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 ${\mathbf{B}}_{\mathit{t}}$ 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×${10}^{10}$ ${\mathrm{cm}}^{\mathrm{-}2}$, 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 ${\mathit{B}}_{\mathit{t}}$ exhibit minima that are accompanied in transport by splitting of the FQHE. For 2/3 the minimum in Δ(${\mathit{B}}_{\mathit{t}}$) is sharp and deep, with Δ reduced by 70%. With ${\mathit{B}}_{\mathit{t}}$ well above its value at the minimum, Δ(${\mathit{B}}_{\mathit{t}}$) for ν=2/3 is linear, with slope ≊g${\mathrm{\mu }}_{\mathit{B}}$ 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 ${\mathrm{\rho }}_{\mathit{x}\mathit{x}}$ 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.

• Received 26 July 1991

DOI:https://doi.org/10.1103/PhysRevB.45.3418