ARPES Study On Iron-based Superconductors And Heavy Fermion Materials

Superconductivity has always been a vibrant field in condensed matter physics since its discovery.As the second family of high temperature superconductors,the ironbased superconductors have attracted extensive attention in the past ten years.Among them,one of the hottest subjects is the iron selenide?FeSe?thin films.Heavy fermion material,as the first “high-temperature” superconductor ever found,provides an important platform to study both strong electron correlations and unconventional superconductivity.In this thesis,using angle-resolved photoemission spectroscopy,the electronic structures of the rare-earth element doped “122” system?Ca,Pr?Fe₂As₂ ironbased superconductor,the iron selenide films prepared by both molecular beam epitaxy?MBE?and pulsed laser deposition?PLD?methods,and CePt2In7 heavy fermion material are studied.The thesis contains the following main contents:1.The discovery of superconductivity and the discovery and development of highT(8 superconductors,including the crystal structure,phase diagram and electronic structure of cuprates,are reviewed.The iron-based superconductors and heavy fermion materials are introduced in details because of their close relation with the content of this thesis.2.The principle and experimental technique of the angle-resolved photoemission spectroscopy?ARPES?are firstly introduced.Then the four vacuum ultra-violet laser-based ARPES systems in our laboratory are introduced in details.3.High resolution ARPES measurements were carried out on the electronic structure of the?Ca,Pr?Fe₂As₂ iron-based superconductor in both the high-T tetragonal structure phase and low-T collapsed tetragonal structure phase?c T?.By comparing the band structure and Fermi surface above and below c T phase,we revealed the details of the band structure reconstructions caused by the collapsed phase transition.We find that the dramatic band reconstruction strictly follows the hysteresis loop.No superconducting gap is observed at low temperature;this indicates that the observed superconductivity up to 47 K is not a bulk effect,and the c T phase is not related to the superconductivity observed in?Ca,Pr?Fe₂As₂.4.The superconductivity of the iron selenide films is one of the hottest subjects in the field of the iron-based superconductors in the past few years.We carried out a series of vacuum annealing and potassium depositing treatments on the3UC-FeSe/SrTiO₃ thin film grown by the MBE method,and performed ARPES measurements to keep track on the evolution of its electronic structures with electron doping during these processes.We found that there is a trend from N phase to S phase with electron doping.Through the temperature dependent ARPES measurements,the nematic transition temperature ?160 K was determined for the 3UC-FeSe/SrTiO₃ films.5.High resolution ARPES measurements are carried out on studying the electronic structure of FeSe/CaF₂ films grown with PLD method.A series of FeSe/CaF₂ film samples are prepared with different compositions,resulting in different superconducting transition temperatures.Through ARPES measurements on three samples with T(8= 4 K,9 K and 14 K,we found that their band structures are roughly the same at the ? points,but exhibit a difference at the M points.The difference between the band splitting sizes around the M point in the three samples indicates that there is competition between nematic phase and superconductivity in the FeSe/CaF₂ thin films by PLD method.We find that the electronic structure of the PLD-grown FeSe/CaF₂ film is similar to that of the20UC-FeSe/SrTiO₃ film prepared by the MBE method.By depositing potassium onto the T(8=4 K and T(8=14 K FeSe/CaF₂ films,we observed the evolution from the N phase to the S phase with increasing electron doping.6.We presented the first high resolution ARPES study on the CePt2In7 heavy fermion system.Clear Fermi surface and band structures are resolved.The measured results show a good agreement with the band structure calculations.It is found that the band renormalization effect in CePt2In7 is rather weak.Compared with CeCoIn₅ in the same family,the renormalization effect is approximately half of that in CeCoIn₅.