Electron-doped Copper Oxide High-temperature Superconductors The Ground State Phase Diagram

The study of the electron-doped cuprates has attracted much attention recently. Compared with the hole-doped ones,the electron-doped cuprate high temperature superconductors(HTSC) show a lot of outstanding differences.Among these,the asymmetry of the phase diagram of the hole-and electron-doped HTSC is one of the notable questions.Especially for the electron-doped cuprates such as NCCO,the antiferromagnetic(AF) insulators remain up to the optimal doping,while the superconducting(SC) order exists only in the narrow region around the optimal doping. Now it is believed that the study of the electron-doped cuprates,especially the well understanding the electron-hole asymmetry,will help us to understand the anomalous properties of the HTSC comprehensively,and shed light on the mechanism of the HTSC untimately.In this thesis,within the extended t-J-U model and the Gutzwiller mean field approximation,the ground state phase diagrams of electron-doped cuprates are studied systemically.The thesis is organized as follows.In chapterⅠ,a brief review about the main and latest experimental results of electron-doped cuprates.In chapterⅡ,firstly,we introduce the t-J-U model,which is a widely accepted model to describe the low-energy physics in the CuO₂ planes;Then we introduce the Gutzwiller mean field approximation which is believed that can treat the strong correlation between the electrons properly.Within the t-J-U mode and Gutzwiller mean-field approximation,the phase diagrams of electron-doped cuprates are studied in ChapterⅢ:Firstly,using the typical experimental data t/J=-2.5,t′=-0.34t,t″=0.23t,which is adopted from the experimental results of NCCO,we get the phase diagrams self-consistently for two different U:U=30t and U=12t.Our results show that,in the large U limit,the AF order parameter is robust until the optimal doping,which is consistent with the real situation.With the decreasing of the U,the AF order parameter shrinks rapidly,and at the same time,the SC order parameter is depressed obviously.This result means electron-doped cuprates are also strong correlating system.Secondly,with the typical parameter t/J=-3,t′=-0.32t,t″=0.16t generally,we calculate the ground state energy and the phase diagrams self-consistently. Our results show that the AF order coexists with the SC order in the large doping region. Our results are agreement with the recent numerical calculations.