Controlling and quantifying two-level systems, disorder and ideality in tetrahedrally bonded amorphous thin films

非技术摘要在凝聚态物理领域，与晶体相比，缺乏结构有序的材料--被认为是“非晶态材料”或“玻璃”--相对没有被探索过。晶体由空间重复的原子组成，这使得可以使用简单的数学形式来计算和预测这些系统的性质。然而，非晶态系统没有这样的结构重复性，因此人们对其了解较少。然而，这种缺乏理解并不排除无序系统的适用性或科学影响；塑料、硅酸盐玻璃和非晶硅光伏都是与日常生活、工业和技术相关的例子。非晶态超导体是一个非凡的例子，说明了一种基本的科学性质是如何超越结构缺陷的。无序材料的性质在很大程度上取决于材料是如何产生的，但目前还不清楚如何描述不同方法产生的不同非晶态结构，即使是对于单一元素的材料，也不清楚完全无序材料中缺陷的性质是什么。显然，无序存在于不同的长度和能量尺度上，从局部的原子大小的无序到更大的尺度。有趣的是，存在着“理想玻璃”的概念，它虽然保持着彻底的无序，但在这种无序中缺乏缺陷，从而接近晶体的独特性，包括其性质的再现性和可预测性。该项目将确定不同制备方法和不同原子产生的无序和缺陷类型之间的关系，以及无序材料的“理想性”的可调性。这样的确定将产生对具有技术和基础科学意义的无序材料的更好的理解和控制。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和研究生为女性，并积极参与努力，使未被充分代表的STEM族裔和社会经济少数群体能够接触到物理学。技术摘要长期和当前感兴趣的问题是非晶态或玻璃态的热力学性质。结构有序的缺乏使得这些系统在数学上不那么容易处理，并使解决无序如何影响热力学性质成为一个挑战。能量格局中的局域和全局极小值(KT)与构型熵有关。理想玻璃具有较低的构型熵，接近于结晶玻璃的构型熵，因此意味着存在一种独特的无序状态，没有缺陷。小得多的局域极小(KT)产生了玻璃的反常低温性质，隧道效应或二能级系统(TLS)很好地描述了这些性质，它们被广泛认为是普遍存在的，尽管对于造成这些小尺度极小的原因存在分歧。更具争议性的是低温和高温热力学性质之间的联系(如果有的话)。近年来，发现了低温普遍行为的显著例外，这表明存在不同类别的无序。一个相关的问题涉及非晶态系统中缺陷的性质和可能的相互依赖；例如，在非晶硅中，TLS和悬挂键都是已知存在的，它们依赖于原子密度，但不直接相关。这种(和其他)四面体键合材料与传统研究的玻璃有根本的不同；它们不能从液体状态淬火，它们的四面体键合导致了一个过度约束的连续随机网络，以及为常规量热法大量生产的困难以前阻碍了大多数热力学测量。有趣的是，这些正是最容易通过气相沉积工艺制成薄膜的材料。不同的生长技术将被用来生产四面体结合材料的薄膜，以研究TLS与密度/结构和无序内的理想性之间的联系；该项目将检验理想玻璃没有TLS或缺陷的假设。使用独特的膜基纳米量热计，将在0.1-1000K的宽温度范围内测量热容和热力学性质。这个温度范围涵盖了非晶硅在低T(1K)下的TLS和所提出的高T玻璃化转变温度。这些量热计极快的加热(10^5 K/秒)和冷却(10^4 K/秒)速度和较宽的温度范围允许进行这些以前不可能进行的实验，包括测定零温构型熵。

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. 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. 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 ethnic and socioeconomic minorities. 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.