Exotic Quantum Responses in Complex Oxide Thin Films

非技术性描述。该项目探索强关联电子系统中的本征电子和光学性质。与目前用于信息技术的半导体不同，这些材料系统中的电子是强耦合的，并共同对外部刺激做出反应，从而提供了超出传统半导体材料的反应。通过改进混合分子束外延技术制备出缺陷浓度极低的钒钛酸盐薄膜，探索了这些超纯材料的电学性质。通过生长实验合成了新的绝缘体BiVO_3薄膜，并最终合成了四元化合物Bi1-xSrxVO_3。其目的是证明在这些材料中可以实现电场对电子性质的控制。对应变薄膜的掺杂进行了探索，从实验上证实了利用外延应变可以诱导和提高超导转变温度。这项研究预计将产生重大的科学和技术影响，为可用于传感器、用于高性能能效计算的逻辑器件和量子计算的电子相变材料提供全新的见解。该项目为研究生和本科生提供电子相变材料薄膜合成和表征领域的多学科培训机会。外展活动针对的是K-12的受众，以培养他们对科学技术的好奇心。该项目致力于与合成钙钛矿结构复合氧化物薄膜相关的生长动力学基础研究。生长实验旨在利用一种组合生长方法，即传统的分子束外延和化学束外延，来绘制获得三元氧化物BiVO_3的自调节生长模式的条件。提出了一种自调节生长BiVO_3的生长策略，使四元化合物Bi1-xSrxVO_3的薄膜合成成为可能。采用角度分辨光电子能谱、透射电子显微镜、扫描隧道显微镜、随温度变化的磁输运和非线性光学光谱等研究方法对四元体系进行了分析。对固溶体Bi1-xSrxVO3电学和光学性质的深入分析，在强关联钒酸盐材料体系中开辟了一个全新的设计空间，其中电场对填充金属到绝缘体转变的量子临界点附近的电子性质的控制是内在的。对应变量子顺电材料SrTiO_3和CaTiO_3的施主掺杂进行了详细的研究，以探索在CaTiO_3中是否可以诱导超导相，以及如果利用外延应变在更高的温度下稳定量子临界行为，是否可以提高SrTiO_3的超导转变温度。该项目的首要目标是探索表现出强电子关联效应的复杂氧化物中的新量子相，并了解它们的奇异响应。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Nontechnical description. The project explores the intrinsic electronic and optical properties in strongly correlated electron systems. Unlike in semiconductors, which are currently utilized in information technology, electrons in these material systems are strongly coupled and collectively respond to external stimuli, thus offering responses that are beyond conventional semiconductor materials. Enabled by the improvement to synthesize vanadate and titanate thin films with exceptionally low defect concentration by hybrid molecular beam epitaxy the electronic properties of these ultrapure materials are explored. Growth experiments are performed to synthesize films of the newly proposed insulator BiVO3 and ultimately the quaternary compound Bi1-xSrxVO3. The goal is to demonstrate that electric field control over electronic properties can be realized in these materials. The doping of the strained thin films is explored to experimentally confirm that the superconducting transition temperature can be induced and enhanced by utilizing epitaxial strain. The research is expected to have significant scientific and technological impact, by providing fundamentally new insights into electronic phase transition materials that can be used in sensors, logic devices for high performance power efficient computing, and quantum computation. The program provides multidisciplinary training opportunities to graduate and undergraduate students in the area of thin film synthesis and characterization of electronic phase transition materials. Outreach activities target K-12 audiences to nurture their curiosity in science and technology.Technical description. The project focuses on fundamental studies of growth kinetics relevant to the synthesis of complex oxide thin films with perovskite structure. Growth experiments aim to map conditions to access a self-regulated growth mode for the ternary oxide compounds BiVO3 using a combinatorial growth approach, i.e. conventional molecular beam epitaxy and chemical beam epitaxy. A growth strategy is developed for the self-regulated growth of BiVO3, which enables the thin film synthesis of the quaternary compound Bi1-xSrxVO3. A collaborative research approach is taken to analyze the quaternary system using angle resolved photoemission spectroscopy, transmission electron microscopy, scanning tunneling microscopy, and temperature dependent magnetotransport and nonlinear optical spectroscopy. The in-depth analysis of electronic and optical properties of the solid solution Bi1-xSrxVO3 opens up an entirely new design space in the strongly correlated vanadate material system, in which electric field control over electronic properties near the quantum critical point of the band-filled metal-to-insulator transition is inherently built-in. Detailed donor doping studies of the strained quantum paraelectric SrTiO3 and CaTiO3 are conducted to explore whether a superconducting phase can be induced in CaTiO3, and if the superconducting transition temperatures in SrTiO3 can be enhanced if the quantum critical behavior is stabilized at higher temperature using epitaxial strain. The overarching goal of the project is to explore new quantum phases in complex oxides exhibiting strong electron correlation effects and to understand their exotic responses.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.