Effect of varying iron content on the transport properties of the potassium-intercalated iron selenide KxFe2-ySe2

We report the successful growth of high-quality single crystals of potassium-intercalated iron selenide KxFe2-ySe2 by the Bridgman method. The effect of iron vacancies on transport properties was investigated by electrical resistivity measurement. With varying iron content, the system passes from a semiconducting or insulating state to a superconducting state. Compared with superconductivity, the anomalous hump effect in the normal-state resistivity is much more sensitive to the iron deficiency. The electrical resistivity exhibits a perfect metallic behavior (R-300 (K)/ R-35 (K) approximate to 42) for the sample with little iron vacancies. Our results suggest that the anomalous hump effect in the normal-state resistivity may be due to the ordering process of the cation vacancies in this nonstoichiometric compound rather than to a magnetic or structure transition. A trace of superconductivity extending up to near 44 K was also detected in some crystals of KxFe2-ySe2, which has the highest T-c of the reported iron selenides.