DISSIPATIVE QUANTUM TUNNELING AND ABSENCE OF QUADRUPOLAR FREEZING IN GLASSY K0.989LI0.011TAO3

Stretched exponential relaxation of stress-induced quadrupolar order is found in K0.989Li0.011TaO3 at temperatures T < 30 K. In contrast to expectations for glasses, however, the stretching exponent increases upon cooling and reaches unity as T --> 0. In parallel with the observed loss of polydispersivity the relaxation time crosses over from Arrhenius to dissipative-quantum-tunneling behavior, tau(T) = tau(0)exp(-alpha-T4) at T < 20 K. The absence of quadrupolar freezing seems to be due to the onset of tunneling and to coupling with the frozen-in dipolar degrees of freedom.