Near-Debye dielectric responses

It is shown on the example of the dielectric data for Er-doped CaF 2 that very slight departures from the true Debye response can be assessed very accurately using the analysis of the real part of the susceptibility. Given adequate precision of measurements, this method is far more sensitive than the plotting of the complex plane Cole-Cole circle or the examination of the loss as a function of frequency. It is shown that a concentration of 1 part Er in 10 4 gives a noticeable departure from the Debye behaviour, while at a concentration of 1 part in 10 5 the susceptibility is so small that insufficient sensitivity remains to enable a meaningful fitting to be obtained. This shows that one cannot move away from many-body interactions by successive dilution of the interacting species, before losing the signal strength in the measuring process.