  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
$\sigma^{ac}=\sigma_1 - i\sigma_2$ 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 $\nu$=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.
