  The record of superconducting transition temperature(Tc) has long been 56K
for the iron-based high temperature superconductors(Fe-HTS's). Recently, in
single layer FeSe films grown on SrTiO3 substrate, signs for a new 65K Tc
record are reported. Here with in-situ photoemission measurements, we
substantiate the presence of the spin density wave(SDW) in FeSe films, a key
ingredient of Fe-HTS that was missed in FeSe before, which weakens with
increased thickness or reduced strain. We demonstrate that the
superconductivity occurs when the electrons transferred from the oxygen-vacant
substrate suppress the otherwise most pronounced SDW in single layer FeSe.
Besides providing a comprehensive understanding of FeSe films and directions to
further enhance its Tc, we establish the phase diagram of FeSe vs. lattice
constant that contains all the essential physics of Fe-HTS's. With the simplest
structure, cleanest composition and single tuning parameter, it is ideal for
testing theories of Fe-HTS's.
