Novel Phases of Transition-Metal Oxides in Heterostructured and Superoxygenated Thin Films

过渡金属氧化物(TMO)是量子有序电子现象的源泉，从高临界温度(TC)超导到巨磁电阻和多铁性。这种广泛的现象反映了各种类型的电子有序之间复杂的相互作用。由于这种相互作用对化学掺杂、晶格应变和电磁场的高度敏感，所以产生了丰富的电子相图。氧化物薄膜外延生长的最新进展使得具有钙钛矿型晶体结构的不同TMO能够结合成具有原子锐化界面的异质结构。这种“乐高乐园”的氧化物合成方法允许不同TMO的不同电子相之间的相互作用可以通过改变每个组成氧化物的化学成分、长度尺度和晶格应变来调节。最近，原子尺度的电子显微镜和光谱学研究发现，在几种TMO异质结构和在超高压氧气中热处理的薄膜中，奇异结构相相互生长。这些奇异的TMO相的形成要么归因于异质外延应变，要么归因于增强氧化，这表明TMO薄膜的热力学相稳定性与块体材料有很大的不同。在这些新范式的指导下，我提出的研究目标是在异质结构和超氧薄膜中产生和检测TMOS的新的电子和结构相。我将重点介绍的材料是铜酸盐、锰酸盐、镍酸盐、Ruthenate和Irates，后者具有很强的自旋-轨道耦合，这对拓扑电子相的形成至关重要。采用脉冲激光沉积的方法制备多层和单层薄膜，并用电输运、X射线衍射和X射线吸收光谱对薄膜进行了表征。为了识别新的TMO相，还将使用几种原子尺度的显微镜和光谱探针：1)扫描电子显微镜；2)电子能量损失光谱；3)d波安德列夫反射光谱；4)扫描隧道光谱。我的总体目标是：1)通过绕过已知的热力学限制，发现具有增强的晶格复杂性和电子性质的新型TMO相；以及2)通过异质结构集成已知的TMO相来实现奇异的界面和杂化现象。对于超氧项目，一个中心目标是通过进一步增加其晶格复杂性来进一步提高铜酸盐的T_c。对于异质结项目来说，一个重要的里程碑是利用邻近效应实现自旋三重态和拓扑超导。这些新颖的TMO相和现象的成功产生和检测将使它们能够被用于量子电子器件，特别是在新兴的超导自旋电子学以及基于应变和基于拓扑的电子学领域。

Transition--metal oxides (TMOs) have been a wellspring of quantum--ordered electronic phenomena, ranging from high critical-temperature (Tc) superconductivity to colossal magnetoresistance and multiferroism. This wide range of phenomena reflects the complex interplay among various types of electronic order.  The high sensitivity of this interplay to chemical doping, lattice strain and electromagnetic fields produces rich electronic phase diagrams. Recent advances in epitaxial growth of oxide thin films have enabled different TMOs with perovskite--based crystal structures to be combined into heterostructures with atomically--sharp interfaces. This "Legoland" approach to oxide synthesis allows the interplay between different electronic phases from different TMOs to be tuned by varying the chemical makeup, length scale and lattice strain of each consitutent oxide.  More recently, atomic--scale electron microscopy and spectroscopy studies have observed intergrowths of exotic structural phases in several TMO heterostructures and in thin films annealed in ultrahigh-pressure oxygen. The formation of these exotic TMO phases is attributed to either heteroepitaxial strain or to enhanced oxidation, indicating that the thermodynamic phase stability of TMO thin films is vastly different than in bulk materials. Guided by these new paradigms, my proposed research aims to generate and to detect novel electronic and structural phases of TMOs in heterostructured and superoxygenated thin films. The materials I will focus on are cuprates, manganites, nickelates, ruthenates and iridates, the latter having strong spin--orbit coupling that is crucial for the formation of topological electronic phases. Multilayer and unilayer thin films will be grown using pulsed laser deposition, and characterized by electrical transport, x-ray diffraction and x--ray absorption spectroscopy. To identify the novel TMO phases, several atomic--scale microscopy and spectroscopy probes will also be used: 1) scanning transmission electron microscopy; 2) electron energy-loss spectroscopy; 3) d-wave Andreev reflection spectroscopy; 4) scanning tunneling spectroscopy. My general goals are: 1) to discover novel TMO phases with enhanced lattice complexity and electronic properties by by--passing known thermodynamic limitations; and 2) to realize exotic interfacial and hybrid phenomena by heterostructurally integrating known phases of TMOs. For the superoxygenation projects, a central objective is to further raise the Tc of cuprates by further increasing their lattice complexity. For the heterostructure projects, a major milestone is to achieve spin--triplet and topological superconductivity using the proximity effect. Successful generation and detection of these novel TMO phases and phenomena will enable them to be harnessed for quantum electronic devices, particularly in the emerging field of superconducting spintronics as well as strain--based and topology--based electronics.