He元素掺杂调控复杂氧化物外延薄膜中的单轴应变及相关物性研究

Strain engineering in epitaxial thin films has been demonstrated as one of the effective means for quantum manipulation. In this project, we will develop a new technique for He doping in oxide epitaxial films via helium/argon co-sputtering process, and then study uniaxial strain effects on the macroscopic properties through independent adjustment of the out-of-plane lattice in the films. Focusing on three typical complex oxides, we will carefully distinguish the distinct roles played by the lattice degree of freedom in each system based upon appropriate mechanisms. In doped manganies displaying colossal magnetoresistance, by controlling Janh-Teller splitting of the degenerate eg electron energy level through the uniaxial strain, abundant performance resulting from the strong competition and subtle balance between the localization and itineration of eg electrons will be investigated. In superconducting cuprates, by changing the out-of-plane lattice in the a- and c-oriented epitaxial films, we will deeply observe the effects of anisotropic Cu-O bond lengths, within and out of the CuO2 planes respectively, on the superconductivity. For multiferroic orthorhombic manganites, we will adjust the out-of-plane a-, b- and c-axes respectively in the epitaxial films with various orientations, and distinguish the effects of three anisotropic super exchange interactions and their different principles of action on the magnetic orders and spontaneous polarizations in the system. The results will be helpful to the comprehensive and in-depth understanding of the strong competition and subtle balance between multiple degrees of freedom, especially the role of lattice degree of freedom, in complex oxides. Furthermore, exploration of the new technique for He doping in oxide epitaxial films will surely enrich the technical means for strain engineering.

外延薄膜的“应变工程”是有效的量子调控手段之一。本项目研发采用氦/氩气共溅射实现氧化物外延薄膜中He掺杂的新工艺，通过单独调整薄膜面外晶格常数研究单轴应变对宏观物性的影响。针对三种典型的复杂氧化物体系，考察晶格自由度通过相应机理所扮演的各种角色。在庞磁电阻掺杂锰氧化物中，通过单轴应变控制eg能级的Janh-Teller劈裂，研究eg电子的局域化和巡游性的平衡关系所带来的丰富表现；在超导铜氧化物中，通过改变a-和c-轴取向薄膜的面外晶格常数，观察CuO2面内和面外各向异性Cu-O键长的变化对体系超导电性的影响；在多铁性正交锰氧化物中，调整不同取向薄膜的面外a-、b-和c-轴，明晰三种各向异性超交换作用对磁有序形态和自发电极化的各自影响等。研究结果将有助于系统、深入理解复杂氧化物体系中多自由度的竞争和平衡关系，特别是晶格自由度所起的作用，并且He掺杂新工艺的探索将丰富“应变工程”的技术手段。

单轴应变调控是外延薄膜应变工程中的一个挑战。由于材料的弹性泊松效应，选择不同单晶基片调整面内双轴应变的同时不可避免地带来面外晶格的相应变化，无法区分单一维度上的效应。本项目探索采用常规磁控溅射技术通过He元素掺杂的方法，实现氧化物外延薄膜面外晶格的独立调控。采取氩/氧/氦混合气氛制备薄膜的工艺，我们发现薄膜面外晶格常数随气氛中氦气比例升高而明显增大。经过对比实验观察，确认加速He+离子透射进入阴极靶材表层是实现沉积薄膜中He掺杂的关键因素。在此基础上采用氦气对阴极制备态薄膜放电的工艺，使La0.7Ca0.3MnO3/SrTiO3外延薄膜的面外晶格伸长0.7%以上。通过独立调控面外晶格，定量研究了掺杂锰氧化物薄膜中体应变和双轴应变（Jahn-Teller畸变）对双交换作用的不同效应。结果表明，双轴应变总是对双交换作用表现出相当强的抑制，而体应变的作用则因料而异。在宽带宽La0.7Sr0.3MnO3材料中，膨胀的晶胞对双交换作用略有抑制，相反在中带宽La0.7Ca0.3MnO3中则是明显有利的。研究了有些La0.8Ca0.2MnO3外延薄膜表现出异常高的转变温度的现象，发现尽管该体系中的双轴应变和体应变效应都比钙0.3最佳掺杂的大2-3倍，但不足以导致该现象，认为薄膜沉积过程中的少量La缺位是可能原因。采用溶胶-凝胶法在LaAlO3单晶基片上制备La0.8Ba0.2MnO3超薄膜，发现其显著的氧活性，进一步研究表明这主要来自晶界的变化。晶界剩余电阻的变化导致低温下明显的电致电阻效应，并在小电流下出现75%的正磁电阻效应。这些独特现象与该超薄膜纳米尺度迷宫状形貌有密切关系。本项目开发常规磁控溅射技术实现外延薄膜He掺杂的新工艺，为薄膜应变工程提供了实现单轴调控的新方法。研究成果有助于深化理解体应变和双轴应变效应对掺杂锰氧化物薄膜中序参量的作用机理，新发现的阻变现象为随机存储器和神经形态仿生应用等方面的研发提供了更多选择。