Exotic properties of spinel oxide superconductor Li_1+xTi ₂O₄

The polycrystalline Li_1+xTi₂O₄ has intricate physical behaviors in normal and superconducting state. High-resolution X-ray diffraction shows that it is the pure spinel phase and there is no structural phase transition down to 20 K. In addition to the field-induced broadening of R(T) near the superconducting regime, an anomalous upturn in R(T) appears near the transition temperature when magnetic field is above 3 T. This anomalous resistivity upturn regime diminishes with the adding of Li, which increases the conductivity of sample. The superconductivity is suppressed at field above 14 T and temperature above 4.2 K, meanwhile the low temperature resistance behaves semiconductor-like. It resembles to the metal-insulator transition observed in La214 under high magnetic field. Furthermore, the sample exhibits other interesting features: a significant positive magnetoresistance below 40 K, e.g., ΔR(H)/R(H=0)∼3% at T=14 K and H=16 T; ZFC and FC curves of magnetization measured below 1000 G split around 170 K; and Hall measurements also show that charge carriers change sign from negative to positive around 170 K. These experimental facts suggest that magnetic property and electronic structure strongly correlate with each other.