Doping evolution of the magnetic susceptibility and transport properties of Fe1+δTe1-xSex single crystals

A study of the doping evolution of the magnetic susceptibility and transport properties was performed on Fe1+delta Te1-xSex (x = 0, 0.22, 0.32, 0.37 and 0.40) single crystals grown by the self-flux method. For x = 0, 0.22 and 0.32 the paramagnetic susceptibility chi(T) in the high-temperature regime can be well fitted with a modified Curie-Weiss law. The Curie-Weiss temperature theta(p) systematically decreases with increasing Se content, indicating the breakdown of the predominant antiferromagnetic (AFM) interactions originating from FeTe (x = 0). Bulk superconductivity is established for x = 0.37 and 0.40, while chi(T) monotonically increases with increasing temperature in the high-temperature regime, as widely observed in iron arsenides. The resistivity rho(ab) simultaneously changes from a semiconducting behavior for x = 0, 0.22, and 0.32 to metallic transport below 200 K for x = 0.37 and 0.40. A sudden drop of the Hall coefficient R-H from positive to negative values signifies the AFM transition of the sample for x = 0 at T-N = 68 K, as evidenced by the magnetic susceptibility measurements. For x = 0.22 and 0.32, R-H is positive and monotonically increases with decreasing temperature. A dramatic change in R-H is observed for x = 0.37 and 0.40, where the positive R-H starts to decrease below T-N = 68 K. A change of sign further occurs at 40 K for x = 0.40. The anomalous doping-dependent behavior for both magnetic susceptibility chi(T) and Hall coefficient R-H can be interpreted with the scenario that the Fe1+delta Te1-xSex system undergoes a critical transition from the (pi, 0) magnetic order of FeTe to the dominant (pi, pi) spin fluctuations of FeSe with the establishment of bulk superconductivity.