unexplored $2.52<\kappa<2.82$ range. In the vicinity of $\kappa = 2.6$ an unexpectedly large change in the density dependence of the energy gap is observed which is suggestive of a transition in the $\nu=5/2$ state. Origins of such a transition are discussed, including the possibility of a topological phase transition.