Metal-to-Insulator Transition in Multi-Orbital Models for A$_x$Fe$_y$Se$_2$

The degree of electron correlations remains a central issue in the iron-based superconductors. Compared to other compounds, the newly discovered A$_x$Fe$_y$Se$_2$ family is unique in some aspects: the Fermi surface consists of only electron pockets, while T$_c$ is as high as 30 K ; the superconducting compound is close to an antiferromagnetically insulating phase with a large magnetic moment. These features suggest that the A$_x$Fe$_y$Se$_2$ system contains stronger electron correlations than pnictides.To investigate the correlation effects in A$_x$Fe$_y$Se$_2$, we study the metal-to-insulator transition in multi-orbital models for this system using slave-spin mean-field method. We show that when electron correlations are tuned, the system undergoes a metal-to-Mott-insulator transition at commensurate electron filling. We also find that the Mott insulator is close to an orbital-selective Mott phase in the phase diagram.