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

Abstract

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 the two-dimensional electron gas. When tuned by hydrostatic pressure, the ν = 5/2 fractional quantum Hall state, believed to be a prototype non-Abelian topological phase, gives way to a quantum Hall nematic phase. Remarkably, this nematic phase develops spontaneously, i.e. in the absence of any externally applied symmetry breaking fields.

Phase transition and intrinsic metric of dipolar fermions in the quantum Hall regime

“…More recently, an exciting possibility of the co-existence of topological order with broken symmetry 18,19 , leading to the "nematic" FQH effect, has also been proposed [20][21][22][23][24] . In this case, the nematic order arises due to spontaneous symmetry breaking, supported by recent numerical calculation 25 and experiments using hydrostatic pressure 26 .…”

Observation of an anomalous density-dependent energy gap of the ν=5/2 fractional quantum Hall state in the low-density regime

“…The closing and re-opening of the energy gap with density was interpreted as a spin transition in the ν = 5/2 FQHS 25 . In another experiment, pressure has drastically altered the ground state at ν = 5/2 near κ 2 from a FQHS to a stripe phase 32 . Since in these experiments an in-situ tuning of the density was not possible, subtle changes in sample properties are virtually impossible to detect.…”

Anisotropy-driven transition from the Moore-Read state to quantum Hall stripes

“…In addition to these theoretical studies, there is good experimental evidence that a modest in-plane field drives the isotropic incompressible state observed in GaAs at ν = 5/2 into a phase with highly anisotropic transport [43][44][45][46][47] , and a recent experiment has even induced a phase transition into an anisotropic phase by tuning isotropic pressure 48 . However, the nature of the resulting anisotropic phase and its connection to the MR state is not fully understood.…”