Thermal and voltage activated excess 1/f noise in FeTe0.5Se0.5 epitaxial thin films

We report on measurements of electric transport and voltage noise in FeTe0.5Se0.5 epitaxial thin films. Detailed structural investigations demonstrate the high quality of the samples, also testified by a superconducting transition temperature of 18 K, larger than the value observed for bulk compounds. An unusual dependence of the 1/f noise is found by varying the applied voltage and the temperature. Above a threshold voltage the 1/f noise shows a nonquadratic dependence on applied voltage with a temperature-dependent exponent. Conversely, at low voltages the 1/f noise amplitude follows always a quadratic law. In the superconductive transition region, a simple percolative model allows one to estimate the dimensionality of the system, which is found to be two-dimensional, with a critical exponent similar to values reported for other high-T-c superconductors. The experimental findings on noise properties give evidence of unusual transport processes, occurring in this new superconducting material, and can provide useful information on possible conduction mechanisms.