Evidence for an even-denominator fractional quantum Hall state in the ground Landau level of an anisotropic system

Even-denominator fractional quantum Hall states (FQHSs) are widely believed to host a novel class of matter that obey non-Abelian statistics and could be of potential use in topological quantum computing. Examples of such states include the ν = 5/2 FQHS in GaAs two-dimensional (2D) electron systems. Even-denominator FQHSs have also been reported in both 2D electron and 2D hole systems confined to wide GaAs quantum wells, in bilayer GaAs 2D electron systems and, more recently, in 2D electrons in ZnO and in graphene. We report here the observation of an even-denominator FQHS at filling factor ν = 1/2 in a 2D electron system confined to a wide AlAs quantum well where the electrons can occupy multiple conduction-band valleys with an anisotropic effective mass. We observe phase transitions from a compressible Fermi liquid to an incompressible FQHS and then to an insulating phase as we tilt the sample in the magnetic field, thus rendering the electron charge distribution more bilayer-like.