Magnetic Correlations and Quantum Critical Points

该个人研究者奖支持实验研究，以探索磁相变附近的空间和时间相关性，磁相变可以通过成分变化、压力或磁场调谐到零温度。将使用中子散射与实验室热容量、磁化和电输运研究相结合的方法。以前的工作集中在局部矩和流动矩磁的相互作用被认为导致零度相变形成的材料上。本项目将探索两个系统的量子临界行为，其中磁性主要是流动的：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由Ce和Yb基等原子金属间系合成新型近藤晶格铁磁体，特别是可以通过压力或磁场调谐的铁磁量子临界点的产生。对交换相互作用和载流子密度改变后的铁磁序稳定性和力矩补偿过程进行了评价。本项目广泛使用国家科研设施，参与研究的学生和博士后将为将来成为这些设施的有效使用者做好充分准备。此外，他们将接受单晶合成技术的培训，这是一个越来越稀缺但有价值的研究专业。我们的主要兴趣是了解通过改变磁性材料的组成和结构来稳定或抑制磁性的方法，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，所有磁铁最终都会失去磁性，经历从组成原子的磁矩排列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。通过类比更熟悉的相变，如冰融化成水，我们对这些时刻如何随着温度的降低而开始排列或排序有了很多了解，首先是在短长度尺度上和短时间内，但最终是在整个样品上和任意长时间内。这个项目解决了最极端的磁铁：那些在零温度极限下变得磁性的磁铁。与在高温下变得磁性的材料不同，磁矩在接近零度但高于零度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相变。它将使用多种实验技术，将中子散射与磁、热、电输运测量相结合。合成这些量子磁体的新家族是这项工作的核心，而高压和高磁场将用于将磁跃迁调整到零温度。该项目广泛使用了国家研究设施，包括国家高磁场实验室和NIST、橡树岭和阿贡国家实验室的中子散射中心。该项目培养的学生和博士后将成为这些设施的未来专家，并熟悉新型体相关电子系统的合成，这是一项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.