Supplementary information files for Two superconductor-insulator phase transitions in the spinel oxide Li1±xTi2 O4-δ induced by ionic liquid gating

Supplementary files for article Two superconductor-insulator phase transitions in the spinel oxide Li1±xTi2 O4-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-δ (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.

补充文件：关于离子液体栅控下尖晶石氧化物Li1±xTi2O4-δ中两种超导体-绝缘体相变的研究。在过渡金属化合物中，涌现的物理现象（如超导性）与3d电子的轨道、电荷和自旋自由度之间的关联引人入胜。在本研究中，我们通过离子液体栅控成功操控了尖晶石氧化物Li1±xTi2O4-δ（LTO）的超导性。构建了一个穹顶形超导相图，其中在重电子掺杂和空穴掺杂区域均揭示了两种绝缘相。在空穴掺杂区域，超导体-绝缘体转变（SIT）可归因于Ti价电子的缺失。在电子掺杂区域，尽管载流子密度增加，但LTO却表现出出乎意料的SIT而非金属行为。此外，在正常状态电阻曲线中观察到热滞后效应，暗示着一级相变。通过将LTO的输运和结构结果与另一种尖晶石氧化物超导体MgTi2O4（MTO）进行比较，并分析电子结构的第一性原理计算，我们推测SIT和热滞后效应源于3d电子相关性的增强以及通过轨道有序化。进一步理解超导性与轨道有序化之间的细致相互作用，将有助于揭示非传统超导配对机制。