Origin of Pinning Disorder in Magnetic-Field-Induced Wigner Solids

Area of Science:

• Condensed Matter Physics
• Materials Science

Background:

• Two-dimensional electron systems (2DES) in GaAs exhibit Wigner solid phases at low Landau level filling factors (ν).
• These Wigner solids are pinned by disorder, leading to a measurable pinning mode frequency.
• The precise origin of this pinning disorder has remained elusive despite decades of research.

Purpose of the Study:

• To investigate the impact of quantum well width on the pinning mode frequency of Wigner solids in ultralow-disorder GaAs.
• To identify the source of disorder responsible for pinning Wigner solids and influencing their characteristic frequencies.

Main Methods:

• Studied a series of GaAs quantum wells with varying widths (d) but otherwise similar ultralow disorder characteristics.
• Measured the pinning mode resonance frequencies (f_{p}) in the low-ν Wigner solid phases.
• Analyzed the relationship between well width and pinning mode frequency.

Main Results:

• Pinning mode frequencies (f_{p}) showed a strong decrease with increasing quantum well width (d).
• The widest quantum wells exhibited pinning frequencies as low as 35 MHz.
• The observed reduction in f_{p} with increasing d was well-explained by considering wave function tails impinging into alloy-disordered barriers.

Conclusions:

• The study identifies wave function overlap with alloy-disordered Al_{x}Ga_{1-x}As barriers as the primary source of disorder pinning Wigner solids.
• Electron Coulomb repulsion in the growth direction is crucial for accurately modeling wave function confinement and predicting pinning frequencies.
• This finding provides a critical understanding of Wigner solid behavior in 2D electron systems and their interaction with disorder.