Study On The Angular Resolution Photoelectron Spectroscopy Of The Electronic Structure Of FeSe Single Crystal Thin Film

Since the discovery of high temperature superconductivity in LaOFeAs in 2008, the iron-based superconductors become an important field in the condensed matter physics community. Exploring higher Tc iron-based superconductor and understanding superconductivity mechanism of iron-based superconductors are two prominent and challenging problems.The FeSe superconductor and its related systems have drawn a lot of attention owing to simple crystal structure and unique electronic/physical properties.In 2012, Q.K. Xue et al. discovered the superconducting gap as large as 20meV in 1-UC (unit cell) FeSe thin film grown on SrTiO3 substrate in their scanning tunneling spectroscopy (STM).Later the superconducting gap corresponding Tc over 65K was confirm by angle-resolved photoemission spectroscopy (ARPES) experiment.So far,1-UC FeSe/SrTiO3 thin film is the iron-based superconductors with the highest Tc.Besides,FeSe/SrTiO3 thin film has an unique electronic structure,layer-dependent behaviors and insulator-superconductor transition, which interest the researchers in condensed matter physics community. We grew FeSe thin films on STO substrates by molecular beam epitaxy (MBE),and conducted the in-suit measurement on the electronic structure of FeSe thin films. Main conclusions are as follow:1. Fe-vacancy disorder-order transition in FeSe multilayer thin filmsWe conducted the in-situ angle-resolved photoemission spectroscopy (ARPES) study of Fe-vacancy disorder-order transition in FeSe multilayer thin films grown on STO substrate by molecular beam epitaxy (MBE). Low temperature annealed FeSe films (550"C for 1h, and 250? for 3h) are identified to be Fe-vacancy disordered phase and electron doped. Further long-time low temperature (250? for 16h) anneal can change the Fe-vacancy disordered phase to ordered phase, which is found to be semiconductor/insulator with ?5�?5 superstructure and can be reversely changed to disordered phase with high temperature anneal. Our results reveal that the disorder-order transition in FeSe thin films can be simply tuned by vacuum anneal and the FeSe thin films with precisely controllable crystal structure, stoichiometry and thickness provide an ideal platform to study the complex electronic structure and superconductivity of FeSe.2. Dirac Cone structures in FeSe multilayer thin filmsHigh quality FeSe thin films were grown on STO substrate by molecular beam epitaxy. We found the existence of Dirac cone structures in multilayer FeSe thin films (2UC or thicker) in nematic state by angle-resolved photoemission spectroscopy. The Dirac cones are located at the dots with high photoemission intensity on the Fermi surface of FeSe thin film, and the apexes of the Dirac cones are sit at-10 meV below Fermi energy, resulting in small electron Fermi pockets. The Dirac cone band dispersions disappear above the nematic transition temperature and are independent from film thickness. When multilayer FeSe thin films was doped with cobalt, the nematicity state will be suppressed obviously, and the Dirac cones vanish in the same time. Our experiments provide some helpful suggestions that the magnetic degrees of freedom may play some kind of role in the nematicity of FeSe, at least strong interplay between the magnetic and orbital degrees of freedom may exist in FeSe.3. elements doping of single-layer FeSe thin filmIn this part,we have made some exploratory research and come to the phased conclusions.We grew single-crystal monolayer FeSe thin films with high quality by molecular beam epitaxy (MBE). FeSe thin film is transferred from MBE chamber to ARPES chamber for in-suit electronic structure characterization. The results of ARPES measurements show that the annealed FeSe thin films have typical features of single-layer FeSe thin film and superconducting with Tc of?30K. Next, the FeSe thin film is doped with Ni, and the result of ARPES showed that the FeSe thin films with Ni doping have the Fermi surface with higher electronic doping level and the bigger electronic pocket around M point whose effective mass is larger. The FeSe thin film with Ni doping is superconducting at 11 K but its quality is too low to judge the precise Tc. Therefore we cannot conclude Ni doping has a favorable or adverse effect on superconductivity,and further work is needed to investigate the effect of Ni doping on FeSe thin film.