Resistance Fluctuations In Superconducting K (X) Fe2-Y Se-2 Single Crystals Studied By Low-Frequency Noise Spectroscopy*

Low-frequency resistance noise spectroscopy is applied to investigate bulk single crystals of the intercalated iron-selenide K (x) Fe2 - y Se-2 superconductors with different iron vacancy orders. Based on a generalized fluctuation model, the well-observed resistance hump above 100 K is interpreted as an insulator-metal phase transition with a characteristic transition energy of 0.1-0.6 eV, indicating a highly inhomogeneous energy distribution configuration. In the superconducting transition regime, we find that the normalized resistance noise scales with resistance R excellently as S-R /R (2) proportional to R (l) (rs) with the noise exponent l (rs) approximate to 1.4. With reduced iron vacancy disordering in enhanced superconductivity K (x) Fe2 - y Se-2 crystals, the level of resistance fluctuations is greatly suppressed, suggesting a geometrical phase transition for conduction channel, which is directly related to the microstructure of the crystals.