High Frequency Conductivity of EuSe and EuO

Abstract

The a.c. conductivity of EuSe and EuO has been measured in the frequency range 103-2.3 × 1010 Hz for the temperatures 4.2-293 K. In both materials the a.c. conductivity is proportional to ωn at temperatures where the d.c. conductivity is low with n = 0.5-0.8 at low frequencies, f ⩽ 106 Hz and n = 1.85-2 at higher frequencies. The results indicate that at these temperatures the frequency dependence of the a.c. conductivity may saturate at microwave frequencies. At temperatures where the d.c.-conductivity is high the frequency dependence of the conductivity is small or may even disappear. This and the relatively high values of carrier concentration and mobility in EuSe at room temperature indicate that the band conduction dominates at high temperatures. Consideration of different models for the frequency dependent conductivity leads to thermally activated hopping as the most likely process at low and high frequencies. Interpreting the high frequency data with a hopping model we have estimated the maximum hopping frequency and the average hopping length in EuSe. Both in EuSe and EuO the strong positive frequency dispersion at temperatures where the d.c. conductivity is low supports the idea of carrier localization.