Comprehensive studies for the crystal structures and electronic properties of the superconducting system Fe1+δSe1-xTex with δ ≃ 0.037 and x ≃ 0.55

Structural aspects and electronic properties for the Fe1+delta Se1-xTex system with 0.04 <= delta <= 0.08 and 0.5 <= x <= 1, especially with the superconducting composition delta similar or equal to 0.037 and x similar or equal to 0.55, are investigated comprehensively with x-ray four-circle diffraction and through measurements of electrical resistivity, thermoelectric power, magnetic susceptibility and nuclear magnetic resonance for Te-125. The crystal structures with an excess Fe site are refined precisely with obvious constraints. For the superconducting composition, the transport properties are explained in terms of the two-band model, where an electron carrier band gives a linear-in-T resistivity and another hole band leads to nearly temperature-independent behaviour. The magnetic susceptibility and the Knight shift are explained with the idea that the electron correlation is enhanced with increasing x and it is reduced with annealing. The spin-lattice relaxation rates for the normal state that show the apparent Korringa relation may also be understood in this framework. These evidences suggest that the superconductivity may emerge in a regime where the correlation is relatively weak in this system.