Mott behavior in KxFe2-ySe2 superconductors studied by pump-probe spectroscopy

We report on a signature indicating a transition to the orbital-selective Mott phase in superconducting KxFe2-ySe2 single crystals by employing dual-color pump-probe spectroscopy. In addition to the multiexponential-decay recovery dynamics of photoinduced quasiparticles, a damped oscillatory component caused by coherent acoustic phonons emerges when the superconducting phase is suppressed at an increased temperature or excitation power. Upon raising the temperature to 150-170 K, the oscillatory component diminishes alongside a significant enhancement of the slow decay component in the recovery traces. These results can be understood as a gap opening in certain k directions, indicating that a vital role is played by electron correlation in iron-based superconductors.