Setting new standards for mobility of 2D holes in GaAs quantum wells*

Modulation-doped two-dimensional (2D) carrier systems hosted in GaAs quantum wells provide nearly ideal testing grounds for exploring exotic low-temperature many-body phenomena. While 2D electron systems have long been at the forefront for exploration of interaction-driven phenomena, 2D hole systems (2DHSs) offer an attractive alternative, owing to their large effective mass, strong and tunable spin-orbit coupling, and complex band-structure. For the first time, in a 2DHS in any material, we obtain peak mobility ≃10 x 10⁶ cm²/Vs at density of only ≃3.8 x 10¹⁰ /cm², at 300 mK which rises to ≃18 x 10⁶ cm²/Vs (implying a mean-free-path ≃57 µm) when measured at 30 mK. We achieve this by optimizing the structure design by systematically adjusting the alloy fraction (x) of the Al_xGa_1-xAs barrier near the quantum well, and the width of the quantum well. Low-temperature magnetotransport data exhibit numerous delicate fractional quantum Hall states which have never been seen before at such low densities in a 2DHS. The achievement of mobilities exceeding 10⁷ cm²/Vs in 2DHSs represents a significant leap, considering that the highest recorded mobilities, until two years ago, were only ≃2 x 10⁶ cm²/Vs. Using transport calculations specific to our structures, we discern the scattering mechanisms that limit the mobility in our new samples.