Band Flattening and Landau Level Merging in Strongly-Correlated Two-Dimensional Electron Systems

TL;DR

This paper reviews experimental evidence of band flattening and Landau level merging in strongly-correlated 2D electron systems, revealing new insights into electron interactions and quantum level behavior under strong magnetic fields.

Contribution

It presents novel experimental observations of spectrum flattening and Landau level merging in high-quality 2D electron systems, highlighting interaction effects at the Fermi level.

Findings

Effective electron mass increases at the Fermi level with density

Observation of Landau level merging in magnetic fields

Indications of quantum level merging of composite fermions

Abstract

We review recent experimental results indicating the band flattening and Landau level merging at the chemical potential in strongly-correlated two-dimensional (2D) electron systems. In ultra-clean, strongly interacting 2D electron system in SiGe/Si/SiGe quantum wells, the effective electron mass at the Fermi level monotonically increases in the entire range of electron densities, while the energy-averaged mass saturates at low densities. The qualitatively different behavior of the two masses reveals a precursor to the interaction-induced single-particle spectrum flattening at the chemical potential in this electron system, in which case the fermion "condensation" at the Fermi level occurs in a range of momenta, unlike the condensation of bosons. In strong magnetic fields, perpendicular to the 2D electron layer, a similar effect of different fillings of quantum levels at the chemical potential -- the merging of the spin- and valley-split Landau levels at the chemical potential -- is observed in Si inversion layers and bilayer 2D electron system in GaAs. Indication of merging of the quantum levels of composite fermions with different valley indices is also reported in ultra-clean SiGe/Si/SiGe quantum wells.