The effects of oxygen in spinel oxide Li_1+xTi_2-xO_4-δ thin films

The evolution from superconducting LiTi₂O_4-δ to insulating Li₄Ti₅O₁₂ thin films has been studied by precisely tuning the oxygen pressure in the sample fabrication process. In superconducting LiTi₂O_4-δ films, with the increase of oxygen pressure, the oxygen vacancies are filled gradually and the c-axis lattice constant decreases. When the oxygen pressure increases to a certain critical value, the c-axis lattice constant becomes stable, which implies that the sample has been completely converted to Li₄Ti₅O₁₂ phase. The two processes can be manifested by the angular bright-field images of the scanning transmission electron microscopy techniques. The transition temperature (T _ch ) of magnetoresistance from the positive to the negative shows a nonmonotonic behavior, i.e. first decrease and then increase, with the increase of oxygen pressure. We suggest that the decrease T _ch can be attributed to the suppressing of orbital-related state, and the inhomogeneous phase separated regions contribute positive MR and thereby lead to the reverse relation between T _ch and oxygen pressure.