| The FeSe-based superconductors have attracted much recent interest because they provide an ideal system to study the interplay between magnetism,nematicity and su-perconductivity.One remarkable discovery is the observation of high temperature superconductivity in the single-layer FeSe/SrTiO3 films which has attracted much at-tention because of its simple crystal structure,distinct electronic structure and record high superconducting transition temperature(TC).Over the past ten years,the field of diluted magnetic semiconductors(DMSs)has evolved into an important branch of materials science.Despite extensive studies,however,the mechanism behind the ferromagnetism is still vigorously debated,and many doubts are related to the elec-tronic structure and Fermi level position.In this thesis,the thin films growth and the morphological characterization of FeSe/STO films,the electronic structure study of single-layer FeSe/STO films and(Ba1-x,Kx)(Zn1-y,Mny)2As2 will be discussed in detail.Following is the main structure of the thesis:1.To draw the starting point of this thesis,a brief introduction of the discovery and development of superconductors has been given,especially for the iron-based high-temperature superconductors.2.Angle-resolved photoemission spectroscopy and its component and principle,is introduced in detail.3.A brief introduction of molecular beam epitaxy technique and the principle of scanning tunneling microscope.Also described here is the growth method and process of FeSe/STO thin films.4.By depositing Se onto the single-layer FeSe/STO films,we find the formation of the second-layer FeSe islands on the top of the first layer during the annealing process at a surprisingly low temperature(?150?)which is much lower than the usual growth temperature(?490?).This observation allows us to develop a reliable method to detect excess Fe and estimate its quantity in the single-layer FeSe/STO films.We find that there is at least 20%excess Fe present that is sur-prisingly high for the superconducting single-layer FeSe/STO films.Our present work provides new insights in understanding the high-temperature superconduc-tivity in the single-layer FeSe/STO films.In establishing reasonable models for the FeSe/STO system,in addition to the STO substrate and single FeSe layer,the presence of a large quantity of excess Fe at the interface should be taken into account.Since magnetism is usually detrimental to superconductivity,the effect of these excess Fe on the superconductivity of the single-layer FeSe/STO films deserves further investigations.Our results provide important insights in understanding high-temperature superconductivity and further enhancing TC in single-layer FeSe/STO films.5.Here we report comprehensive high-resolution angle-resolved photoemission spectroscopy measurements on the electronic structure evolution with doping in single-layer and multiple-layer FeSe/STO films.We find that the single-layer FeSe/STO films have a unique parent phase and a route of doping evolution to superconductivity that are distinct from multiple-layer FeSe/STO films.In multiple-layer FeSe/STO films,superconductivity occurs by suppressing the ne-matic order in the parent compound with electron doping.In the single-layer FeSe/STO films,the parent phase is a Mott-like insulator without a nematic order;its doping evolution is similar to doping a Mott insulator.These ob-servations indicate that there is a strong electron correlation in the single-layer FeSe/STO films and the doping evolution is distinct from that of all the other iron-based superconductors.Our results provide key insight in understanding the high temperature superconductivity in single-layer FeSe/STO films.6.We first report a systemic high-resolution angle-resolved photoemission study of Mn-doped ?-?-? DMSs system.The experiments were carried out on a series of samples(Ba,K)(Zn,Mn)2As2,Ba(Zn,Mn)2As2,(Ba,K)Zn2As2 and BaZn2As2.The theoretical spectrum was given by the density-functional theory in the local density approximation.Comparing spectra of the series of samples and combin-ing experimental data with theoretical calculation,we clarify that(?)BaZn2As2 forms a narrow bandgap.In the case of(Ba,K)(Zn,Mn)2As2,via(Ba,K)sub-stitution to dope hole carriers and(Zn,Mn)substitution to introduce spin,its Fermi level is located around As 4p valence-band.(?)the main Mn 3d-related density of states(DOS)is located at?3.2 eV;(?)in the Mn-doped samples((Ba,K)(Zn,Mn)2As2 and Ba(Zn,Mn)2As2),we identify the indication of As 4p orbital splitting which may correspond to p-d hybridization.(iV)our obser-vations favor the p-d exchange mechanism.We conclude that the strong hy-bridization between the Mn 3d and the As 4p orbitals plays an important role in inducing high temperature ferromagnetism in(Ba1-xKx)(Zn1-y,Mny)2As2 system. |
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