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Abstract

A study of the doping evolution of the magnetic susceptibility and transport properties was performed on Fe(1+delta)Te(1-x)Se(x) (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 Fe(1+delta)Te(1-x)Se(x) 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.