We measure the chemical potential jump across the fractional gap in the low-temperature limit in the two-dimensional electron system of $\mathrm{GaAs}/\mathrm{AlGaAs}$ single heterojunctions. In the fully spin-polarized regime, the gap for filling factor $\nu =1/3$ increases linearly with the magnetic field and is coincident with that for $\nu =2/3$, reflecting the electron-hole symmetry in the spin-split Landau level. In low magnetic fields, at the ground-state spin transition for $\nu =2/3$, a correlated behavior of the $\nu =1/3$ and $\nu =2/3$ gaps is observed.

• Received 16 February 2007

DOI:https://doi.org/10.1103/PhysRevLett.99.086802