The effect known as ferroelectricity arises when forces between polarizable ions in a solid produce a spontaneous displacement of these ions which results in a lattice polarization below some characteristic (Curie) temperature. Fluctuations in this polarization may be thermally induced as in the case of classical ferroelectrics, or if the Curie temperature is near O K, the fluctuations can be due to quantum-mechanical zero-point motion. The term "quantum ferroelectric" is applied to those systems where fluctuations in the polarization result from the zero-point motion. Experimental determinations of variations in the dielectric constant, spontaneous polarization, and elastic compliance as a function of temperature and impurity concentration are reported for

K_{1 - x} {Na}_{x} Ta O_{3}

and

K {Ta}_{1 - y} {Nb}_{y} O_{3}

, and these results show that the physical properties of quantum ferroelectrics differ from those of classical ferroelectrics in the following ways: First, for a quantum ferroelectric, the transition temperature depends on impurity concentration (i.e., on an effective order parameter) as

T_{c} \propto {(x - x_{c})}^{\frac{1}{2}}

, as opposed to

T_{c} \propto (x - x_{c})

for the classical case. Second, the inverse dielectric constant varies with temperature as

ε^{- 1} \propto T^{2}

for the quantum-mechanical case, instead of

ε^{- 1} \propto T

. Finally, the distribution of transition temperatures in a given macroscopic sample with a Gaussian impurity concentration distribution is

p (T_{c}) \propto T_{c} \exp (- α T_{c}^{4})

for the quantum ferroelectric, as opposed to a Gaussian for the classical situation. These results are in agreement with previous theoretical predictions of some of the distinguishing characteristics of quantum ferroelectricity.

Publications with shared reference:

0 publications found