Arpes Studies Of Electron Correlation Effects In AFe₂As₂?A=Cs,Rb?and La_2-2xSr_1+2xMn₂O₇?x?0.59?

The transition metal compounds have attracted great attention because of the novel physical and chemical properties.Especailly,the interaction between orbit,lattice,charge and spin have always been a hot topic in condensed matter physics.The novel physical and chemical properties of transition metal compounds could be modulated by doping,temperature,pressure and other parameters,such as the competition between anti-ferromagnetic order and superconductivity in phase diagram of iron-based superconductors and the temperature driven metal-insulator transition in manganese oxides.Because electron structure near Fermi level determines physical and chemical properties of many materials,the research of electron structure is very important.Angle resolved photoemission spectroscopy(ARPES)is the most direct and effective tool of detecting band dispersion,which could reveal the microscopic origin of much novel physical and chemical properties.In particular,ARPES based on synchrotron radiation light source have the advantages of variable photon energy,which can probe the three-dimensional electron structure of the material.In this paper,we use ARPES study the heavily doped "122" compounds and complex manganese oxide "La2-2xSr1+2XMn2O7(x?0.59)".The corresponding results achieved are summarized as follow:1.By studying the electron structure character of Fe-based superconductors AFe2As2(A=Cs,Rb),we observed the spectrum weight is distributed in one-Fe Brillouin zone,however,at most cases,the spectrum weight is distributed in two-Fe Brillouin zone.The electronic excitation in one-Fe Brillouin is obvious different from two-Fe band structure.Based on theoretical calculations,we believe that the larger distance between adjacent layers of AFe2As2(A=Cs,Rb)could weaken the interlayer coupling and ultimately make the Fe containing one unit cell appears.2.Using ARPES,we find that the Fermi surface of electron doped CaCo2As2 is consist of several electron pockets and the Fermi level is higher than the "122" iron-based compounds.The temperature dependenced measurements of CaCo2As2 show that Fermi surface and band dispersion did not change significantly though the phase transition(TN=70K).This phenomenon illustrated the correlation effect between local magnetic moment and conduction electron is not strong.3.Using ARPES,we study on the temperature dependenced metal-insulator transition in manganese oxide La2-2xSr1+2xMn2O7(x?0.59).By removing the temperature-independent background,we found that the spectral weight of La2-2xSr1+2xMn2O7(x?0.59)appears transfer at a large energy scale with the temperature increase,which leads to the metal-insulator transition.Different from the traditional metal-insulator transition,we found that the effective mass of carrier decreases with the increase of temperature,which mainly due to the increase of temperature weaken the electron phonon coupling effect.4.In order to extend the function of the ARPES experimental station and increase the capability of in-situ preparation and characterization of sample,we build and optimize a set of pulsed laser deposition system(PLD).First,we design a double-light path device for transporting laser to film deposition cavity,which could achieve thin film samples in-situ spectral analysis.Then,we use optimized sample holder instead of the substrate,which could change mechanical environment of thin single crystal samples.At last,we design an independent vacuum chamber,which could transfer the film samples form PLD to ARPES under ultra vacuum condition.