Abstract
FeSe superconductor withonly FeSe4 tetrahedral layer by layerstructure is an ideal candidate to investigated the mechanism of superconductivity.However, whether there exist the parent compounds for FeSe and potassium-intercalated FeSe have been controversial. In thisthesis, we have demonstrated by transport measurements that non-superconducting K2Fe4Se5 with Fe-vacancy ordered isthe parent compound of the superconducting K1-xFe2-ySe2, in which the superconductivity emerges after the iron vacancy becoming disorder.We also observed in theFe0.8Se nanowires both metallic and insulating behaviors in temperature dependence of resistance. This difference can also be attributed to the state of iron vacancy in Fe4Se5 and Fe24Se25 phases, respectively. The resistance of the non-superconducting K2Fe4Se5can be fit by the two-dimensional Mott variable-range-hoppingA pronounce transition at 120 K with hysteretic behavior is observed in this vacancy-ordered sample by magnetic measurement. This transition is associated with the presence of charge/spin order due to the iron vacancy order.The upper critical field of superconducting K1.9Fe4.2Se5 is about 93 Tesla, which corresponds to acoherence length of about 1.77 nm. Magnetoresistance (MR) measurements on superconducting K1.9Fe4.2Se5show linear dependencein magnetic field at temperatures between 90K-200K. We propose that this linear relationship comes from the combination of two-band model term and the negativeMR signals form Mott variable-range-hopping term (or the quantum limit of Dirac states proposed by Abrikosov.Detailed vortex studies show thatthere are non-negligible contribution from grain boundary to MR at low field, following by slow increase from thermal activated flux flow, and finally flux flows at high field.