Acoustic Measurements of the Stripe and the Bubble Quantum Hall Phases

TL;DR

This study uses surface acoustic waves to probe anisotropic conductivity in quantum Hall stripe and bubble phases, revealing compressible behavior that challenges existing models of incompressibility in these phases.

Contribution

It provides the first acoustic measurements of the stripe and bubble phases, highlighting anisotropic and isotropic compressible behaviors that question current theoretical assumptions.

Findings

Stripe phase shows anisotropic compressibility with SAW along the easy direction.

Bubble phase exhibits isotropic compressibility in both directions.

Results challenge models assuming incompressible domains in these phases.

Abstract

We launch surface acoustic waves (SAW) along both the $<110>$ and the $<1\bar{1}0>$ directions of a Hall bar and measure the anisotropic conductivity in a high purity GaAs 2-D 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" $<1\bar{1}0>$ 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.