The Study Of The Infrared Spectra On The Electron Doped Superconductor Nd_1.85-xR_xCe_0.15CuO_4±δ (R=Gd And Sm)

Transition metal compounds become one of main and hot topic in condensed matter physics for their interesting and abnormal properties, such as high-TC superconductivity, IR et al, which have attracted much attention of researching. By now, there are still no mature theories can explain such singular natures of the compoundsThe element substitutions have played an important role on the investigation of the physical properties and mechanism of high-TC superconductivity. Much work has been focus on the N-type superconductors with doping in the building block in order to find the origin of the suppression of TC。In this dissertation, we have done some research about X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectra, and the in-plane Resistivityρof Nd_1.85-xR_xCe_0.15CuO_4±δ (R = Gd and Sm) single crystals.In chapter 1, we have described the structure and phase diagram of the high-TC superconductors. Their anomalous properties and the recent development in experimental and theoretical physics about this topic are briefly discussed. The effects of element-substitution on the physical properties of high-TC superconductors are also been mentioned.In chapter 2, we have reviewed the principles and analysis of infrared spectrum. We also discussed the development and the present situation of IR research.In chapter 3, we present the method of the growth of Nd_1.85-xR_xCe_0.15CuO_4±δ (R = Gd and Sm) single crystals. There are no any impurity peaks in the X-ray diffraction patterns of Nd_1.85-xR_xCe_0.15CuO_4±δ (R = Gd and Sm) single crystals and the full width at half maximum (FWHM) are very small, which indicates that all the samples are of high quality and lay an important foundation for our further study.In chapter 4, The X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectra, and resistivityρare studied on Nd_1.85-xR_xCe_0.15CuO_4±δ (R = Gd and Sm) single crystals. The motivation is to study the evolution of the infrared spectra with increasing doping content and to investigate whether the infrared spectra could contribute to shed light on the origin of the depression of TC. The X-ray diffraction patterns of Nd_1.85-xR_xCe_0.15CuO_4±δ (R = Gd and Sm) single crystals show that there are no any impurity peaks in the experimental rang and the full width at half maximum (FWHM) are very small, which indicates that the samples are of high quality. The lattice constant c for Gd substitution decreases more sharply with increasing the doping concentration than that for Sm doping. The transmission peaks shift to high frequency for Gd doping but nearly unchanged for Sm doping by comparing the infrared spectra of Nd_1.85-x Gd_xCe_0.15CuO_4±δ with Nd_1.85-xSm_xCe_0.15CuO_4±δ single samples, which indicates that substituting Nd with Gd will make the Cu-O distance short, while the Cu-O distance is nearly unchanged when we substitute Nd with Sm. The decrease of lattice constant c and the Cu-O distances make the volume of the unit cell V decrease more sharply for Gd doping than that for Sm doping, which in turn makes the density of states at EF decrease faster for Gd doping than that for Sm doping, so that a fast change of suppression rate of TC for Nd_1.85-xGd_xCe_0.15CuO_4±δ compared to Nd_1.85-xSm_xCe_0.15CuO_4±δ become possible. We have compared the superconducting transition temperature TC versus doping concentration of Nd_1.85-xGd_xCe_0.15CuO_4±δ and Nd_1.85-xSm_xCe_0.15CuO_4±δ single crystals, the superconducting transition temperature TC for Nd_1.85-xGd_xCe_0.15CuO_4±δ and Nd_1.85-xSm_xCe_0.15CuO_4±δ systems are all suppressed with increasing the doping content. However, the superconducting TC is strongly depressed for Gd doping, and the TC decrease slowly for Sm doping. This trend is very corresponding to the change of the volume of the unit cells and the change of density of states at EF mentioned above for two series.