Correlated states in ultra-high-quality two-dimensional electron systems.

Two-dimensional electron systems (2DESs) under a strong, perpendicular magnetic
field exhibit a plethora of correlated phenomena like the fractional quantum Hall (FQH) effect
and Wigner crystallization. Advancements in the quality of these 2DESs have led to the
realization of fragile, exotic states that show unique, topological characteristics. Recent
developments in molecular-beam-epitaxy have enabled the growth of ultra-high-quality
GaAs/AlGaAs quantum wells, producing 2DESs with mobilities exceeding 50 million; a
substantial enhancement in quality from the previous generation high-mobility 2DESs by a
factor of 2. More importantly, improvements in low-density-2DESs have allowed us to probe the
physics at very small Landau-level filling factors within achievable laboratory magnetic fields.
In this talk, I will focus on the new physics we see at these very low fillings. In particular, we see
new, fragile FQH phases at fillings where the magnetically induced Wigner crystal dominates.
These emerge as competing FQH states amidst the strong insulating background of the Wigner
crystal. Transport measurements in the Wigner crystal phase show evidence of the quantum
nature, and is consistent with the theoretically predicted correlated composite-fermion crystal.
Additional non-linear I-V and spectral noise measurements indicate the presence of several
transport regimes, revealing a rich dynamic phase diagram of driven Wigner crystals.