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Resonance of polarisation mechanisms in alternating fields
Below its resonance frequency, the ionic mode of polarisation
oscillates as seen previously.
When the frequency of the applied field is equal to the
resonance frequency of the ionic polarisation mechanism, resonance occurs
and the amplitude of the oscillations increases. This causes an increase
in the polarisation and hence in the dielectric constant.
If the frequency is raised further, to above the resonance
frequency of the mechanism, then as we have seen before the ionic mechanism
can no longer keep up with the field switching and ceases to contribute
to polarisation, leading to a drop in the dielectric constant.
A similar thing happens for the electronic mode of polarisation.