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In this work, we study the A$_{x}$Fe$_{2-y}$Se$_2$ (A=K, Rb) superconductorsusing angle-resolved photoemission spectroscopy. In the low temperature state,we observe an orbital-dependent renormalization for the bands near the Fermilevel in which the dxy bands are heavily renormliazed compared to the dxz/dyzbands. Upon increasing temperature to above 150K, the system evolves into astate in which the dxy bands have diminished spectral weight while the dxz/dyzbands remain metallic. Combined with theoretical calculations, our observationscan be consistently understood as a temperature induced crossover from ametallic state at low temperature to an orbital-selective Mott phase (OSMP) athigh temperatures. Furthermore, the fact that the superconducting state ofA$_{x}$Fe$_{2-y}$Se$_2$ is near the boundary of such an OSMP constraints thesystem to have sufficiently strong on-site Coulomb interactions and Hund'scoupling, and hence highlight the non-trivial role of electron correlation inthis family of iron superconductors.

Observation of Temperature-Induced Crossover to an Orbital-Selective Mott Phase in A$_{x}$Fe$_{2-y}$Se$_2$ (A=K, Rb) Superconductors