Controlling and Quantifying Two-Level Systems, Disorder and Ideality in Tetrahedrally Bonded Amorphous Thin Films

控制和量化四面体键合非晶薄膜中的二能级系统、无序和理想性

• 批准号: 1411315
• 负责人: Frances Hellman
• 金额: $ 16.49万
• 依托单位: University of California-Berkeley
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-07-15 至 2016-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1411315&HistoricalAwards=false
• 关键词: Controlling; Quantifying; Two; Level; Systems

Non-technical AbstractIn the field of Condensed Matter Physics, materials that lack structural order -- deemed amorphous materials or glasses -- are, compared to crystals, relatively unexplored. Crystals consist of spatially repeated atoms, which permit mathematically simple formalisms that can be used to calculate and predict properties of these systems. Amorphous systems, however, have no such structural repeatability, and thus are less understood. This lack of understanding, however, does not preclude the applicability or scientific impact of disordered systems; plastics, silicate glasses, and amorphous silicon photovoltaics are examples that are pertinent to daily life, industry, and technologies, and amorphous superconductors are a remarkable example of how a fundamental scientific property transcends structural imperfection. The properties of a disordered material depend strongly on how the material was produced, but it is not clear how to describe the different amorphous structures produced by different methods, even for a single element material, nor what the nature of a defect is in a fully disordered material, unlike crystals. It is clear that disorder exists on different length and energy scales, ranging from local, atomic-sized disorder to larger scales. Intriguingly, there exists the notion of an ideal glass, which while remaining thoroughly disordered, lacks imperfections in that disorder and thus approaches the uniqueness of a crystal, including reproducibility and predictability of its properties. The project will determine the relationship between types of disorder and defects produced by different preparation methods and for different atoms, and the tunability of the ideality of disordered materials. Such a determination will yield improved understanding and control of disordered materials of technological and fundamental scientific significance. The project will also educate and train students and help to increase diversity participation in science; the PI and graduate student are women, and actively engage in efforts to make physics accessible to underrepresented STEM groups.Technical AbstractA problem of both longstanding and current interest is the thermodynamic nature of the amorphous or glassy state. The lack of structural order makes these systems less mathematically tractable and makes it a challenge to resolve how disorder affects the thermodynamic properties. Local and global minima on a broad scale (kT) in the energy landscape are relevant to the configurational entropy. An ideal glass has low configurational entropy, approaching that of the crystalline counterpart, thus implying the existence of a unique disordered state that lacks defects. Local minima on a much smaller scale (kT) produce anomalous low temperature properties of glasses that are well described by tunneling or two level systems (TLS), which are widely considered universal although disagreement exists as to what causes these small scale minima. Even more controversial is the connection (if any) between the low and high temperature thermodynamic properties. In recent years, significant exceptions to low temperature universal behavior have been found, suggesting that different classes of disorder exist. A related question concerns the nature and possible interdependence of defects in an amorphous system; e.g. in amorphous silicon, both TLS and dangling bonds are known to exist, and are dependent on atomic density, but are not directly correlated. This (and other) tetrahedrally-bonded materials are fundamentally different than the traditionally studied glasses; they cannot be quenched from the liquid state, their tetrahedral bonding leads to an overconstrained continuous random network, and the difficulty in producing large quantities for conventional calorimetry has previously prevented most thermodynamic measurements. Intriguingly, these are precisely the materials most easily made as thin films by vapor deposition processes. Various growth techniques will be used to produce thin films of tetrahedrally bonded materials to study the link between TLS and density/structure and ideality within disorder; the project will test the hypothesis that ideal glasses do not have TLS or defects. Using unique membrane-based nanocalorimeters, heat capacity and thermodynamic properties will be measured over a wide temperature range, 0.1-1000K. This temperature range covers the TLS at low T (1K) and the proposed high T glass transition temperature for amorphous silicon. The enormously fast heating (10^5 K/sec) and cooling (10^4 K/sec) rates of these calorimeters and wide temperature range permit these previously impossible experiments, including determination of zero temperature configurational entropy.

与晶体相比，非技术抽象的凝结物理物理学领域，缺乏结构秩序的材料 - 被认为是无定形材料或玻璃的材料。晶体由空间重复的原子组成，该原子允许数学上简单的形式主义，可用于计算和预测这些系统的性质。但是，无定形系统没有这种结构可重复性，因此知之甚少。但是，缺乏理解并不排除无序系统的适用性或科学影响。塑料，硅酸盐玻璃和无定形硅光伏是与日常生活，工业和技术有关的例子，并且无定形超导体是一个重要的例子，说明了基本科学特性如何超越结构性不完美。无序材料的特性在很大程度上取决于如何产生材料，但尚不清楚如何描述不同方法产生的不同的无定形结构，即使是单一元素材料，也不清楚缺陷的性质在完全与晶体不同，材料无序。显然，疾病的长度和能量尺度的存在，范围从局部，原子大小的疾病到较大的尺度。 有趣的是，存在理想玻璃的概念，尽管玻璃仍然存在彻底的混乱，但在该疾病中缺乏缺陷，因此接近了晶体的独特性，包括其性质的可重复性和可预测性。该项目将确定疾病类型与不同制剂产生的缺陷和不同原子产生的缺陷之间的关系，以及无序材料的理想性的可调性。 这种确定将对技术和基本科学意义的无序材料产生改进的理解和控制。 该项目还将对学生进行教育和培训，并帮助增加对科学的多样性参与； PI和研究生是女性，并积极努力使代表性不足的STEM群体可以使用物理学。长期以来和当前兴趣的技术抽象问题是无定形或玻璃状态的热力学性质。 缺乏结构秩序使这些系统在数学上的处理程度降低，因此解决疾病如何影响热力学特性的挑战。 能源景观中广泛的本地和全球最小值（KT）与构型熵有关。 理想的玻璃具有较低的构型熵，接近晶体的熵，因此暗示存在缺乏缺陷的独特无序状态。较小的局部最小值（KT）产生的玻璃的异常低温特性通过隧道或两个级别的系统（TLS）很好地描述，尽管存在分歧，但这些系统被广泛认为是普遍的，这是什么导致这些小规模的最小值。更具争议的是低温和高温热力学特性之间的联系（如果有）。近年来，发现低温普遍行为的显着例外，表明存在不同类别的疾病。 一个相关的问题涉及在无定形系统中缺陷的性质和可能相互依存。例如在无定形硅中，已知TLS和悬空键都存在，并且取决于原子密度，但并非直接相关。这种（和其他）四面体键的材料与传统研究的眼镜根本不同。它们不能从液态中淬灭，它们的四面体粘结会导致过度限制的连续网络，并且难以生产大量传统量热法的困难以前已经阻止了大多数热力学测量。有趣的是，这些正是通过蒸气沉积过程最容易成为薄膜的材料。各种生长技术将用于生产四面体键合材料的薄膜，以研究TLS与疾病内密度/结构和理想性之间的联系；该项目将测试理想眼镜没有TL或缺陷的假设。使用独特的基于膜的纳米钙化器，将在0.1-1000k的宽温度范围内测量热容量和热力学特性。该温度范围覆盖低T（1K）的TLS和无定形硅的拟议高T玻璃过渡温度。这些热量计和宽的温度范围的非常快的加热（10^5 k/sec）和冷却（10^4 K/sec）允许这些以前不可能的实验，包括确定零温度构型熵。