Electronic structure and nematic phase transition in superconducting multiple-layer FeSe films grown by pulsed laser deposition method

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 T_c 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.