Spin Fluctuations at Exposed Quantum Critical Points

暴露量子临界点处的自旋涨落

• 批准号: 1610349
• 负责人: Johnpierre Paglione
• 金额: $ 35万
• 依托单位: University of Maryland, College Park
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-09-15 至 2019-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1610349&HistoricalAwards=false
• 关键词: Spin; Fluctuations; Exposed; Quantum; Critical; Spin; Fluctuations; Exposed; Quantum; Critical

Non-technical AbstractQuantum materials promise to revolutionize future technologies. The study of quantum criticality plays a pivotal role in research of a poorly understood class of such materials called unconventional superconductors. Quantum criticality appears to be related to the occurrence of unconventional superconductivity. But important questions remain about the nature of this relationship, which often remains hidden because superconductivity "masks" the relevant quantum phenomena. This program focuses on using advanced measurements performed at ultra-low temperatures to fully characterize the relation between quantum criticality and superconductivity. The broader impact of this program involves the inclusion of high-school students, undergraduate students, graduate students, and postdoctoral scientists in interdisciplinary research and areas of scientific and technological significance, including collaborative and exchange programs with external institutions. An ongoing participation in the Graduate Resources Advancing Diversity with Maryland Astronomy and Physics (GRADMAP) program will involve undergraduates in research exposure programs designed to attract a broader audience to graduate studies. Researchers are also integrated into the UMD Physics department's prominent existing outreach programs, including Physics is Phun, Physics Lecture Demo, and the Summer Girl's Project. Ongoing participation in the NIST SURF (Summer Undergraduate Research Fellowships) Program will also continue to integrate facilities, mentorship and training between UMD and the NIST Center for Neutron Research.Technical Abstract:There are two closely related but disparate mechanisms for superconductivity in the vicinity of a quantum critical point: 1) an enhancement of the bosonic coupling strength due to the increase in fluctuations of the suppressed order parameter, and 2) an indirect enhancement of the pairing strength due to an increase in normal state entropy near a critical point that favors a lower-energy ground state. Both scenarios are quite important but are difficult to discern if superconductivity intervenes, but can be elucidated by carefully studying fluctuations in non-superconducting systems. This program focuses on the study of quantum criticality in systems that do not exhibit bulk-phase superconducting instabilities upon approach to their critical points. The approach is from three directions, utilizing three related systems that each entail uniquely different magnetic behavior: a) a benchmark electron-doped iron-pnictide material that, surprisingly, does not harbor bulk superconductivity upon suppression of antiferromagnetic order; b) an overdoped iron pnictide material that exhibits telltale signatures of quantum criticality, again without superconductivity; and c) a binary iron-pnictide material that harbors a metallic spin-density wave magnetic order that can be suppressed with chemical substitution toward a metallic ground state. The full characterization of critical behavior via measurements of both thermodynamic properties (specific heat, thermal transport, nuclear magnetic resonance) and dynamic susceptibilities using inelastic neutron scattering techniques, allows for an understanding of the failure to stabilize Cooper pairing in select systems, and hence helps elucidate the pairing mechanism in known families of high-temperature superconductors.

非技术抽象素材材料有望改变未来的技术。对量子临界的研究在研究不佳类别的类似于非常规超导体的材料的研究中起着关键作用。量子关键性似乎与非常规超导性的发生有关。但是关于这种关系的性质仍然存在的重要问题，这种关系通常仍然隐藏，因为超导性“掩盖”了相关的量子现象。该程序的重​​点是使用在超低温度下执行的高级测量，以完全表征量子关键性和超导性之间的关系。该计划的更广泛影响涉及包括跨学科研究以及科学和技术意义领域的高中生，本科生，研究生和博士后科学家，包括与外部机构的协作和交流计划。持续参与马里兰州天文学和物理学（GradMap）计划的研究生资源的参与将涉及本科生参加旨在吸引更多受众群体学习研究生学习的研究曝光计划。研究人员还纳入了UMD物理部门的著名现有外展计划，包括Physics是Phun，Physics演讲演示和夏季女孩的项目。持续参与NIST冲浪（夏季本科研究奖学金）计划还将继续整合UMD和NIST中子研究中心之间的设施，指导和培训。技术摘要：技术摘要：两种密切相关但有分散的机制，可用于超导性的超导性，以增强量子的良好效果：1）增强了bossonic couplosic counsic的效果，以增强bossonic的效果。参数和2）由于临界点附近正常状态熵的增加，对配对强度的间接增强了，该临界点有利于较低的能量基态。两种情况都很重要，但是如果超导性干预，很难辨别，但是可以通过仔细研究非责任系统的波动来阐明。该计划的重点是研究在接近其临界点时不会表现出散装相关不稳定性的系统中的量子关键。该方法来自三个方向，利用三个相关的系统，每个系统都需要独特的磁性行为：a）基准电子掺杂的铁刺皮材料，令人惊讶的是，在抑制抗磁磁序的情况下，它们不会在散装超导范围内具有散装的超导性； b）一种过多的pnictide铁材料，表现出量子关键的明显特征，同样没有超导性。 c）具有金属旋转密度波磁序的二元铁刺皮材料，可以用化学替代靠向金属基态进行抑制。通过测量热力学特性（特定的热量，热传输，核磁共振）和使用非弹性中子中子散射技术的动态敏感性，通过测量临界行为的全面表征，可以理解未能稳定精选系统中的库珀配对，因此有助于阐明已知的高灵化度超级尺寸高位符合能力的配对机制。