Arpes Studies Of The Electronic Structure Of "Na-111" Type Iron- Based Superconductor

As a novel macroscopic quantum phenomena, superconductivity has become one of the hottest scientific research field since its discovery, Superconductivity with its unique fascination continuously attracts attentions from experimental and theoretical scientists. The discovery of high-temperature superconductor cuprates in 1986 made the exploring of physical properties of unconventional superconductivity the frontier of condensed matter physics. Until now, it is still one of the most important challenges to find out new superconductors with higher transition temperature and the retical understand the high temperature superconducting mechanism in the field of condensed matter physics. Angle resolved photoelectron spectroscopy (ARPES), which can directly probe the momentum and energy dependence of the electronic structure of the crystal, is considered one of the most powerful tools for unraveling these mysterics.The discovery of iron-based superconductors (iron pnictides) in 2008, as the second type high-temperature superconductor except cuprates until now, has drawn more and more attentions from scientists all over the world. The various high transition temperature iron pnictides and their novel properties as well as potential application renew the high-temperature superconductivity research field, open up a new path in high temperature superconducting research. Investigation of the mechanism of iron-based superconductors, is an important part of the high temperature superconducting mechanism. Studies of the superconducting mechanism are to find out the reason how the superconducting electrons pair, and give an accurate theoretical description, predict new phenomenon. The difference of the current two major theories of the iron-based superconductors is the correlated strength. At the beginning of studies of iron-based superconductors, it is doubtful whether iron-based superconductors (and their parent compounds) can be regarded as a strongly correlated electron system. Especially, with the discovery of iron selenide compounds, weakly correlated theories are challenged.Therefore, in order to understand the correlated strength of iron-based superconductors, understanding its electronic structure is a very essential step. Angle resolved photoemission spectroscopy, a very powerful technique to direct probe the band structure of materials, is used in our papers to study the electronic structure of "Na-111" iron-based high temperature superconductors from the view of the balance between itinerancy of mobile carriers and local interactions by angle-resolved photoelectron spectroscopy method.In Chapter 1, we first review the photoelectric effect, and the theoretical description of the electrons escaped from crystal. Then we introduced the angle resolved photoemission spectrometer in our group and other related laboratory equipment and how to conduct ARPES experiment. At last, we briefly introduce an advanced experimental method that our group is developing:Pump-probe ARPES. With the pump and probe technique, we can measure the electronic structure information that is not occupied above the Fermi energy, so you can get more information about lattice, spin or charge, even the information of charge dynamics.In Chapter 2, we first briefly introduce the history of superconductors, the progress of related theories, as well as the cuprates high temperature superconductors. Then we introduc the discovery of iron-based superconductors, and the crystal structure and phase diagram are introduced about identified 4 main types of iron-based superconductors. After that we briefly review the comparison between cuprates high temperature superconductors and the iron-based superconductors. Meanwhile, the electronic structure of the parent compound NaFeAs that we need to study in this thesis are briefly introduced. Finally, we review the current two main theories of iron-based superconductors:the local interaction mechanisms and itinerant mechanismsIn Chapter 3, we use angle resolved photoemission spectroscopy (ARPES) to study the evolution of electronic structure of NaFe1-xoxAs from an optimally doped superconducting compound (x= 0.028) to a heavily overdoped nonsuperconducting one (x= 0.109). Our data suggest that the Co dopant in NaFe1-xCoxAs supplies extra charge carriers and shifts the Fermi level accordingly. The overall band renormalization remains basically the same throughout the doping range we studied, suggesting that the local magnetic and electronic correlations are not affected by carrier doping. In the x= 0.109 compound, the holelike bands around the zone center Γ move to deeper binding energies and an electron pocket appears instead, resulting in a Fermi surface topology similar to that of AFe2-ySe2 (A= K, Cs, Rb, T1). Our data suggest that a balance between itinerant properties of mobile carriers and local interactions plays an important role for the superconductivity in these materials.In Chapter 4, we studied the electronic structure of NaFe1-xCuxAs (x= 0.019,0.045,0.14). With increasing the doping concentration, we found that the Cu dopant introduces extra charge carriers. The overall band dispersions barely change with doping, suggesting that the Cu substitution does not affect local correlations. Similar to the case of NaFe1-xCoxAs, one electron pocket emerges at the Brillouin zone center at high doping levels. Moreover, the near-EF spectral weight decreases with increasing the Cu dopant, which explains why the NaFe1-xCuxAs shows a poor electronical conductivity at high doping levels.