Figure 3: Sequence of dc conductivity measurements of a highest purity grade MgO single crystal under the most stringent clean conditions. (a) Shows an anomalous increase in conductivity during the first heating, which is not observed as shown in (b) when the crystal is reheated immediately after cooling. However, if the crystal is annealed for several hours, the anomalous conductivity reappears as shown in (c). Panel (d) shows that this anomalously high conductivity is due to the activation of charge carriers that conduct electricity with an activation energy ~1 eV [45].