Current-Induced Anisotropy and Reordering of the Electron Liquid-Crystal Phases in a Two-Dimensional Electron System

The correlated phases in a two-dimensional electron system with a high index partially filled Landau level are studied in transport under nonequilibrium conditions by imposing a dc-current drive. At filling $1/4$ and $3/4$ of these Landau levels, where the charge density wave picture predicts an isotropic bubble phase, the dc drive induces anisotropic transport behavior consistent with stripe order. The easy axis of the emerging anisotropic phase is perpendicular to the drive. At half filling the anisotropic stripe phase is stabilized by the dc drive provided drive and easy-axis directions coincide.

Figure 1

$R$ as a function of $\nu$ and $I_{\mathrm{dc}}$ for the four $(I_{\mathrm{dc}},I_{\mathrm{ac}})$ configurations. These current configurations are drawn schematically to the right of the data panels. Black lines mark the orientation of the CDW stripe phase at zero dc current, and blue lines illustrate the current channeling and spreading for the injected ac current. In the data panels, white contours enclose regions of negative $R$. Cartoons of the equilibrium phases in the CDW and ELC models are drawn in the relevant filling regions at the top. The top right inset helps to identify the important features in data of (a) and (d), which provide evidence for the stabilizing influence of a dc current when aligned along the easy direction at filling $9/2$. The easy axis is marked by a set of parallel white lines. The bottom right inset highlights those features in (c) and (b), which represent evidence for anisotropic transport at $I_{\mathrm{dc}}>40\mathrm{nA}$ and filling 4.75. The easy axis is aligned perpendicular to the dc-current drive direction.

Figure 2

Color renditions of $R$ for the current configurations of Figs. 1a (left panels) and 1d (right panels) and samples of different sizes. The rhombs with red borders at fillings $\nu =4.25$ and 4.75 in the left panels scale with the side length, whereas the red area centered around $9/2$ does not. The top right panel illustrates line traces of the differential resistance $R$ when sweeping $I_{\mathrm{dc}}$ up or down (solid and dashed lines) for a $90\mu \mathrm{m}$ sample (different heterostructure as the other panels) at some fillings. Hysteresis is observed around $\nu =4.25$.