Thermal and voltage activated excess1/fnoise in FeTe0.5Se0.5epitaxial thin films

We report on measurements of electric transport and voltage noise in FeTe${}_{0.5}$Se${}_{0.5}$ 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.