High frequency (ac) conductivity in the single quantum well AlGaAs/GaAs/AlGaAs with high mobility was investigated by contactless acoustic methods in the fractional quantum Hall effect regime in perpendicular and tilted magnetic fields. We studied the dependence of ac conductivity $sigma^{ac}=sigma_1 - isigma_2$ on both the temperature and magnetic field tilt angle. Tilting the magnetic field relative to the sample surface enabled us to change the position of the conductivity oscillation minimum at $ u$=2/3. We measured the temperature dependence of ac conductivity for each tilt angle and for the 2/3 state we calculated the activation energy $Delta E$ which was derived by constructing the Arrhenius plot ln $sigma_1$ against 1/$T$. Analyzing behavior of the activation energy in total magnetic field for the filling factor 2/3 we observed a distinct minimum which can be interpreted as the spin unpolarized-polarized phase transition.
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