Single Crystal Growth And High Pressure Raman Study Of Ti Doped Topological Insulator Bi₂Se₃ And Cuprate Superconductor Nd_1.87Ce_0.13CuO₄

Topological insulators and cuprate superconductors,as new quantum materials,have attracted much attention in scientific community since their discovery,mainly because of their unique physical properties different from those of traditional insulators and/or superconductors.For example,topological insulators have an non-energy gap metal surface state with electron spins being in opposite directions,while their inner body is insulating and has an energy gap.The surface electronic states of topological insulators are protected by time inversion symmetry and are not affected much by impurities or defects in the body,so that electrons can conduct on the surface without scattering or energy consumption.Different from topological insulators,cuprate superconductors are derived from antiferromagnetic insulators,with no topological surface state,but they become high-temperature superconductors（nontraditional superconductors）when dopped certain amount of electrons and/or holes.Therefore,they are of great scientific value in fields of electronics and energy.Bi₂Se₃ and Nd2-xCexCuO₄ are proved to be this type of materials that have novel properties.The main work of this thesis is to study the Raman spectroscopy of Ti doped topological insulator Bi₂Se₃ and Nd doped cuprate superconductor Nd_1.87Ce_0.13CuO₄ on basis of our high quality single crystal growth.It consists of four major parts.In the first part,the growth of new topological materials was explored.The single crystal sample of Ti_0.1Bi₂Se₃,a qualified Ti-doped topological insulator Bi₂Se₃,was successfully prepared for the first time by temperature gradient method,with the control of the experimental parameters such as doping level and temperature.The composition and quality of the samples were verified by experiments of single crystal XRD,EDXRF and HRTEM.In the second part,by controlling and adjusting sintering temperature in the double temperature zone of an sintering furnace,high quality electron doped cuprate superconducting single crystals of Nd_1.87Ce_0.13CuO₄（NCCO）was successfully grown by liquid phase transfer method.The quality of the single crystals was also measured and proved by experiments of single crystal XRD and HRTEM.The third part is for the study of Raman spectra of Ti_0.1Bi₂Se₃ under high pressure.The Raman frequency-intensity curves of Ti_0.1Bi₂Se₃ single crystals are found to be similar to those of NbxBi₂Se₃ and SrxBi₂Se₃.However,under high pressure,the spectral lines show blue shift with the increase of pressure,and the spectral line shape and spectral line width（FWHM）have a significant change at 14.6 GPa.This indicates that Ti_0.1Bi₂Se₃ has undergone high pressure crystal structure phase transition.When the pressure is released,the experimental results show that the crystal structure phase transition at 14.6 GPa is reversible,and a pressure delay effect exists in the process of pressure release.In the fourth part,the Raman spectra of Nd_1.87Ce_0.13CuO₄ under high pressure were studied.By the measurement of Raman frequency,it was found that within high pressure 18 GPa,there is no new spectrum peak appearance,the spectrum intensity of Nd_1.87Ce_0.13CuO₄ decreases slightly at high frequencies,and the change of Raman shift is not obvious.This indicates that Nd_1.87Ce_0.13CuO₄ exhibits rather strong structure stability under high pressure below 18 GPa.By the high pressure Raman spectroscopy experiment,different spectral properties of Ti_0.1Bi₂Se₃ and Nd_1.87Ce_0.13CuO₄ under high pressure are shown.The former exhibits crystal structure phase transition under high pressure,while the latter shows a rather stable structure.The Raman shift of the former increases with the increase of pressure,while that of the latter decreases with the increase of pressure.This difference indicates that their internal intermolecular forces are affected differently by external high pressure.In addition,the spectral linewidth of the Raman spectrum of Nd_1.87Ce_0.13CuO₄ has a linear relation with high pressure,while the one for Ti_0.1Bi₂Se₃ shows more complex.These results provide important reference for future theoretical and experimental studies.