Using controlled disorder to probe quantum phase transitions under the dome of superconductivity

非技术摘要现代技术依赖于实验室中专门针对所需功能而创建的新型材料的不寻常特性。特别令人感兴趣的是表现出看似不相容特性的共存和相互作用的材料，例如电子的磁性、电子和超导有序性。有时，一种秩序支配并压制另一种秩序。它在零温度下发生的点称为量子临界点。抑制序不会轻易放弃，并且该点附近的量子临界区充满了令人惊讶且很大程度上未经探索的特性。该项目的目标是识别这些功能并利用它们不寻常的特性来实现未来的技术、设备和应用。具体来说，该团队正在研究超导性与电荷密度波共存的复杂材料。在这种特殊的状态下，电子密度被空间调制，同时超导。通过引入人工散射中心来研究潜在的量子临界点，可以调整这两种排序趋势之间的相互作用。这项研究的最终目标是了解如何利用量子临界点的不寻常物理学。该研究项目为实验研究各个方面的多元化本科生和研究生群体提供了一个优秀的实践培训平台。该项目的基本概念和结果将被纳入高级物理课程，以强调量子临界性在现代凝聚态物理中的地位。 技术摘要该提案正在系统地研究电荷密度波 (CDW) 和超导性 (SC) 之间的相互作用，不是在单一化合物（例如经过充分研究的 NbSe2）中，而是在新型 3-4-13 锡化物 (Ca,Sr)3(Ir1-xRhx)4Sn13 中。在这些化合物中，CDW 通过改变成分不断调整至结构量子临界点 (QCP)。结果与自旋密度波（SDW）与超导共存的材料（特别是（Ba，K）（Fe，T）2（As，P）2（T = Co，Ni，Rh）进行了比较。其中一些系统中超导圆顶下的量子点量子点已经建立。然而，对其结构和对紊乱的反应的了解有限。具体来说，QCP 对结构点状无序的鲁棒性如何尚不清楚。理论预测范围从 QCP 完全消失到变得鲁棒甚至因无序而稳定。该研究涉及结构、热力学和输运测量。特别是 X 射线散射、μ 子自旋旋转光谱、电和热传输、伦敦穿透深度、磁化强度和弹性电阻率。测量是在温度成分相图上进行的，强调了正常相和超导相中 QCP 附近的量子临界行为。兆电子级电子辐照引入的受控点状无序将扰乱所研究的固定化学、电子和磁性构型的系统。正在解决的具体问题是：（1）SC 会削弱还是保护 QCP？ (2) SC 态中的 QCP 是否比普通金属中的 QCP 更能抵抗无序？ (3) SC相内QCP的通用性等级与正常状态下相同吗？ (4) 是否存在与无序相关的新颖的新兴现象，例如导致 QCP 附近的格里菲斯奇点的大而罕见的区域？该奖项反映了 NSF 的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

Non-technical abstractModern technologies rely on the unusual properties of novel materials created in labs purposefully targeting needed functionalities. Of particular interest are materials exhibiting coexistence and interplay of seemingly incompatible properties, such as magnetic, electronic, and superconducting ordering of electrons. Sometimes one type of order dominates and quenches the other. The point where it happens at zero temperature is called a quantum critical point. The suppressed order does not give up easily, and the quantum critical regime in the vicinity of this point is full of surprising and largely unexplored properties. It is the goal of this project to identify these features and harness their unusual properties for future technologies, devices, and applications. Specifically, the team is studying complex materials where superconductivity coexists with charge-density wave. In this peculiar state, the electron density is spatially modulated while simultaneously superconducting. The interplay between these two ordering tendencies is tuned by the introduction of artificial scattering centers to study the underlying quantum critical point. The ultimate goal of this research is to learn how to harness the unusual physics of the quantum critical point. The research program provides an excellent hands-on training platform for a diverse group of undergraduate and graduate students in all aspects of experimental research. The underlying concepts and results of the project will be incorporated into an upper-level physics course to highlight the place of quantum criticality in modern condensed matter physics. Technical abstractThis proposal is systematically studying the interplay between charge-density-wave (CDW) and superconductivity (SC), not in a singular compound such as well-studied NbSe2, but in novel 3-4-13 stannides (Ca,Sr)3(Ir1-xRhx)4Sn13. In these compounds, the CDW is tuned continuously to a structural quantum critical point (QCP) by changing the composition. The results are compared with materials where spin-density-wave (SDW) coexists with superconductivity, specifically, (Ba,K)(Fe,T)2(As,P)2 (T=Co, Ni, Rh). QCPs under the dome of superconductivity in some of these systems have already been established. However, there is limited knowledge of their structure and response to disorder. Specifically, it is unknown how robust QCPs are to structural point-like disorder. Theoretical predictions range from the QCP completely disappearing, to being robust and even stabilized by disorder. The research involves structural, thermodynamic and transport measurements. In particular, x-ray scattering, muon spin rotation spectroscopy, electrical and thermal transport, London penetration depth, magnetization, and elastoresistivity. The measurements are performed across temperature-composition phase diagrams, emphasizing the quantum critical behavior near the QCP in the normal and the superconducting phases. Controlled point-like disorder introduced by MeV-range electron irradiation will perturb the studied systems at fixed chemical, electronic, and magnetic configurations. Specific questions being addressed are: (1) does SC weaken or protect the QCP? (2) is the QCP inside the SC state more robust against disorder than in the normal metal? (3) is the universality class of the QCP inside the SC phase the same as in the normal state? (4) are there novel emergent phenomena associated with disorder, such as large, rare regions leading to Griffiths singularities near the QCP?This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.