Unexpected thermal activation behavior of electrical transport in K<sub>6</sub>C<sub>60</sub>

Among C60 compounds, A6C60 is regarded as an insulator because the t1u derived LUMO is fully filled. Although C60 compounds have been extensively studied for many years, the physical properties of A6C60 were not investigated due to experimental difficulties. In present study, we successfully detected the temperature dependence of electrical resistivity of K6C60 in bulk under dry condition. Contrary to expectation, experimentally estimated Eg was significantly smaller than those of previous reports. On the other hand, the thermal activating behavior did not obey Arrhenius law and VRH models in whole temperature range. This anomalous behavior may be attributed to lattice dynamics. The resistivity data based on our experimental breakthrough could provide us with critical information for universal understanding in fulleride.