The dielectric constant, also called permittivity
describes the relation
between electric displacement and field strength. For the general case it is a
tensor dependent on the frequency and external influences like magnetic fields.
In isotropic materials, this tensor reduces to a scalar. The permittivity is
usually given as the product of the dimensionless relative permittivity
and the vacuum permittivity
.
Special interest is devoted to the static dielectric constant
and
the high frequency dielectric constant
. While the static dielectric
constant enters Poisson's equation as well as the models for several
scattering mechanisms, both constants are employed in the description of polar
optical scattering.
The static dielectric constant of lead telluride is unusually high and is
dependent on temperature as illustrated in Fig. 5.1.
Experimental data are mostly available for the low temperature range
[198,199,200]. Values for room temperature and higher
are given in [200,194] but are not sufficient to give us a
clear picture of the dependence of
in that range. The low
temperature data from Nishi [198], Tennant
[200], and Dashevsky [201] can
be modeled by a simple power-law
The high-frequency dielectric constant is about
times lower than the
static dielectric constant [194,203].