Dissipative quantum tunneling and absence of quadrupolar freezing in glassy ${\mathrm{K}}_{0.989}$${\mathrm{Li}}_{0.011}$${\mathrm{TaO}}_3$

Stretched exponential relaxation of stress-induced quadrupolar order is found in ${\mathrm{K}}_{0.989}$ ${\mathrm{Li}}_{0.011}$ ${\mathrm{TaO}}_3$ 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, τ(T)=τ(0)exp(-${\mathit{aT}}^4$) 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.