Exploring the Influence of Frustration near Quantum Criticality through Uniaxial Pressure

非技术摘要：数十种高度相关的材料表现出相似但绝不相同的行为模式。二十多年来，对这些行为及其发生的条件进行分类一直是一个高度优先的问题，它可能被证明是最终制造包括室温超导体在内的有用材料的关键。这个项目探索了磁阻的作用，在这种情况下，系统倾向于相互不兼容的局部磁配置。系统必须满足一些局部约束，但不满足其他约束，从而导致大量具有相同或非常相似能量的状态。沿着单一方向挤压样本可以将能级转移到更大的范围，从而减少挫折对样本行为的影响。这个项目施加了这种单向压力，并测量了它如何改变样品的低温电和磁行为。了解挫折的影响可能有助于未来设计具有特定所需性能的材料。专门的冷却和加压技术为进行实验的研究生和本科生提供了极好的培训。技术摘要：Yb基重费米子化合物的低温行为的一个主要问题是，是否会发生近藤击穿，费米面的大小出现不连续的跳跃。更广泛地说，应该确定几何效应的作用，如几何受挫和晶体维度。对于这一目的，单轴压力是一种自然的技术，因为它对挫折感和维度的影响比流体静压更强烈。这项研究涉及在单轴压力高达1 Gpa、温度低至30 MK、磁场高达10特斯拉的情况下进行比热、磁化率和输运测量。这些测量建立了压力-温度-场相图，揭示了几何在许多相互竞争的影响中的重要性。特别值得注意的是T*的行为，T*是传导电子和局域力矩在单轴压力下杂交的起始温度。与流体静压和化学掺杂相关的调谐技术相比，T*具有惊人的稳定性。这项建议的目标之一是测试T*线的稳定性是否也发生在单轴压力下。

Nontechnical Abstract:Dozens of strongly correlated materials exhibit similar but by no means identical behavior patterns. Classifying the behaviors and the conditions under which they occur has been a high priority for more than two decades, and it may prove key to eventually fabricating useful materials including room-temperature superconductors. This project explores the role of magnetic frustration, where a system favors local magnetic configurations which are not mutually compatible. The system must satisfy some local constraints but not others, leading to a large number of states with the same or very similar energy. Squeezing a sample along a single direction can shift the energy levels to a broader range, thereby reducing the influence of frustration on the sample's behavior. This project applies such unidirectional pressure and measures how it changes a sample's low-temperature electrical and magnetic behavior. Understanding the effects of frustration may help in designing materials with specific desired properties in the future. The specialized cooling and pressure techniques provide excellent training for the graduate and undergraduate students who perform the experiments.Technical Abstract:A major question in the low-temperature behavior of Yb-based heavy-fermion compounds is whether Kondo breakdown occurs, with a discontinuous jump in the size of the Fermi surface. More generally, the role of geometrical effects, such as geometic frustration and crystal dimensionality, should be ascertained. Uniaxial pressure is a natural technique for this purpose, since it affects both frustration and dimensionality more strongly than does hydrostatic pressure. This research involves specific heat, magnetic susceptibility, and transport measurements under uniaxial pressure up to 1 GPa, temperatures down to 30 mK, and magnetic fields up to 10 Tesla. These measurements establish a pressure-temperature-field phase diagram which reveals the importance of geometry among many competing influences. Of special note is the behavior of T*, the onset temperature for hybridization between the conduction electrons and local moments, under uniaxial pressure. T* is surprisingly stable to the related tuning techniques of hydrostatic pressure and chemical doping. One of the goals of this proposal is to test if the stability of the T* line occurs for uniaxial pressure as well.