  We report comprehensive angle-resolved photoemission investigations on the
electronic structure of single crystal multiple-layer FeSe films grown on CaF2
substrate by pulsed laser deposition (PLD) method. Measurements on FeSe/CaF2
samples with different superconducting transition temperatures Tc of 4 K, 9 K
and 14 K reveal electronic difference in their Fermi surface and band
structure. Indication of the nematic phase transition is observed from
temperature-dependent measurements of these samples; the nematic transition
temperature is 140-160 K, much higher than 90 K for the bulk FeSe. Potassium
deposition is applied onto the surface of these samples; the nematic phase is
suppressed by potassium deposition which introduces electrons to these FeSe
films and causes a pronounced electronic structure change. We compared and
discussed the electronic structure and superconductivity of the FeSe/CaF2 films
by PLD method with the FeSe/SrTiO3 films by molecular beam epitaxy (MBE) method
and bulk FeSe. The PLD-grown multilayer FeSe/CaF2 is more hole-doped than that
in MBE-grown multiple-layer FeSe films. Our results on FeSe/CaF2 films by PLD
method establish a link between bulk FeSe single crystal and FeSe/SrTiO3 films
by MBE method, and provide important information to understand
superconductivity in FeSe-related systems.
