New Phase Transition between Partially and Fully Polarized Quantum Hall States with Charge and Spin Gaps at $\mathit{\nu }=\frac{2}{3}$

The average electron spin polarization $P$ of a two-dimensional electron gas confined in GaAs/GaAlAs multiple quantum wells was measured by NMR near the fractional quantum Hall state with filling factor $\nu =\frac{2}{3}$. Above this filling factor ( $\frac{2}{3}\le \nu <0.85\mathrm{}$), a strong depolarization is observed corresponding to two spin flips per additional flux quantum. The most remarkable behavior of the polarization is observed at $\nu =\frac{2}{3}$, where a quantum phase transition from a partially polarized ( $P\approx \frac{3}{4}$) to a fully polarized ( $P=1\mathrm{}$) state can be driven by increasing the ratio between the Zeeman and the Coulomb energy above a critical value ${\eta }_{\mathrm{c}}=\frac{{\Delta }_{\mathrm{Z}}}{{\Delta }_{\mathrm{C}}}=0.0185\mathrm{}$.