Advanced Solid State Physics

Figure 103: Response of a mass spring system

with

ω 0 =

√

k

m

Fig. 103 shows an expected response of a mass spring system. At low frequencies (below the resis-
tant frequency) the mass will move in phase, at much higher frequencies the system will oscillate
out of phase. The Fourier transform of the impulse response function is the generalized
susceptibility.

13.1.1 Dielectric Response of Insulators

Fig. 104 shows the real (doted line) and the imaginary part of a dielectric response of an insulator.
The peak of the imaginary part indicates energy losses at a particular frequency.

Calculation of the dielectric constant:

 = 1 +χ = 1 +

P

 0 E

(215)

with

P = nex

The+1in eqn. (215) shifts the response function in the positive direction, which means that in
comparison to fig. 104 the dielectric constant of insulators goes to+1instead to zero at higher