Interacting Bilayer Electron System

Abstract

This thesis summarizes the explorations of interacting bilayer electron systems which provide a versatile, extended platform to study electron-electron interaction beyond single layers. By setting each layer at a different density, the bilayer electron system hosts different electronic phases in each layer; moreover, we can study the role of interaction between these phases when the two layers are closely spaced.

We first present a new technique and experimental results directly probing the magnetic-field-induced Wigner crystal (WC) through its interaction with another many-body phase which is comprised of composite fermions (CFs). We measure the magnetoresistance of a bilayer electron system where one layer has a very low density and is in the WC regime, while the other (probe) layer is near half Landau level filling and hosts a sea of CFs. The data exhibit commensurability oscillations in the magnetoresistance of the CF layer, induced by the periodic potential of WC electrons in the other layer, and provide a unique, direct glimpse at the symmetry of the WC, its lattice constant, and melting. They also demonstrate a striking example of how one can probe the many-body state of two-dimensional electrons using another many-body state with exotic quasiparticles.

We also report experiments demonstrating that the layer densities of an asymmetric bilayer electron system oscillate as a function of perpendicular magnetic field that quantizes the energy levels. At intermediate fields, this interlayer charge transfer can be well explained by the alignment of the Landau levels in the two layers. At the highest fields where both layers reach the extreme quantum limit, however, there is an anomalous, enhanced charge transfer to the majority layer. Surprisingly, when the minority layer becomes extremely dilute, this charge transfer slows down as the electrons in the minority layer condense into a WC. Furthermore, by examining the quantum capacitance of the dilute layer at high fields, the screening induced by the CFs in an adjacent layer is unveiled. The results highlight the influence of strong interaction in interlayer charge transfer in the regime of very high fields and low Landau level filling factors.

By further scrutinizing the details of the interlayer charge transfer as a function of magnetic field and its termination as mentioned above, we are able to measure the critical filling factor (CFF) below which a magnetic-field-induced, quantum WC forms in a dilute, two-dimensional electron layer when a second, high-density electron layer is present in close proximity. The data reveal that the WC forms at a significantly smaller CFF compared to the CFF (≃ 0.20) in single-layer two-dimensional electron systems. The measured CFF exhibits a strong dependence on the interlayer distance, reflecting the interaction and screening from the adjacent, high-density layer.