唯一尖晶石氧化物超导薄膜多维电子态相图的研究

Among hundreds of spinels, LiTi2O4 (LTO) is the only known spinel oxide superconductor. Its superconducting transition temperature (Tc) is ~ 13 K and the superconductivity is mainly dominated by 3d electrons of Ti atoms. Except the cuprate superconductors and iron-based superconductors, LTO possesses the highest Tc in all the transition-metal oxide superconductors. Accordingly, the understanding of superconductivity in LTO is complementary to those in high-Tc cuprates and ironpnictides. However, the lack of high quality single crystal samples inhibits the approach to the nature of its superconductivity. Recently, our group succeeded in synthesizing high quality LTO thin films by pulsed laser deposition, and a thorough study of electrical transport and tunneling spectroscopy had been conducted on (001) LTO films. Consequently, we reported the first complete electronic phase diagram of LTO superconductors, for which the community has been yearning since 1973. Nevertheless, the subsequent experimental results led to more puzzles: 1) The mixture of Ti3+ and Ti4+ is good for superconductivity, but pure Ti3+ or Ti4+ in LTO leads to a transition to insulator; 2) There is remarkable difference between tunneling spectra along the [001] direction and these along the [111] direction; 3) Its neighbor LiV2O4 belongs to rare 3d heavy fermion systems; 4) The in-plane angular dependent magnetoresistance exhibits a two-fold anisotropy, and occasionally a four-fold symmetry below Tc; and etc. Therefore, a comprehensive picture of LTO is urgent, by tuning oxygen/lithum, carrier numbers, and so on. In this project, we will carry out more careful electrical transport measurements as well as the point contact spectra on both (001) and (111) LTO films. We will develop thermal transport probes like Nernst and Seebeck, which are more sensitive to the energe dependence of density states near the Fermi level. Routine collaborations with several teams are also expected, such as Raman, ultrafast optical spectroscopy, TEM. Undoubtedly, our expertise in fine controlling the growth of high quality LTO thin films on substrates of different kinds and directions can guarantee a step closer to the mechanism of superconductivity in LTO.

LiTi2O4（LTO）是唯一的尖晶石结构氧化物超导体。除铜氧化物和铁基体系外，LTO在3d过渡金属氧化物中拥有最高的超导转变温度（~13K），因此对比研究其超导电性有助于加深对高温超导机理的认识。但该体系一直缺乏高质量单晶样品且退化效应明显，没有完整电子态相图的报道。最近，本项目团队通过脉冲激光沉积技术获得高质量稳定的[001] 取向LTO薄膜，通过电输运和点接触隧道谱研究获得该体系第一幅完整的电子态相图。但进一步，该体系的费米面是各向同性吗？锂离子调控如何影响超导？自旋、轨道等与超导有什么关系？深入探究这些关键科学问题必将推动对3d电子超导的全面认识。本项目将在前期工作基础上进一步生长不同取向的LTO薄膜，通过多场调控（磁场、电场等）下多类输运手段构建LTO的高维相图，揭示其费米面结构、自旋/轨道相关态以及Li离子浓度对超导的影响，为进一步对比理解高温超导奠定基础。

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，并开展了以下研究工作：（1）通过研究不同取向LiTi2O4的隧道谱，首次揭示出该体系存在明显的各向异性电声耦合，实验证实其来源于氧空位增强的Jahn-Teller畸变；（2）通过生长条件控制，获得了从LiTi2O4-δ到Li4Ti5O12的一系列薄膜，发现随着氧压的增加，欠氧样品先后发生补全氧空位和超导-绝缘转变两个过程；（3）通过精确控制薄膜中的氧含量，实现了LiTi2O4-δ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；（4）利用离子液体调控技术对LiTi2O4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；（5）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。本项目的成果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。