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Chapter V Dielectric Study of ……
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occurring at multiple frequencies. The dielectric function (ω) must have poles
for frequencies with positive imaginary parts and, therefore, satisfies the
Kramers- Kronig relations. At a given frequency, the imaginary part (”) of the
permittivity leads to absorption loss or gain depending on its sign; leads to
absorption loss, if its value is positive and leads to gain, if its value is
negative. More commonly, the imaginary part of the eigen values of the
anisotropic dielectric tensor should be considered.
5.4.2 Classification of Materials
Materials can be classified as per their permittivity.
(1) Materials with negative real part of permittivity.
Metals are usually having negative real part of permittivity and
propagation of electromagnetic waves exists.
(2) Materials with positive real part of permittivity
Dielectric materials exhibit this nature.
Distinction is often made for semiconductors and dielectrics. A non-
polar material is having no permanent dipoles, for example, silicon,
germanium and carbon (diamond). The III-V compounds such as GaAs, InSb
and GaP share their valance electrons in such a manner that the ions forming
the lattice tend to be positive (group-V) or negative (group- III). Hence, the
lattice is a mass of permanent dipoles whose moment can be changed by
applied field. There are compounds, such as hydrocarbons (C6H6 and
paraffin) having permanent dipole arrangements but surprisingly zero net
dipole moment. There are molecules like water and many transformer oils that
have permanent dipole moments and the total dipole moment is determined
by their orientational polarizability.
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