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

现代技术依赖于实验室中创造的新材料的不同寻常的特性，有目的地针对所需的功能。特别令人感兴趣的是材料表现出共存和相互作用的看似不相容的性质，如磁性，电子，和超导电子有序。有时，一种秩序支配并扼杀了另一种秩序。它在零温度下发生的点被称为量子临界点。被抑制的秩序不会轻易放弃，在这一点附近的量子临界状态充满了令人惊讶的和很大程度上未被探索的性质。这个项目的目标是识别这些特性，并利用它们不同寻常的特性为未来的技术、设备和应用服务。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，电子密度在空间上被调制，同时具有超导性。通过引入人工散射中心来研究潜在的量子临界点，调节了这两种有序趋势之间的相互作用。这项研究的最终目标是学习如何利用量子临界点的不寻常物理。该研究项目为本科生和研究生在实验研究的各个方面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，以突出量子临界在现代凝聚态物理中的地位。技术摘要：本提案系统地研究了电荷密度波（CDW）和超导性（SC）之间的相互作用，而不是在单一的化合物中，如已被充分研究的NbSe2，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通过改变组成连续地调谐到结构量子临界点（QCP）。结果与自旋密度波（SDW）与超导性共存的材料进行了比较，特别是（Ba，K）（Fe，T）2(As，P)2 （T=Co, Ni, Rh）。在这些系统中，超导穹顶下的量子cps已经建立起来。然而，对它们的结构和对紊乱的反应的了解有限。具体而言，尚不清楚qcp对结构点状紊乱的鲁棒性。理论预测的范围从QCP完全消失，到强健甚至因无序而稳定。这项研究涉及结构、热力学和输运测量。特别是x射线散射、介子自旋光谱、电和热输运、伦敦穿透深度、磁化和弹性。测量是在温度-组成相图中进行的，强调在正常相和超导相中QCP附近的量子临界行为。由mev范围的电子辐照引入的受控的点状无序将使所研究的系统在固定的化学、电子和磁构型下发生扰动。要解决的具体问题是：(1)SC是削弱还是保护QCP？(2) SC态的QCP是否比正常金属更能抵抗无序？(3) SC相内QCP的普适性是否与正常状态相同？(4)是否存在与无序相关的新涌现现象，例如在QCP附近导致Griffiths奇点的大而罕见的区域？该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识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.