Acoustic measurements of the stripe and the bubble quantum Hall phase

M E Msall,W Dietsche

doi:10.1088/1367-2630/17/4/043042

Abstract

We launch surface acoustic waves (SAW) along both the and the 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’ direction sense a compressible behavior (finite resistance) which is seen in standard transport measurement only if current flows along the ‘hard’ 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.

Highlights

The physics of a two-dimensional electron system (2DES) in high magnetic fields is dominated by the interactions between the charges
We find an enhanced resistivity if the surface acoustic waves (SAW) propagates along the ‘easy’ 〈110〉 direction while standard resistance measurements show the the enhancement when current flows along the perpendicularly oriented ‘hard’ 〈11 ̄0〉 direction
The anisotropy measured with standard resistance measurements is qualitatively reflected in the SAW, if one deduces the σxx value relevant for the SAW from the resistivity ρyy perpendicular to the respective SAW propagation direction

Summary

Introduction

The physics of a two-dimensional electron system (2DES) in high magnetic fields is dominated by the interactions between the charges. Interactions lead to the correlated states of the fractional quantum Hall effect [1] but can lead to a spatial modulation of the electron density into geometrical patterns or domains. In the regime of the integer quantum Hall effect, interactions lead to quantum Hall ferromagnetism [6, 7] when Landau levels with different spin-polarization cross. SAW provide a new window into the composition of the quantum Hall stripe and bubble phases in the filling-factor region 4–5. Our experiment uses pairs of bifrequency transducers to measure the SAW propagation along straight line paths through the bulk of the 2DES along two principle axes at very low temperatures (≈10 mK). In the bubble phase our new SAW data yield a finite conductivity while standard transport data show the vanishing conductivity of a quantum Hall effect

The stripe and the bubble phase

SAW and resistivity measurements

SAW and the bubble phase

Findings

Conclusions