The Preparation And Study On Physical Properties Of Transition Metal Oxide Superconducting Films

Among transition metal oxides,LiTi₂O₄ is the only spinel oxide superconduc-tor possessing superconducting transition temperature?13 K.The main work of this paper includes the preparation of a series of high-quality LiTi₂O₄ thin films by pulsed laser deposition?PLD?technique via varying substrate material,sub-strate orientation,growth temperature and oxygen pressure.The structures and electrical transport properties of LiTi₂O₄ films were also studied as well.Anoth-er work is to prepare SrCoO_2.5/La_2-xCe_xCuO_4±??LCCO?multilayer films,and study the effect of annealing and strain on electron-doped cuprates LCCO films.The main contents are as follows:?1?The high-quality epitaxial LiTi₂O₄ films were prepared on SrTiO₃,KTaO₃ and MgAl₂O₄ substrates of different orientations by PLD technique,and their elec-trical transport properties were studied in detail.We find that when the tempera-ture is less than 100 K,the angle dependence of in-plane magnetoresistivity shows a twofold symmetry.In addition,the sign of magnetoresistivity?MR?changes from positive to negative at 50 K.We suggest that the positive MR origins from the orbit-related state because the magnetic field may cause the fluctuation of the orbital magnetic moment in the antiferromagnetic orbital order background,thus enhancing the electron scattering.Once the temperature is higher than 50 K,the local spin fluctuations of the LiTi₂O₄ samples are suppressed by the magnetic field,which weakens the scattering.The electrical transport results of[110]and[111]epitaxial films are consistent with those of[001]films.?2?A series of high-quality LiTi₂O₄ films have been prepared by PLD under different oxygen pressures in growth.Their structures and electrical transport results were analyzed as well.With the increase of oxygen pressure,the out-of-plane?c-axis?lattice parameter decreases gradually.When the oxygen pressure reaches a critical value,the c-axis lattice does not change anymore,which implies that a new phase starts to form.Electrical transport results show that there is a transition from the superconductor to the insulator with the increase of oxygen content.We thus confirm that the new phase is Li₄Ti₅O₁₂.As seen from the spherical aberration corrected transmission electron micrograph,there are plenty of oxygen vacancies in the LiTi₂O_4-? sample while no vacancies are obviously observed in the Li₄Ti₅O₁₂ phase.Therefore,we conclude that the process of superconductor-to-insulator transition is related to not only the content of Li ions,but also the generation of Li₄Ti₅O₁₂ along with the filling of oxygen vacancy.?3?The SrCoO_2.5/La2-xCexCuO_4±??LCCO?multilayer films have been pre-pared on SrTiO₃ substrate by PLD.As shown in the XRD results,the structure of the LCCO layer changes from T' phase to T phase with increasing the number of interfaces.The resistance of the samples also shows a transition from the super-conductor to the insulator.Because the in-plane lattice constant of LCCO is large than that of SrCoO_2.5,the in-plane compressive stress to LCCO layers increases gradually with the increase of interfaces.The apical oxygen becomes more stable so that the LCCO film is formed in T phase and not affected by annealing in vacuum.