Spin polarization of the ν=12/5 fractional quantum Hall state

We have carried out tilt magnetic field (B) studies of the ν=12/5 fractional quantum Hall state in an ultrahigh-quality GaAs quantum well specimen. Its diagonal magnetoresistance R _xx shows a nonmonotonic dependence on tilt angle (θ). It first increases sharply with increasing θ, reaches a maximal value of ∼70 Ω at θ ≈ 14°, and then decreases at higher tilt angles. Correlated with this dependence of R _xx on θ, the 12/5 activation energy (Δ _12/5) also shows a nonmonotonic tilt dependence. Δ _12/5 first decreases with increasing θ. Around θ=14, Δ _12/5 disappears as R _xx becomes nonactivated. 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.