Universality and phase diagram around half-filled Landau levels.

Gated GaAs/AlxGaAs1−x heterostructures were used to determine the low temperature behavior of the two-dimensional electron gas near filling factor ν = 1 2 in the disorder-magnetic field plane. We identify a line on which σxy is temperature independent, has value σxy = 0.5±0.02 (e /h), and a distinct line on which ρxy = 2±0.04 (h/e ). The phase boundaries between the Hall insulator and the principal quantum Hall liquids at ν = 1 and 1 3 show levitation of the delocalized states of the first Landau level for electrons and composite fermions. Finally, data suggests that there is no true metallic phase around ν = 1 2 . PACS number(s):73.40.Hm, 71.30.+h, 72.30.+q Typeset using REVTEX