Magnetic and Superconducting Properties in Single Crystalline Fe1+δTe1-xSex (x < 0.50) System

The spin-fluctuation effect in the Se-substituted single crystalline Fe1-delta Te1-xSex (x = 0, 0.05, 0.12, 0.20, 0.30, 0.33, 0.45, and 0.48; 0 <= delta <= 0.12) and the polycrystalline Fe1.11Se has been studied by the measurements of the X-ray diffraction, the magnetic susceptibility under high magnetic fields and the electrical resistivity under magnetic fields up to 14 T. The samples with x = 0.05, 0.12, 0.20, 0.30, 0.33, 0.45, and 0.48 show superconducting transition temperatures in the ranger of 10-14 K. We obtained their intrinsic susceptibilities by the Honda-Owen method. A nearly linear-in-T behavior in magnetic susceptibility of Se-rich superconducting samples was observed, indicating the antiferromagnetic spin fluctuations have a strong link with the superconductivity in this series. The upper critical field mu H-0(c2)orb for T -> 0 was estimated to exceed the Pauli paramagnetic limit. The Kadowaki-Woods and Wilson ratios indicate that electrons are strongly correlated in this system. Furthermore, the superconducting coherence length and the electron mean free path were also discussed. These superconducting parameters indicate that the superconductivity in the Fe1-delta Te1-xSex system is unconventional.