Observation of a transition from a topologically ordered to a spontaneously broken symmetry phase

Until the late 1980s, phases of matter were understood in terms of Landau's symmetry-breaking theory. Following the discovery of the quantum Hall effect, the introduction of a second class of phases, those with topological order, was necessary. Phase transitions within the first class of phases involve a change in symmetry, whereas those between topological phases require a change in topological order. However, in rare cases, transitions may occur between the two classes, in which the vanishing of the topological order is accompanied by the emergence of a broken symmetry. Here, we report the existence of such a transition in a two-dimensional electron gas hosted by a GaAs/AlGaAs crystal. When tuned by hydrostatic pressure, the nu = 5/2 fractional quantum Hall state, believed to be a prototypical non-Abelian topological phase, gives way to a quantum Hall nematic phase. Remarkably, this nematic phase develops spontaneously, in the absence of any externally applied symmetrybreaking fields.