Spin Polarization of the 12/5 Fractional Quantum Hall Effect

Abstract

We have carried out tilt magnetic field (B) studies of the ν=12/5 fractional quantum Hall state in an ultra-high quality GaAs quantum well specimen. Its diagonal magneto-resistance Rxx shows a non-monotonic dependence on tilt angle (θ). It first increases sharply with increasing θ, reaches a maximal value of ~ 70 ohms at θ~ 14^o, and then decreases at higher tilt angles. Correlated with this dependence of Rxx on θ, the 12/5 activation energy (Δ_{12/5}) also shows a non-monotonic tilt dependence. Δ_{12/5} first decreases with increasing θ. Around θ= 14^{o}, Δ_{12/5} disappears as Rxx becomes non-activated. With further increasing tilt angles, Δ_{12/5} reemerges and increases with θ. This tilt B dependence at ν=12/5 is strikingly different from that of the well-documented 5/2 state and calls for more investigations on the nature of its ground state.