Acoustic measurements of the stripe and the bubble quantum Hall phase

Abstract

We launch surface acoustic waves (SAW) along both the 〈 〉 110 and the 〈 〉 110 directions of a Hall bar and measure the anisotropic conductivity in a high purity GaAs two-dimensional electron system in the quantum Hall regime of the stripe and the bubble phases. In the anisotropic stripe phase, SAW propagating along the 'easy' 〈 〉 110 direction sense a compressible behavior (finite resistance) which is seen in standard transport measurement only if current flows along the 'hard' 〈 〉 110 direction. In the isotropic bubble phase, the SAW data show compressible behavior in both directions, in marked contrast to the incompressible quantum Hall behavior seen in transport measurements. These results challenge models that assume that both the stripe and the bubble phase consist of incompressible domains and raise important questions about the role of domain boundaries in SAW propagation.

Onset of quantum criticality in the topological-to-nematic transition in a two-dimensional electron gas at filling factor ν=5/2

“…The anisotropic magnetoresistance we observe at ν = 5/2 and P = 8.71 kbar are identical in all aspects to that of the prototype stripe phase forming at ν = 9/2 and at other filling factors [27][28][29][30][31][32][33][34][35][36][37] . Indeed, anisotropy at both of these filling factors develops in the absence of the application of any in-plane B-field and in a very limited range of filling factors of width ∆ν ≈ 0.15 around the half-integer value 26,[30][31][32][33][34][35][36][37] . In the absence of an energy gap in the density of states, the stripe phase at ν = 5/2 is compressible [30][31][32][33][34][35][36][37] .…”

“…While we do not observe any obvious signatures of density gradients in our sample 60 , it is possible that small variations around the mean pressure result in small density fluctuations which may also influence the magnetoresistance. In contrast to the behavior of R xx and R yy taken above 40 mK, R xx and R yy sharply deviate one from another at lower temperatures [30][31][32][33] . As seen in Fig.2b, the R xx /R yy ratio of the resistances in the two different crystallographic directions exceeds three orders of magnitude at the lowest temperatures.…”

“…Our recent measurements 26 revealed that the ground state at ν = 5/2 undergoes a pressure-driven quantum phase transition from the FQHS to a stripe phase [27][28][29][30][31][32][33][34][35][36][37] . In this experiment the two-dimensional electron gas was not exposed to an in-plane magnetic field or any other externally applied symmetry breaking fields 26 .…”

“…This FQHS, similarly to all other FQHSs 3,4 , is topologically ordered, it is believed to belong to the Pfaffian universality class [5][6][7][8][9][10][11][12][13][14][15][16][17] , and it is thought to support non-Abelian excitations 5 . Within the framework of the composite fermion theory 18,19 , the ν = 5/2 FQHS can be understood as being due to pairing of composite fermions, a pairing driven by the residual attractive interactions between the composite fermions 5,20-25 .Our recent measurements 26 revealed that the ground state at ν = 5/2 undergoes a pressure-driven quantum phase transition from the FQHS to a stripe phase [27][28][29][30][31][32][33][34][35][36][37] . In this experiment the two-dimensional electron gas was not exposed to an in-plane magnetic field or any other externally applied symmetry breaking fields 26 .…”

“…Indeed, anisotropy at both of these filling factors develops in the absence of the application of any in-plane B-field and in a very limited range of filling factors of width ∆ν ≈ 0.15 around the half-integer value 26,[30][31][32][33][34][35][36][37] . In the absence of an energy gap in the density of states, the stripe phase at ν = 5/2 is compressible [30][31][32][33][34][35][36][37] . The stripe phase we observe at ν = 5/2 is likely related to nematic phases developing in the two-dimensional electron gas but under different experimental conditions, specifically with the application of an in-plane B-field.…”