Angle Resolved Photoemission Spectroscopy Study On Single-Layer FeSe/SrTiO₃ Films

Singlelayer FeSe films grown on the SrTiO3 substrate(FeSe/STO)have attracted much attention because of their possible recordhigh superconducting critical temperature(Tc).The distinct electronic structures and simple crystal structure make singlelayer FeSe/STO films ideal for investigating the physics of superconductivity in ironbased superconductors.In this thesis,we prepared high-quality single-layer FeSe/STO flims by using molecular beam epitaxy technique and investigates the electronic structures and superconductivity of singlelayer FeSe/STO films by highresolution angleresolved photoemission measurements.Following parts are included in this thesis1.A brief introduction of the history of superconductivity researches is given,in particular,the study of iron-based superconductors.To further draw the motivation of this thesis,the background about single-layer FeSe/STO films is reviewed2.The principle,experimental systems and operation of angleresolved photoemis-sion spectroscopy(ARPES)and spin-resolved ARPES systems are introduced3.A brief introduction of the principle of molecular beam epitaxy technique and scanning tunneling microscope are given.The process of growing highquality single-layer FeSe/STO films is described4.By preparing pure single-layer FeSe/STO film,we find direct spectroscopic ev-idence that the parent phase of the single-layer FeSe/SrTiO3(FeSe/STO)films is insu-lating.It represents the first clear case in the iron-based superconductors that the parent compound is an insulator.We also find that the single-layer FeSe/STO films exhibit a distinct doping evolution to superconductivity that is similar to doping a Mott insulator in cuprate superconductors.These observations provide key insights in understanding the record high-TC superconductivity in single-layer FeSe/STO films and point to a new route of realizing superconductivity in iron-based superconductors5.Observe spectroscopic evidence of superconductivity pairing at 83 K in single-layer FeSe/STO films.By preparing high-quality single-layer FeSe/STO films,we observe for the first time strong superconductivity-induced Bogoliubov back-bending bands that extend to rather high binding energy～100 meV by high-resolution angle-resolved photoemission measurements.The Bogoliubov back-bending band provides a new definitive benchmark of superconductivity pairing that is directly observed up to 83 K in the single-layer FeSe/STO films.Moreover,we find that the superconduc-tivity pairing state can be further divided into two temperature regions of 64-83 K and below 64 K.We propose the 64-83 K region may be attributed to superconductivity fluctuation while the region below 64 K corresponds to the realization of long-range superconducting phase coherence.These results indicate that either Tc as high as 83 K is achievable in iron-based superconductors,or there is a pseudogap formation from superconductivity fluctuation in single-layer FeSe/STO films6.A patent of invention is introduced that is a vacuum system used to transfer samples between different sample holders in the ultra-high vacuum condition...