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We have investigated the influence of Fe-excess on the electrical transportand magnetism of Fe1+yTe0.5Se0.5 (y=0.04 and 0.09) single crystals. Bothcompositions exhibit resistively determined superconducting transitions (Tc)with an onset temperature of about 15 K. From the width of the superconductingtransition and the magnitude of the lower critical field Hc1, it is inferredthat excess of Fe suppresses superconductivity. The linear and non-linearresponses of the ac-susceptibility show that the superconducting state forthese compositions is inhomogeneous. A possible origin of this phase separationis a magnetic coupling between Fe-excess occupying interstitial sites in thechalcogen planes and those in the Fe-square lattice. The temperature derivativeof the resistivity drho/dT in the temperature range Tc < T < Ta with Ta beingthe temperature of a magnetic anomaly, changes from positive to negative withincreasing Fe. A log 1/T divergence of the resistivity above Tc in the samplewith higher amount of Fe suggests a disorder driven electronic localization.

Disorder-driven electronic localization and phase separation in superconducting Fe1+yTe0.5Se0.5 single crystals