Potassium Absorption And Superconducting Properties Of Ultrathin FeSe Films On SrTiO₃

The concept of interface superconductivity was introduced in 1950 s.Recent advances in atomically controlled thin film growth techniques make it possible to produce atomically flat interface,and enable the study of interface superconductivity to a new level of sophistication.In 2012,our group reported a superconducting gap as large as 20 meV in one unit-cell?UC?thick FeSe films grown on SrTiO₃?001?substrate by molecular beam epitaxy?MBE?,which evoked great interests in the field of high temperature superconductivity.To investigate the mechanism of interface enhanced superconductivity,we have grown ultrathin FeSe films on SrTiO₃ substrates by molecular beam epitaxy and investigated the influence of potassium adsorption on the superconducting properties of 1-4 UC FeSe films with scanning tunneling microscopy.Our work reveals that electron doping from K atoms and interface enhanced electron-phonon coupling both play vital roles in enhancing Tc of ultrathin FeSe films on SrTiO₃.The main results are summarized as follows:?1?2-4 UC FeSe films all exhibit a superconducting like gap larger than 11 meV at appropriate surface potassium doping level.The optimized gap decreases with increasing film thickness.A superconducting gap of 14.5 meV is also observed on 1.5-UC Kx Fe2Se2 on SrTiO₃.However,surface potassium doping always suppresses superconductivity of 1-UC FeSe film on SrTiO₃.The results suggest that electron doping is a key factor in enhancing Tc of ultrathin FeSe films on SrTiO₃.The experiment also indicates that the FeSe/SrTiO₃ interface has additional enhanced effects except for electron doping.?2?We studied electron-phonon interaction in superconducting FeSe films on SrTiO₃ by investigating bosonic modes in superconducting tunneling spectroscopy.Two characteristic bosonic modes at ¹1.0 meV and ²1.5 meV are determined.The energies of bosonic modes do not change with potassium doping level and film thickness.According to Raman scattering and neutron scattering measurements,the two bosonic modes are probably phonons.Our results strongly suggest that interface electron-phonon coupling plays a vital role in enhancing Tc of ultrathin FeSe films on SrTiO₃ substrates.In summary,electron doping and interface enhanced electron-phonon coupling are both key factors in enhancing Tc of FeSe ultrathin films on SrTiO₃.Superconducting gaps are enhanced to ⁹ me V by electron doping,and further enhanced to 15-20 meV by interface enhanced electron-phonon coupling.Our results demonstrate that interface enhancement in artificial heterostructures provides a new way to raise superconducting transition temperature.