Composite fermions mass in an anisotropic two-dimensional electron system

Recent advances in molecular beam epitaxy capabilities have stablished a new record in the quality of AlAs two-dimensional electron systems (2DESs). This breakthrough allows us to study the temperature dependence of fractional quantum Hall states up to ν = 7/15 and 7/13 in the extreme quantum limit. We report on the energy gaps and Shubnikov-de Haas mass (m_CF) of composite fermions in samples with density n = 1.04 x 10¹¹ cm^-2 and quantum well widths of 35 and 50 nm. The measured m_CF are 0.68 and 1.2 (in units of free electron mass) for the 35 and 50 nm wide quantum well near ν = ½, respectively. We contrast our results with the previously reported energy gaps and m_CF of 2DESs in GaAs and ZnO, and the 2D hole system in GaAs. Our experiments on AlAs 2DESs demonstrate the important role of electron layer thickness in determining the energy gaps and m_CF.