Spin Fluctuations at Exposed Quantum Critical Points

量子材料有望彻底改变未来的技术。对量子临界性的研究在研究一种鲜为人知的称为非常规超导体的材料中起着关键作用。量子临界性似乎与非常规超导性的发生有关。但是，关于这种关系的性质仍然存在重要的问题，因为超导性“掩盖”了相关的量子现象，所以这种关系往往被隐藏起来。该计划的重点是使用在超低温下进行的先进测量，以充分表征量子临界性和超导性之间的关系。该计划的更广泛的影响涉及高中生，本科生，研究生和博士后科学家在跨学科研究和科学和技术意义的领域，包括与外部机构的合作和交流计划。正在进行的参与研究生资源推进多样性与马里兰州天文学和物理学（GRADMAP）计划将涉及本科生的研究曝光计划，旨在吸引更广泛的观众研究生学习。研究人员也融入了UMD物理系现有的突出外展计划，包括物理是Phun，物理讲座演示和夏季女孩的项目。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：在量子临界点附近有两种密切相关但不同的超导机制：1）由于被抑制的序参量的波动的增加而引起的玻色子耦合强度的增强，以及2）由于在临界点附近正常态熵的增加而间接增强了配对强度，该临界点有利于较低能量基态。这两种情况都很重要，但很难辨别超导性是否介入，但可以通过仔细研究非超导系统中的波动来阐明。该计划的重点是研究系统中的量子临界性，这些系统在接近临界点时不会表现出体相超导不稳定性。该方法是从三个方向，利用三个相关的系统，每一个都需要独特的不同的磁行为：a）一个基准的电子掺杂的铁-磷属元素化物材料，令人惊讶的是，不港口的反铁磁秩序抑制体超导性; B）过掺杂的铁-磷属元素化物材料，表现出量子临界性的迹象，再次没有超导性;以及c）二元铁-磷属元素化物材料，其具有金属自旋密度波磁序，该磁序可以通过朝向金属基态的化学取代而被抑制。通过测量的热力学性质（比热，热输运，核磁共振）和动态磁化率使用非弹性中子散射技术的临界行为的充分表征，允许在选择系统中的失败，以稳定库珀配对的理解，因此有助于阐明在已知的高温超导体的家庭的配对机制。

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.