The Observation of Small Polaron Tunnelling in the ab-Plane of K_0.85Fe_1.66Se_2.0

The temperature-dependent conductivity spectrum from 3×10² Hz to 1×10⁸ Hz in the ab-plane of a K_0.85Fe_1.66Se_2.0 single crystal is probed. The dc measurements indicate that the charge transport can be described well by the variable range hopping model. From the ac conductivity spectrum, clear evidence for small polaron tunnelling with an activation energy of about 155 meV in ab-plane is revealed. With decreasing temperature, the real part of the dielectric spectrum shifts towards lower frequencies and also shows a wider distribution of the relaxation time. Below 140 K, the peak position of the dielectric loss spectra begins to deviate from thermal excitation behavior, reflecting the freezing of the phonon which assists the tunnelling.