Competing quantum Hall phases in the second Landau level in the low-density limit

In this Rapid Communication we present the results from two high-quality, low-density GaAs quantum wells. In sample A of electron density n = 5.0 x 10(10) cm(-2), anisotropic electronic transport behavior was observed at nu = 7/2 in the second Landau level. We believe that the anisotropy is due to the large Landau level mixing effect in this sample. In sample B of density 4.1 x 10(10) cm(-2), strong 8/3, 5/2, and 7/3 fractional quantum Hall states were observed. Furthermore, our energy gap data suggest that, similar to the 8/3 state, the 5/2 state may also be spin unpolarized in the low-density limit. The results from both samples show that the strong electron-electron interactions and a large Landau level mixing effect play an import role in the competing ground states in the second Landau level.