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

We present here the results from two high quality, low density GaAs quantum wells. In sample A of electron density n $=$ 5.0 x 10$^{\mathrm{10}}$ cm$^{\mathrm{-2}}$, anisotropic electronic transport behavior was observed at $\nu =$7/2 in the second Landau level. We believe that the anisotropy is due to the large Landau level mixing effect in this sample. In sample B of density 4.1 x 10$^{\mathrm{10}}$ cm$^{\mathrm{-2}}$, strong 8/3, 5/2, and 7/3 fractional quantum Hall states were observed. Furthermore, our energy gap data obtained in various samples of different densities suggest that the 5/2 state may 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. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.