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化的时刻或电荷，以及这些复杂系统如何接近它们的最终基态，反映了它们的电子局域化或离域化的程度。 在f电子系统中，最明显的是重电子化合物，最初位于f轨道上的电子可以离域，扩大费米面。 特别感兴趣的是当这种定域-离域发生在T=0量子临界点时，其将电子被定域并且不参与费米表面的磁有序状态与f电子完全并入费米表面的顺磁性和强相互作用金属分离。 该项目将跟踪从量子临界点到更高温度的这一时刻解除限制过渡，并研究其与近藤相干性的关系，其中f电子被认为是通过单独近藤补偿时刻的杂交而离域的。 该项目将在YbTX化合物中进行这一计划，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会将磁局域化与量子临界点分开。 这种可能性将在R2 T2 X（R=Ce，Yb）Shastry-Sutherland化合物中进行研究。 这个计划将结合联合收割机的新f-电子化合物的合成，他们的调查，使用中子散射实验以及实验室为基础的磁力，比热，和电输运测量。该项目广泛利用国家研究设施，受训学生将为今后成为有效的用户做好充分准备。通过学习各种不同的合成和表征技术，参与的学生将开发一套有价值的和非常便携的技能，并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，以维持他们在未来的职业生涯中作为材料启发的科学家。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;材料的其他性质也可以通过改变温度、压力或磁场而发生类似的变化：金属到绝缘体、磁体到非磁体、导体到超导体。 控制这些相变的能力是实现新一代传感器的核心，例如，小磁场将导体转变为非导体，或者温度的微小变化可能导致磁体变成超导体。 当相变发生在最低温度时，这些效应变得最强，并且本研究的目的是研究最极端的磁性相变，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磁性材料，这些材料具有改进的功能，适用于磁性数据存储和能量控制等各种应用。 该项目将开发新的材料家族，以实现这项研究，其中磁性的强度和起始温度将因成分而异。 我们将记录材料导热和导电能力的相应变化，以及当我们将材料推向不再可能具有磁性的组成时，材料中感应的磁性强度。在这种磁不稳定性本身附近，磁性只存在于短暂的时间内，并且只在很短的尺度上存在。 中子可以用来在显微镜下探测这些磁波动。散射实验将在国家中子散射设施中进行，如位于马里兰州盖瑟斯堡的NIST和位于田纳西州橡树岭的Sputterus中子源。 越来越多的人认识到，缺乏合成技术方面的科学家，使美国在材料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非常特定功能的新材料。 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。 参加该计划的学生开发了一套备受追捧的，非常便携的技能，这已经证明了对自己和未来雇主的巨大价值。

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.