Moment Localization and Delocalization in f-Electron Compounds

相关电子材料的基本性质，如它们的导热或导电能力，它们是否维持静态和局域力矩或电荷，以及这些复杂系统如何接近其最终基态，反映了它们的电子局域或非局域的程度。在f电子系统中，最显著的是重电子化合物，最初定位在f轨道上的电子可以离域，扩展费米表面。特别有趣的是，当这种局域化-离域化发生在T=0量子临界点时，它将电子局域化且不参与费米表面的磁有序态与f电子完全融入费米表面的顺磁性强相互作用金属分开。该项目将跟踪从量子临界点到更高温度的矩解定跃迁，并研究其与近藤相干的关系，在近藤相干中，f电子被认为是通过单个近藤补偿矩的杂化而离域的。该项目将在YbTX化合物中进行这一程序，通过适当选择过渡金属T和主族元素X，我们可以跨越重电子相图的所有区域。也有人提出，与几何挫折相关的强量子涨落可能使磁局域化与量子临界点分离。这种可能性将在R2T2X (R=Ce,Yb) Shastry-Sutherland化合物中进行研究。该项目将结合新的f电子化合物的合成，并利用中子散射实验、实验室磁强计、比热和电输运测量对它们进行研究。该项目广泛利用国家研究设施，培养的学生将为将来成为有效的使用者做好充分准备。通过学习各种不同的合成和表征技术，参与的学生将发展一套有价值且非常便携的技能，并且通过对自己发明的材料进行第一次测量的兴奋，将获得动力，使他们在未来的职业生涯中成为受材料启发的科学家。正如改变温度可以使水从根本上改变其性质，从固体到液体再到蒸汽；材料的其他特性也可以通过温度、压力或磁场的变化而发生类似的变化：金属变为绝缘体，磁铁变为非磁铁，导体变为超导体。控制这些相变的能力是实现新一代传感器的核心，例如，小磁场可以将导体转变为非导体，或者温度的微小变化可以导致磁铁变为非磁性。当相变发生在最低温度时，这些效应变得最强，而本研究的目的是研究最极端的磁相变，其中磁性仅在零温度下稳定。这项研究的目标是了解控制磁性稳定性的潜在因素，这些信息将用于设计新一代具有改进功能的磁性材料，用于磁性数据存储和能量控制等多种应用。该项目将开发新的材料家族来实现这项研究，其中磁性强度和起始温度将在成分上发生变化。我们将记录材料导热和导电能力的相应变化，以及当我们将其推向不再可能具有磁性的成分时，材料中产生的磁性强度。非常接近这种磁不稳定性本身，磁性只是短暂地存在，并且只在很短的长度尺度上存在。中子可以用来在微观上探测这些磁波动。散射实验将在国家中子散射设施进行，如马里兰州盖瑟斯堡的NIST和田纳西州橡树岭的散裂中子源。人们越来越认识到，缺乏在合成技术方面受过训练的科学家，正使美国在材料启发研究方面的竞争力处于危险之中。该项目的重点是合成具有特定功能的新材料。参与的本科生和研究生将学习各种不同的合成技术，以及证明样品高质量所需的实验技术库。参加这个项目的学生培养了一套非常受欢迎和非常实用的技能，这些技能已经被证明对他们自己和他们未来的雇主都有很大的价值。

Technical AbstractThe fundamental properties of correlated electron materials such as their ability to conduct heat or electricity, whether they sustain static and localized moments or charges, and indeed how these complex systems approach their ultimate ground states reflect the degree to which their electrons are localized or delocalized. In f-electron systems, most notably heavy electron compounds, electrons initially localized on f-orbitals can be delocalized, expanding the Fermi surface. Of special interest is when this localization - delocalization occurs at a T=0 quantum critical point, which separates a magnetically ordered state where the electron is localized and does not participate in the Fermi surface, from a paramagnetic and strongly interacting metal where the f-electron is fully incorporated in the Fermi surface. This project will track this moment deconfinement transition away from the quantum critical point to higher temperatures and investigate its relationship to Kondo coherence, where f-electrons are thought to delocalize by the hybridization of individually Kondo compensated moments. The project will pursue this program in the YbTX compounds, where with an appropriate choice of transition metal T and main group element X we can span all regimes of the heavy electron phase diagram. It has also been suggested that strong quantum fluctuations associated with geometrical frustration may separate magnetic localization from the quantum critical point. This possibility will be investigated in the R2T2X (R=Ce,Yb) Shastry-Sutherland compounds. This program will combine the synthesis of new f-electron compounds, and their investigation using neutron scattering experiments as well as lab-based magnetometry, specific heat, and electrical transport measurements. The project makes extensive use of national research facilities, and the students trained will be well prepared to become effective future users. By learning a variety of different synthesis and characterization techniques, participating students will develop a valuable and very portable set of skills, and through the excitement of performing the first measurements on materials of their own invention, will gain the motivation to sustain them in their future careers as materials-inspired scientists. Non-Technical AbstractJust as changing temperature can cause water to fundamentally change its properties from solid to liquid to vapor; other properties of materials can similarly be transformed by varying temperature, pressure, or magnetic field: metal to insulator, magnet to non-magnet, conductor to superconductor. The ability to control these phase transitions is central to implementing new generations of sensors, where for instance small magnetic fields transform a conductor into a nonconductor, or a small change in temperature can cause a magnet to become nonmagnetic. These effects become strongest when the phase transitions occur at the lowest temperatures, and it is the purpose of this research to study the most extreme of magnetic phase transitions, where magnetism is stable only at zero temperature. The goal of this research is to understand the underlying factors which control the stability of magnetism, information which will be used to design new generations of magnetic materials with improved functionality for applications as diverse as magnetic data storage and energy control. This project will develop new families of materials to enable this research, where the strength of the magnetism and its onset temperature will be varied compositionally. We will document the corresponding changes in the material's ability to conduct heat and electricity, and the strength of the magnetism induced in the material as we drive it ever closer to the composition where magnetism is no longer possible. Very near this magnetic instability itself, the magnetism exists only ephemerally, and over only short length scales. Neutrons can be used to microscopically probe these magnetic fluctuations. Scattering experiments will be performed at national neutron scattering facilities such as those at NIST in Gaithersburg MD and the Spallation Neutron Source in Oak Ridge TN. It is increasingly recognized that the dearth of scientists trained in synthesis techniques is placing US competitiveness in materials inspired research at risk. The project focus is to synthesize new materials with very specific functionality. Participating undergraduate and graduate students will learn a variety of different synthesis techniques, as well as the arsenal of experimental techniques required to certify the high quality of the samples. Students participating in this program develop a highly sought and very portable skill set, which has already proven of great value to themselves and their future employers.