SrTiO3: An intrinsic quantum paraelectric below 4 K

The dielectric constants ${\epsilon }_{〈110〉}$ and their change with uniaxial $〈1\overline{1}0〉$ stress in monodomain $〈001〉$SrTi${\mathrm{O}}_3$ samples have been measured as a function of temperature. Between 4 K and 0.3 K these quantities are independent of temperature. In one sample $\epsilon \mathrm{}\left(T\right)\mathrm{}$ was measured to 0.035 K and found to be constant. This proves the quantum-mechanical stabilization of the paraelectric phase below 4 K with a very high dielectric constant $\epsilon$. The crossover from classical to quantum behavior on lowering the temperature is discussed, and it is shown that the coupling of the ferroelectric mode to acoustic ones is important. The $\epsilon \mathrm{}\left(T\right)\mathrm{}$ dependence is compared to earlier and recent theories. The former, yielding a coth ($\frac{\hslash \Omega }{\mathrm{kT}}$) term, are found to be inadequate for the crossover region. A recent biquadratic ferroelectric mode-coupling theory fits the data better.