Screening properties of bubble phases in excited Landau levels

Bubble phases, electron solids that possibly contain more than one electron per site, form in very high quality (low-disorder) two-dimensional (2D) electron systems in the excited Landau levels. At the lowest temperatures, magneto-transport traces exhibit a vanishing longitudinal resistance at certain filling factors, implying insulating states associated with the bubble phases. As the temperature is raised, a peak in resistance is observed, which is interpreted to signal the melting of the bubble phase. Other techniques such as nonlinear transport, microwave spectroscopy, surface acoustic wave propagation, and thermo-power measurements support the picture of an electron solid. By employing a capacitance technique and using very high-quality GaAs 2D electron systems, we report an unexplored bulk property of the bubble phases, namely, their screening efficiency, measured as their response to a perpendicular electric field. We find that the bulk of the 2D electron system does not screen the electric field and behaves as an insulator at the lowest temperatures. As the temperature is raised, the screening reaches a local maximum close to the temperature at which the resistance shows a peak. These observations are consistent with the electron solid interpretation of the bubble phases.