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

职业生涯：操纵和研究原型二维金属系统中的量子相和相变的多方面方法

• 批准号: 1454304
• 负责人: Utpal Chatterjee
• 金额: $ 52.85万
• 依托单位: University of Virginia Main Campus
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-05-01 至 2022-04-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1454304&HistoricalAwards=false
• 关键词: CAREER; Multifaceted; Approach; Manipulation; Investigation

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.

非技术性解释：通常，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变成冰。也有相变的例子，它们仅由非热参数控制，如压力，磁场和成分无序。二维金属系统量子相变的关键原理尚未完全描述，这对于实现铜酸盐和磷属元素化物高温超导体、铼酸盐和重费米子等具有重要意义和潜在技术价值的材料的有趣行为至关重要。本研究计划旨在结合一套互补的实验技术，对模型系统中的QPT进行基础研究。根据其对该国世界一流科学，技术，工程和数学劳动力发展的坚定承诺，该项目除了支持研究生的博士研究外，还通过暑期实习为早期阶段的本科生和当地高中生提供实践，研究型教育。该项目将教育与研究相结合所产生的协同作用预计还将完成一项重要任务：为美国同步加速器研究领域的下一代科学家做好准备。该项目的进一步教育和推广工作包括为非专家开发与先进材料表征技术有关的基于网络的教育材料，以及通过针对当地非科学受众的讲座演示更广泛地传播PI小组的研究活动。技术描述：二维金属体系相图中的量子相变的实验研究是相当复杂的。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经常掩盖了相关的量子临界点，阻碍了直接的实验访问其量子临界性。为了解决这个问题，本研究项目采用了一种直接的方法：（i）找到一个具有明确定义的有序状态的简单模型系统，（ii）通过改变某些非热参数连续地将该状态的临界温度调谐到零，以及（iii）通过量子临界点直接测量各种物理性质沿着的相关序参数的变化。目前研究的感兴趣的系统是一组层状过渡金属二硫属化物，即2 H-NbSe 2，2 H-TaS 2和2 H-TaSe 2，它们具有简单的晶体和电子结构。这些化合物中的每一种都通过在空间中形成具有较高和较低电荷密度的交替区域的波而表现出明确的相变，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节到零。在第一种研究途径中，这些化合物的CDW顺序将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紊乱来量子力学地熔化。在第二条研究路线中，单晶样品的厚度将逐渐减小，最终将其转化为几层至单层厚的晶体。利用角分辨光电子能谱、X射线衍射和电阻率测量的组合，通过（a）揭示这些材料的结构、电学和电子性质的演化以及它们的CDW序参数沿着通过温度无序相图中的量子临界点，以及（B）研究量子限制对在几层至单层厚样品中的CDW不稳定性和相变的影响。 在这种情况下，金属量子临界性的精确理论处理是非常具有挑战性的，因为人们需要考虑单粒子激发和序参量波动在同一个立足点。在这个项目中的结构，光谱和运输探针的拟议集成提供了一个独特的机会，这样做的实验。此外，该项目提供了一个定义明确的方法来研究复杂的障碍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，这些材料对外部刺激表现出强烈的增强反应，可用于能源和设备应用。