Magnetic Correlations and Quantum Critical Points

这一个人研究人员奖支持探索磁相变附近的空间和时间相关性的实验研究，磁相变通过成分变化、压力或磁场调节到零温度。将使用中子散射与基于实验室的热容、磁化和电传输研究相结合的方法。以前的工作主要集中在局域磁矩和巡游磁矩相互作用导致零温度相变形成的材料上。这个项目将探索两个以巡游为主的磁性系统的量子临界行为：T=0量子反铁磁体Cr0.966V0.0034和铁磁体Zr0.95Nb0.05Zn2。该提案的第二个主题是从Ce和Yb基等原子金属间化合物系列合成新颖的近藤晶格铁磁体，特别是产生可通过压力或磁场调节的铁磁量子临界点。当交换相互作用和载流子密度改变时，铁磁有序的稳定性和磁矩补偿过程将被评估。该项目广泛使用了国家研究设施，参与研究的学生和博士后将做好充分准备，成为这些设施未来的有效用户。此外，他们将接受单晶合成技术方面的培训，这是一种日益稀缺但有价值的研究专业。我们的主要兴趣是了解如何通过改变磁性材料的组成和结构来稳定或抑制磁性，这是合理设计新型磁性材料的核心问题。在足够高的温度下，所有磁体最终都会失去其磁性，经历从组成原子的磁矩排列和静止的低温状态到磁矩波动并指向随机方向的高温状态的相变。通过类比更常见的相变，比如冰融化成水，我们很清楚这些时刻是如何随着温度的降低而开始排列或排序的，首先是在短长度的尺度上，时间很短，但最终是在整个样本上，时间任意长。这个项目解决了最极端的磁体：那些在零温度极限下变得有磁性的磁体。与在较高温度下变得有磁性的材料不同，在零温度附近但在零温度以上的磁矩的波动是由于它们的量子力学性质。这个项目试图描述这种量子磁性相变的特征。它将使用多种实验技术，将中子散射与磁、热和电传输测量相结合。这些量子磁体的新系列的合成是这一努力的核心，而高压和强磁场将被用来将磁跃迁调整到零温度。该项目广泛利用了国家研究设施，包括国家高磁场实验室以及设在NIST、橡树岭和阿贡国家实验室的中子散射中心。在该项目下培训的学生和博士后将成为这些设施未来的专家用户，并熟悉新的块状关联电子系统的合成，这是一种罕见但非常有价值的实验技能。

This individual investigator award supports experimental investigations to explore the spatial and temporal correlations near magnetic phase transitions, which are tuned to zero temperature by compositional variation, pressure, or magnetic field. A combination of neutron scattering with lab-based heat capacity, magnetization, and electrical transport studies will be used. Previous work focused on materials where the interplay of local moment and itinerant moment magnetism is thought to lead to the formation of the zero temperature phase transition. This project will explore the quantum critical behaviors of two systems where the magnetism is predominantly itinerant: the T=0 quantum antiferromagnet Cr0.966V0.0034, and the ferromagnet Zr0.95Nb0.05Zn2. The second theme of the proposal is the synthesis of novel Kondo lattice ferromagnets from the Ce and Yb based equiatomic intermetallic series, especially the generation of ferromagnetic quantum critical points that can be tuned by pressure or magnetic field. The stability of ferromagnetic order and the process of moment compensation as the exchange interaction and carrier density are modified will be assessed. This project makes extensive use of national research facilities, and the students and postdocs involved in the research will be well prepared to become effective future users of these facilities. Further, they will be trained in single crystal synthesis techniques, an increasingly scarce but valuable research specialty. Our primary interest is to understand the ways in which magnetism can be stabilized or alternatively suppressed by modifications to the composition and structure of magnetic materials, an issue that is central to the rational design of new classes of magnetic materials. All magnets ultimately lose their magnetic properties at sufficiently large temperatures, undergoing a phase transition from a low temperature state where the magnetic moments of the constituent atoms are aligned and static, to a high temperature state where the moments fluctuate and point in random directions. By analogy to more familiar phase transitions, such as the melting of ice into water, much is known about the way in which these moments begin to align or order with reduced temperature, first on short length scales and for short times, but ultimately over the entire sample and for arbitrarily long times. This project addresses the most extreme magnets: those that become magnetic in the limit of zero temperature. Unlike materials that become magnetic at higher temperatures, the fluctuations of the magnetic moments near but above zero temperature are due to their quantum mechanical nature. This project seeks to characterize this sort of quantum magnetic phase transition. It will use a variety of experimental techniques combining neutron scattering with magnetic, thermal, and electrical transport measurements. Synthesis of new families of these quantum magnets is central to this effort, while high pressures and high magnetic fields will be used to tune magnetic transitions to zero temperature. This project makes extensive use of national research facilities, including the National High Magnetic Field Laboratory and the neutron scattering centers at NIST, Oak Ridge, and Argonne National Laboratories. Students and postdocs trained under this project will be expert future users for these facilities, as well as being conversant in the synthesis of novel bulk correlated electron systems, a rare but highly valued experimental skill.