Emergence of superconductivity and magnetic ordering tuned by Fe-vacancy in alkali-metal Fe chalcogenides Rb_xFe_2-ySe₂

Samples of Rb_xFe_2-ySe₂ exhibiting superconductivity [superconducting (SC) samples] undergo a phase-separation into two phases, a Fe-vacancy ordered phase with antiferromagnetic (AFM) transition at T_N1∼500 K (AFM1 phase) and a phase with little Fe- vacancy and SC transition at T_c∼30 K (SC phase). The samples of Rb_xFe_2-ySe₂ exhibiting no SC behaviour (non-SC samples) are phase-separated into three phases, the AFM1 phase, another AFM phase with T_N2 ∼150 K (AFM2 phase), and a paramagnetic phase with no SC transitions (paramagnetic non-SC phase). In this paper, we present the experimental results of magnetic susceptibility, electrical resistivity, and NMR measurements on single crystals of Rb_xFe_2-ySe₂ to reveal physical properties of these co-existing phases in the SC and non-SC samples. The ⁸⁷Rb and ⁷⁷Se NMR spectra show that the Fe vacancy concentration is very small in the Fe planes of the SC phase, whereas the AFM2 and paramagnetic non-SC phases in non-SC samples have larger amount of Fe vacancies. The randomness induced by the Fe vacancy in the non-SC samples makes the AFM2 and paramagnetic non-SC phases insulating/semiconducting and magnetically active, resulting in the absence of the superconductivity in Rb_xFe_2-ySe₂