Magnetism and Superconductivity in Rb$_{1-x}$Fe$_{2-y}$Se$_2$

We report on the structural, magnetic, and superconducting properties of Rb$_{1-x}$Fe$_{2-y}$Se$_2$ single crystals. The system exhibits a strongly anisotropic antiferromagnetic behavior below 400~K. For 1.53 $<2-y<$ 1.6 superconductivity is found, whereas for Fe concentrations $2-y<$ 1.5 and $2-y>$ 1.6 insulating and semiconducting behavior is observed, respectively. The sharpest transition to the superconducting state and the highest transition temperature $T_c$ of 32.4~K is found for compositions close to Rb$_2$Fe$_4$Se$_5$. Comparison of the magnetic behavior of non-superconducting and superconducting samples provides evidence for the coexistence of superconductivity and static antiferromagnetic order [1]. THz time-domain transmission spectroscopy in superconducting samples evidences a metallic response and the superconducting transition of the system [2].\\[4pt] [1] V. Tsurkan et al., Phys. Rev. B 84, 144520 (2011).\\[0pt] [2] A. Charnukha et al., arXiv:1108.5698.