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

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.

非技术摘要量子材料有望彻底改变未来技术。量子临界性的研究在人们知之甚少的一类称为非常规超导体的材料的研究中发挥着关键作用。量子临界性似乎与非常规超导性的发生有关。但关于这种关系的本质仍然存在重要的问题，这种关系常常被隐藏起来，因为超导性“掩盖”了相关的量子现象。该计划的重点是使用在超低温下进行的先进测量来充分表征量子临界性和超导性之间的关系。该计划更广泛的影响包括让高中生、本科生、研究生和博士后科学家参与跨学科研究和具有科学技术意义的领域，包括与外部机构的合作和交流项目。持续参与马里兰天文学和物理学研究生资源促进多样性 (GRADMAP) 计划将使本科生参与旨在吸引更广泛受众参加研究生学习的研究接触计划。研究人员还参与了UMD物理系现有的著名外展项目，包括Physics is Phun、物理讲座演示和暑期女孩项目。持续参与 NIST SURF（夏季本科生研究奖学金）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：在超导附近有两种密切相关但又不同的机制。量子临界点：1）由于抑制序参数波动的增加而增强了玻色子耦合强度，2）由于法线的增加而间接增强了配对强度接近临界点的状态熵有利于较低能量的基态。这两种情况都非常重要，但很难辨别超导是否介入，但可以通过仔细研究非超导系统的波动来阐明。该计划重点研究在接近临界点时不表现出体相超导不稳定性的系统中的量子临界性。该方法从三个方向出发，利用三个相关系统，每个系统都具有独特的不同磁性行为：a）基准电子掺杂铁磷化物材料，令人惊讶的是，在抑制反铁磁有序时不具有体超导性； b) 一种过度掺杂的铁磷化物材料，它表现出量子临界性的明显特征，但同样没有超导性； c）二元铁-磷化物材料，具有金属自旋密度波磁序，可以通过向金属基态的化学取代来抑制该磁序。通过使用非弹性中子散射技术测量热力学性质（比热、热传输、核磁共振）和动态磁化率来全面表征关键行为，可以了解选定系统中稳定库珀配对的失败，从而有助于阐明已知高温超导体家族中的配对机制。