Low-Temperature ac Conductivity of Chalcogenide Glasses

The conductivity of amorphous ${\mathrm{As}}_2$ ${\mathrm{Te}}_3$ and ${\mathrm{As}}_2$ ${\mathrm{Te}}_3$Ge was measured down to 4.2 K at frequencies ranging between 0.1 and 100 kHz. The ac conductivity below 100 K, which is totally unaffected by the dc conductivity, is proportional to ${\omega }^s{T}^n$ with $s=1\mathrm{}$ and $n\simeq 0.5$ for as-deposited films and $n\simeq 0.1$ for annealed films. These experiments are interpreted in terms of Elliott's theory based on electron pairs hopping over a barrier between both paired and random defects.