The Study Of The Electronic Structure And T_c On The Electron Doped Superconductor Nd_1.85-xLn_xCe_0.15CuO₄ (Ln=Gd And Sm)

Transition metal compounds is a main topic in condensed matter physics for their interesting and abnormal properties, such as high-TC superconductivity, XPS et al, which have attracted much attention of researching. But by now there are no mature theories can explain such character.Rare-earth cuprates of composition R2CuO4 (R=Pr, Nd, Sm, and Eu) with the tetragonal T'structure play a unique role among cuprates, becoming so-called N-type superconductors when suitably doped. Since element substitutions have displayed great importance in the investigation on the physical properties and mechanism of high-TC superconductivity, much work has been done in the N-type superconductors with doping in the building block in order to know the origin of the suppression of TC, but by now no consistent result has been achieved.In chapter 1, we have reviewed the structure and phase diagram of the high-TC superconductors and listed some strange characters of the high-TC superconductors, such asρ.In chapter 2, we introduced the X-ray-photoelectron spectroscopy (XPS) theory and its qualitative analysis et al., and we also summarized the XPS of the high-TC superconductors.In chapter 3, we presented the method of the growth of single-crystal ( Nd_1.85-xLn_xCe_0.15CuO_4-δ(Ln=Sm,Gd)) .In chapter 4, we performed the resistivity experiments and XPS measurements on Nd_1.85-xLn_xCe_0.15CuO_4±δ (Ln=Gd and Sm) with the doping in the building block. The motivations is to study the evolution of the TC and the electron structure with the doping, and to investigate whether the electron states density contributes to and sheds light on the origin of the suppression of TC. Our particular interests are the effect of Gd and Sm substitution on the core level of Cu and its valance states. It is found that the fraction of monovalent of Cu does not change with the increasing of Gd or Sm content, which indicates that such doping does not change the doped carrier density. Gd doping leads to the enhancement of the density of states at the 6-8 eV for superconducting Nd_1.85-xGd_xCe_0.15Cu_4±δ, which could arise from the Gd 4f signal, and Sm doping makes the density of states around 4 eV increased, which could results from the Sm 4f signal. Moreover, a weakness in the density of states at the Fermi level is observed for Gd doping, whereas, the changes are not observed for Sm doping. We infer that Gd doping induces more disorder than Sm doping, and the more disorder localizes the electron carriers, making the density of states decreased at the Fermi level. In resistivity experiments, we also find that for Gd doping the superconducting TC is strongly depressed with increasing doping concentration, while for Sm doping TC is nearly unchanged with the increasing of Sm content. Based on that we ascribe the suppression of TC to the more localization induced by Gd doping.