Uniaxial Pressure Measurements Near Quantum Criticality

量子临界点是由零温度下的量子涨落驱动的相变，影响金属体系的许多不寻常的低温行为。在量子临界点附近的样品表现出复杂的相图，包括超导性、反铁磁性、铁磁性、自旋玻璃行为和非费米液体状态。这些相之间的转变通常由压力、合金化或磁场等参数控制。实现对量子临界现象的定量理解将大大提高我们对多体系统的把握。这个项目利用一种罕见的单轴压力技术的特殊能力来研究几个重费米子系统。通过压力调整样品的性质，并与合金化调整的结果进行比较，将有助于区分量子临界点的影响和无序的后果。测量固有的各向异性样品将允许控制晶格常数和传导电子/f电子杂化的变化，这是确定强相关系统基态的关键参数。最后，在压力变化期间将样品温度保持在300 mK以下的能力允许研究低温自旋玻璃相的滞后效应。接触低温设备和压力技术将为研究生和本科生在学术界、工业界或国家实验室的进一步工作做好准备。最简单的金属理论忽略了材料中电子之间的相互作用。因此，这些理论不能解释依赖于电子-电子相关的许多有趣和重要的现象，其中包括超导性和磁性。在一类被称为“重费米子”的材料中，强电子-电子相互作用主导着这种行为。这些化合物通常表现出不止一个高度相关的相，并且可以通过磁场或压力从一个相调整到另一个相。这使得重费米子成为寻找超导或磁相所需精确条件的绝佳试验场。对重费米子的研究将提高对其近亲——高tc超导体的理解，并可能指导对其他超导体的研究。这个项目通过一种罕见的压力技术来研究重费米子系统，这种技术提供了一种控制样品在不同阶段之间调整的方法。压力对于改变原子间的距离特别有用，而原子间的距离又决定了电子的相互作用。这项工作将阐明晶体缺陷的作用，晶体内平面结构的重要性，以及对相变的记忆效应。接触压力和低温下的测量技术将为研究生和本科生在学术界、工业界或国家实验室的进一步工作做好准备。

Quantum critical points, phase transitions driven by quantum fluctuations at zero temperature, influence many unusual low-temperature behaviors of metallic systems. Samples near a quantum critical point exhibit complex phase diagrams including superconductivity, antiferromagnetism, ferromagnetism, spin glass behavior, and non-Fermi liquid regimes. Transitions among these phases are often controlled by parameters such as pressure, alloying, or magnetic field. Achieving a quantitative understanding of quantum critical phenomena will greatly improve our grasp of many-body systems. This project uses the special capabilities of a rare uniaxial pressure technique to study several heavy-fermion systems. Tuning sample properties by pressure and comparing with results from tuning by alloying will help distinguish the influence of a quantum critical point from consequences of disorder. Measuring inherently anisotropic samples will allow controlled variation of lattice constants and conduction electron/f-electron hybridization, key parameters in determining the ground state of a strongly correlated system. Finally, the ability to maintain the sample temperature below 300 mK during pressure changes allows study of hysteretic effects in low-temperature spin glass phases. Exposure to cryogenic equipment and pressure techniques will prepare graduate and undergraduate students for further work in academia, industry, or national laboratories.%%%The simplest theories of metals ignore interactions among the electrons in a material. As a result, these theories cannot explain the many interesting and important phenomena which depend on electron-electron correlations, among them superconductivity and magnetism. In the class of materials known as "heavy fermions," strong electron-electron interactions dominate the behavior. These compounds often exhibit more than one highly correlated phase, and can be tuned from one phase to another by using magnetic fields or pressure. This makes the heavy fermions an excellent testing ground for finding the precise conditions needed for superconducting or magnetic phases. Studies on heavy fermions will improve understanding of their relatives, the high-Tc superconductors, and may guide searches for other superconductors. This project investigates heavy-fermion systems through a rare pressure technique that provides a controlled means of tuning a sample among phases. Pressure is particularly useful for varying the interatomic distances, which in turn determine the electron interactions. This work will elucidate the role of crystal defects, the importance of planar structures within crystals, and memory effects upon phase changes. Exposure to techniques for measurements under pressure and at low temperature will prepare graduate and undergraduate students for further work in academia, industry, or national laboratories.