Structure, superconductivity, and magnetism in Rb1-xFe1.6Se2-zSz

The single-crystal growth, stoichiometry, and structure of Rb1-xFe2-ySe2-zSz crystals with substitution of Se by S are reported. The variation of the magnetic and thermodynamic parameters of samples was studied by differential-scanning calorimetry, magnetic susceptibility, conductivity, and specific heat. The experimental results are discussed within a T-z phase diagram, which includes vacancy-ordered and vacancy-disordered antiferromagnetic (AFM), superconducting (SC), and non-superconducting phases. The structural study revealed change in the local environment of the Fe tetrahedrons depending on substitution: a reduction of the Fe-Fe and Fe-Ch(chalcogen) bond lengths and a tendency for the six out of eight bond angles to approach values for a regular tetrahedron suggesting a reduction of structural distortions with substitution. With increasing substitution, a lowering of the superconducting transition temperature Tc was observed; the percolation threshold for the SC state is located at the substitution z = 1.2. The SC state was found to coexist with the AFM state that persists in all samples independent of substitution. The temperature of the transition into the AFM state TN shows a monotonous decrease indicating a weakening of the AFM interactions with increasing substitution. The AFM phase exhibits an iron-vacancy-ordered structure below the structural transition at Ts. The temperature Ts shows a non-monotonous variation: a decrease with increasing z up to 1.3, followed by an increase for further increasing z. The suppression of the superconductivity with substitution is accompanied by a significant reduction of the density of states at the Fermi energy and a weakening of the electronic correlations in the studied system.
Comment: 32 pages, 22 figures