New results of ultra-low temperature experiments in the second Landau level in a two-dimensional electron system

In the second Landau level around nu = 5/2,filling of an extremely high quality 2D electron system and at temperatures T down to 9 mK we observe a very strong even-denominator fractional quantum Hall effect at Landau level filling nu = 5/2 and its energy gap is large and Delta similar to 0.45K. A clear FQHE state is seen at v = 2 + 2/5, with well-quantized R-xy. A novel, even-denominator FQHE state at nu = 2 + 3/8 seems to develop, as deduced from the T-dependence of dR(xy)/dB. In addition, four fully developed re-entrant integral quantum Hall effect (RIQHE) states are also observed. At low temperatures, the wide RIQHE plateau around at nu = 2 + 2/7 is interrupted by a dip, indicating an additional reentrance. Finally, the tilted magnetic field experiment at an ultra-low temperature of 10 mK was carried out to examine the spin-polarization of the nu = 5/2 FQHE state.