CAREER: A Multifaceted Approach for Manipulation and Investigation of Quantum Phases and Phase Transitions in Prototypical 2-D Metallic Systems

非技术解释：通常情况下，材料会因温度变化而发生相变。一个典型的例子是水在零摄氏度下转化为冰。也有相变的例子，它完全由非热参数控制，如压力、磁场和成分无序。在二维(2-D)金属系统中，量子相变(QPTs)背后的关键原理的完整表征尚未完成，这被认为对于实现各种具有基本兴趣和潜在技术意义的材料的有趣行为至关重要，如铜酸盐和磷酸盐高温超导体、Ruthenate和重费米子。本研究项目旨在通过结合一套互补的实验技术，对模型系统中的QPTs进行基础研究。根据其对发展国家世界级科学、技术、工程和数学劳动力的坚定承诺，该项目除了支持研究生的博士学习外，还通过暑期实习为本科生和当地高中生提供实践性、研究性的教育。该项目将教育与研究相结合所促进的协同效应也有望完成一项重要任务：培养美国同步加速器研究领域的下一代科学家。这个项目的进一步教育和推广努力包括为非专家开发与先进材料表征技术有关的网络教育材料，并通过讲座演示更广泛地传播PI小组的研究活动，目标是当地的非科学受众。技术描述：对二维(2-D)金属系统相图中的量子相变(QPT)的全面实验研究通常相当复杂。这主要是因为这些系统中相互竞争的相互作用的微妙相互作用往往掩盖了相关的量子临界点，阻碍了对其量子临界性的直接实验访问。为了解决这个问题，本研究项目采用了一种简单的方法：(I)找到一个定义良好的有序态的简单模型系统，(Ii)通过改变某些非热学参数，连续地将这种态的临界温度调到零，以及(Iii)通过量子临界点直接测量各种物理性质的变化以及相应的有序参数。目前研究的是一组具有简单晶体结构和电子结构的层状过渡金属二卤化物，即2H-NbSe_2、2H-TaS_2和2H-TaSe_2。这些化合物中的每一个都通过在空间中形成具有高和低电荷密度的交替区域的波而表现出明确的相变，被称为电荷密度波(CDW)。此外，它们的CDW转变温度可以通过各种非热过程连续调谐到零。在第一个研究途径中，通过在这些材料的单晶样品中有条不紊地引入电子和结构无序，这些化合物的CDW有序将被量子力学熔化。在第二条线的研究中，单晶样品的厚度将逐渐减小，最终将它们转变为几层到单层的厚晶体。利用角度分辨光电子能谱、X射线衍射和电阻率测量相结合的方法，通过(A)通过温度-无序相图中的量子临界点揭示这些材料的结构、电学和电子性质以及它们的CDW序参数的演化，以及(B)研究量子限制对少数到单层厚样品中CDW不稳定性和相变的影响，从而获得对二维系统中量子临界性和量子相变的关键见解。在这种背景下，金属中量子临界性的精确理论处理是非常具有挑战性的，因为人们需要在相同的基础上同时考虑单粒子激发和序参数涨落。该项目中拟议的结构、光谱和传输探测器的集成为在实验中做到这一点提供了独特的机会。此外，这个项目提供了一种明确的方法来研究复杂无序对物理系统的电子和结构特性的作用。这是开发材料的核心，这种材料对外部刺激表现出强烈的增强反应，在能量和设备应用中都很有用。

Non-Technical Explanation: Commonly, materials undergo phase transitions due to change in temperature. A typical example is the transformation of water into ice at zero degrees Celsius. There are also examples of phase transitions, which are controlled solely by non-thermal parameters such as pressure, magnetic field, and compositional disorder. A complete characterization of the key principles behind quantum phase transitions (QPTs) is yet to be accomplished in two-dimensional (2-D) metallic systems, which is believed to be crucial to the realization of intriguing behaviors in various materials of fundamental interest and potential technological import such as cuprate and pnictide high temperature superconductors, ruthenates and heavy fermions. This research project aims at fundamental studies of QPTs in model systems by incorporating a set of complementary experimental techniques. In accordance to its strong commitment to the development of world-class science, technology, engineering and mathematics workforce in the country, this project, in addition to supporting the PhD studies of a graduate student, provides hands-on, research-based, education to undergraduate students at an early stage and local high school students through summer internship. The synergy fostered by this project's integration of education into research is also anticipated to accomplish an important task: preparing the next generation of scientists in the field of synchrotron-based research in the United States. Further education and outreach endeavors of this project comprise development of web based educational materials pertaining to advanced materials characterization techniques for non-experts and broader dissemination of the research activities of the PI's group through lecture-demonstrations, targeted at local non-scientific audiences.Technical Description: Comprehensive experimental investigations of quantum phase transitions (QPTs) in the phase diagrams of two-dimensional (2-D) metallic systems, in general, are rather intricate. This is mostly because the subtle interplay of competing interactions in these systems often shrouds the relevant quantum critical points stymieing direct experimental access to their quantum criticality. In order to address this issue, this research project adopts a straightforward approach: (i) finds a simple model system with a well defined ordered state, (ii) continuously tunes critical temperature of this state to zero by changing certain non-thermal parameter, and (iii) directly measures changes in various physical properties along with the pertinent order parameter through the quantum critical point. The systems of interest for the current studies are a set of layered transition metal dichalcogenides, namely 2H-NbSe2, 2H-TaS2 and 2H-TaSe2, which possess simple crystal and electronic structures. Each of these compounds exhibits a well defined phase transition via the formation of waves in space with alternating regions of higher and lower density of charges, known as a charge density wave (CDW). Moreover, their CDW transition temperatures can continuously be tuned to zero through various non-thermal processes. In the first avenue of research, CDW orders of these compounds are to be melted quantum mechanically via methodical introduction of electronic and structural disorders in single crystal samples of these materials. In the second line of research, the thickness of single crystal samples are to be gradually diminished transforming them eventually into few- to single-layer thick crystals. Using a combination of Angle Resolved Photoemission Spectroscopy, X-ray diffraction and electrical resistivity measurements, critical insights into quantum criticality and quantum phase transitions in 2-D systems are to be attained by (a) unveiling evolution of structural, electrical and electronic properties of these materials along with their CDW order parameters through quantum critical points in temperature-disorder phase diagram, and (b) investigating the impact of quantum confinement on CDW instability and phase transition in few- to single-layer thick samples. In this context, exact theoretical treatment of quantum criticality in metals is highly challenging because one needs to take into account both single particle excitations and order parameter fluctuations in the same footing. The proposed integration of structural, spectroscopic and transport probes in this project provides a unique opportunity to do so experimentally. Furthermore, this project provides a well-defined methodology to investigate the role of complex disorders on electronic and structural properties of a physical system. This is at the heart of developing materials, which exhibit strongly enhanced responses to external stimuli, useful in energy as well as in device applications.