Scanning Tunneling Microscope Study Of Iron Selenium Films Under Extreme Conditions

Iron-based high-temperature superconductors have been the focus of condensed mat-ter research since their discovery in 2008.Despite thousands of papers have been pub-lished per year in the last 11 years,these materials remain not well understood.The electronic structure and phase diagram of these materials are very rich.The most distinct one is Fe Se,which has the simplest structure and the highest superconducting transition temperature(the transition temperature of single-layer Fe Se/Sr Ti O₃is 65K).Specially,long-range magnetic order is absent in bulk Fe Se.In this work,we grow Fe Se films on graphene of different thickness by molecular beam epitaxy(MBE)and explore the mys-tery of Fe Se by scanning tunneling microscope(STM).(1)Compared with those of bulk Fe Se,the sizes of electron and hole pockets become smaller in monolayer Fe Se because the absence of inter-layer coupling tends to narrow the energy band and decrease the overlap between electron and hole pockets in energy.For Fe Se monolayer grown on graphene/Si C(0001)surface,we find the Fermi energy is in the range of a few me V.In addition,we demonstrate an approach to tuning the Fermi energy by the thickness of graphene layers.At the base temperature of STM,the film grown on trilayer graphene becomes superconducting with?=0.62me V.In contrast,no signature of superconductivity has been detected in the d I/d V spectra of Fe Se monolayer on bilayer graphene.The Fe Se monolayer on graphene/Si C(0001)has the potential to study the intriguing physics where the Fermi energy,the superconducting gap and the Zeeman energy in magnetic field are in the same order of magnitude.(2)The Fermi wave vector of single-layer Fe Se is very small,resulting in a very long Fermi wavelength.Due to size effect,electrons or holes of d-orbital forming stand-ing waves originate from the intererence of quantum states and usually created by scat-tering off defects and step edges with large potential barriers.The Fermi surface of Fe Se is contributed by several d orbitals,and the spectral weight of quasiparticle of dorbital is much larger than that of other orbitals.As a result,quasiparticle interference is mainly contributed by dorbital electrons.The formation of quantum well states and stand-ing waves could also have orbital selectivity.Fe Se monolayer presents free-eletron/hole gas behaviour and shows the quantized Landau level under the high magnetic field.The electron-electron interaction may become stronger under a strong magnetic field,and novel phenomena may emerge.(3)The ratio of the superconducting gap and the Fermi energy of Fe Se is close to1,indicating the emergence of BCS-BEC crossover.There are two stacking order of trilayer graphene.The work function of ABA stacking is 0.3?0.6e V larger than that of ABC stacking.The Fermi energy of Fe Se can be tuned by trilayer graphene with different stacking,resulting in a transition of monolayer Fe Se from the BCS limit to the BEC limit.The evolution of BCS-BEC crossover with temperature and magnetic field is inverstigated by using STM.