基于尖晶石超导氧化物的异质结输运性质及其电场调控研究

LiTi2O4 (LTO) is the only known spinel oxide superconductor, the superconductivity of which is mainly dominated by 3d electrons of Ti atom. Compared to copper-oxide superconductors and iron-based superconductors, LTO has distinct crystal structure and magnetic structure. The research of its transport properties and related electric-field manipulation effects, is not only helpful to supplement the understanding of superconductivity in 3d transition metal oxides, but can also open an attractive way for the potential application in novel information storage devices and energy source materials. In this proposed project, we will employ the technique of pulsed laser deposition to synthesize high quality LTO epitaxial thin films on PMN-PT piezoelectric single crystal substrates with different orientations. Based on these high quality samples, deeply researches on electrical transport and electrical control effect will be carried out, so that we can achieve the electric-field manipulation of transport properties of LTO thin films mediated by epitaxial strain. At the same time, we will prepare the heterostructures combined LTO and CeO2 (or other oxide insulator). The oxygen content in LTO layer can be mediated by resistive switching effect so that the nonvolatile manipulation transport properties of LTO layer can be achieved. This research can further establish the electronic phase diagrams of LTO, which will lead to comprehensive and embedded studying the superconducting mechanism in LTO.

LiTi2O4（LTO）是迄今发现的唯一尖晶石氧化物超导体，其晶体结构和磁结构等与钙钛矿结构铜氧化物超导体不尽相同，研究其输运性质的电场调控效应不仅对理解该体系的电子能态结构及其演化规律、建立完善的高温超导图像具有重要的科学意义，在信息、能源等领域也有潜在的应用前景。本项目（1）将在研究组前期工作的基础上，在不同取向的PMN-PT压电单晶衬底上生长LTO薄膜，通过外延应变为媒介，实现电场对LTO薄膜输运性质的调控；（2）构建LTO与CeO2（或其他氧化物绝缘体）的异质结，通过电致阻变效应调节LTO薄膜中的氧含量，从而调控LTO薄膜的输运性质，为进一步完善LTO电子能态结构图像、理解LTO的超导机理奠定基础。

LiTi2O4（LTO）曾被认为是近半个世纪以来唯一被发现的尖晶石氧化物超导体，类似已发现的铜氧化物高温超导体和铁基高温超导体，其超导电性同样来源于3d电子，对比研究其超导电性有助于加深对高温超导机理的认识。项目团队在前期给出LTO首幅电子态相图并给出轨道相关态存在的证据的基础上，进一步系统研究了高质量LTO薄膜的制备工艺，对其物性进行外场调控研究，取得了以下研究进展：.（1）通过精确控制薄膜中的氧含量，实现了LTO薄膜上临界磁场超过100%的提升（11 T→26 T），并揭示这一提升是由于轨道拆对为主过渡到自旋拆对效应，这对超导材料临界参数优化具有借鉴意义；.（2）通过人工设计多层膜结构，抑制轨道有序，在MgTi2O4中实现体超导电性（Tc ~ 5 K，且有提升空间），这是在物态丰富的尖晶石氧化物家族中发现的第二例超导体；.（3）利用离子液体调控技术对LTO进行了精细的电子掺杂和空穴掺杂，建立了该体系穹顶型电子态相图，并在重电子掺杂和重空穴掺杂区域分别观察到超导-绝缘转变，发现电子掺杂的LTO与轨道有序绝缘体MgTi2O4的高度相似性；.（4）通过晶向调制，在(011)取向的MAO上获得了外延稳定的新型正交的Mg: Ti9O10超导体。.本项目的研究进展增进了对过渡金属氧化物超导体多维电子态相图的理解和认识，也为研究竞争序与超导电性的竞争关系提供了平台。