Fractional quantum Hall effect at ν=2/3 and 3/5 in tilted magnetic fields

We report on measurements of electron transport for the fractional quantum Hall effect (FQHE) at filling factors {ν}=2/3 and 3/5, in magnetic fields Bₜ tilted by angles {θ} with respect to the normal to the sample plane. Our device was prepared at an electron density of only 2.4×{}10¹⁰ cm^-2, but still exhibited a well-developed FQHE at {ν}=2/3 and 3/5. This exceptionally low density allowed us to access very low total fields, where the spin is less likely to be completely polarized. For many tilt angles, we obtained gap energies {Δ} from the temperature dependence of the diagonal conductivity on the FQHE minima. For both 2/3 and 3/5, plots of {Δ} versus Bₜ exhibit minima that are accompanied in transport by splitting of the FQHE. For 2/3 the minimum in {Δ}(Bₜ) is sharp and deep, with {Δ} reduced by 70%. With Bₜ well above its value at the minimum, {Δ}(Bₜ) for {ν}=2/3 is linear, with slope {}gμB for GaAs, indicating an increase in the two-dimensional electron-system Zeeman energy on excitation. We present a detailed survey of the evolution of the splitting of the FQHE with angle, and find that local ρₓₓ minima that are shifted up to 6% upfield of {ν}=2/3 at {θ}{}23^∘{} evolve continuously into an unsplit FQHE at {ν}=2/3 at {θ}{}0^∘{}. The split and shifted FQHE's that we observe are interpreted as effects of phase separation associated with ground-state spin transitions.