Two superconductor-insulator phase transitions in the spinel oxide Li1±xTi2O4-δ induced by ionic liquid gating

The associations between emergent physical phenomena (e.g., superconductivity) and orbital, charge, and spin degrees of freedom of 3d electrons are intriguing in transition metal compounds. Here, we successfully manipulate the superconductivity of spinel oxide Li1 +/- xTi2O4-delta (LTO) by ionic liquid gating. A dome-shaped superconducting phase diagram is established, where two insulating phases are disclosed both in heavily electron-doping and hole-doping regions. The superconductor-insulator transition (SIT) in the hole-doping region can be attributed to the loss of Ti valence electrons. In the electron-doping region, LTO exhibits an unexpected SIT instead of a metallic behavior despite an increase in carrier density. Furthermore, a thermal hysteresis is observed in the normal state resistance curve, suggesting a first-order phase transition. We speculate that the SIT and the thermal hysteresis stem from the enhanced 3d electron correlations and the formation of orbital ordering by comparing the transport and structural results of LTO with the other spinel oxide superconductor MgTi2O4 (MTO), as well as analyzing the electronic structure by first-principles calculations. Further comprehension of the detailed interplay between superconductivity and orbital ordering would contribute to the revealing of unconventional superconducting pairing mechanism.