Resistance fluctuations in superconducting K_xFe_2-ySe₂ 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_xFe_2-ySe₂ 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/R² ∝ R^{l _rs} with the noise exponent l_rs≈ 1.4. With reduced iron vacancy disordering in enhanced superconductivity K_xFe_2-ySe₂ 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.