Abstract
We have investigated the influence of Fe excess on the electrical transport and magnetism of ( and 0.09) single crystals. Both compositions exhibit resistively determined superconducting transitions with an onset temperature of about 15 K. From the width of the superconducting transition and the magnitude of the lower critical field , it is inferred that excess of Fe suppresses superconductivity. The linear and nonlinear responses of the ac susceptibility show that the superconducting state for these compositions is inhomogeneous. A possible origin of this phase separation is a magnetic coupling between Fe excess occupying interstitial sites in the chalcogen planes and those in the Fe-square lattice. The temperature derivative of the resistivity in the temperature range with being the temperature of a magnetic anomaly, changes from positive to negative with increasing Fe. A divergence of the resistivity above in the sample with higher amount of Fe suggests a disorder-driven electronic localization.
- Received 14 July 2010
DOI:https://doi.org/10.1103/PhysRevB.82.144523
©2010 American Physical Society