  Phase separation of metallic and antiferromagnetic (AFM) insulating phases in
alkaline iron selenides (AxFe2-ySe2) continues to attract intense interest
because the relationship between two peculiar features probably is a key to
clarifying the pairing mechanism of AxFe2-ySe2 superconductors. Here we report
that the 3D visualizations of nano-scale phase separation in
(Na0.32K0.68)0.95Fe1.75Se2 single crystals are revealed by hybrid
focused-ion-beam scanning electron microscopy and the superconducting paths are
fully percolative in 3D. Moreover, the phase-related ultrafast dynamics in
(Na0.32K0.68)0.95Fe1.75Se2 is studied by dual-color pump-probe spectroscopy.
The anomalous changes in the electron and acoustic phonon components of
transient reflectivity change ({\Delta}R/R) identify two characteristic
temperatures T*~100 K (the onset temperature of coupling between nano-metallic
and AFM phases) and TH~230 K (the onset temperature of
metallic-interface-phase). An energy-transfer channel between the nano-metallic
and AFM phases is inferred. This proposed channel provides a new insight into
the pairing mechanism of alkaline iron selenide superconductors.
