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有目的地针对所需功能创造的新材料的不同寻常的特性。特别令人感兴趣的是表现出表面上不相容的性质的共存和相互作用的材料，例如电子的磁性、电子性和超导有序性。有时，一种秩序支配另一种秩序。它在零温度下发生的点称为量子临界点。被压抑的秩序不会轻易放弃，在这一点附近的量子临界区域充满了令人惊讶的和在很大程度上未被探索的性质。本项目的目标是识别这些功能，并将其不同寻常的特性用于未来的技术、设备和应用。具体地说，该团队正在研究超导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，同时超导。这两种有序趋势之间的相互作用是通过引入人工散射中心来研究潜在的量子临界点来调节的。这项研究的最终目标是学习如何利用量子临界点的不同寻常的物理学。该研究项目为不同群体的本科生和研究生在实验研究的各个方面提供了一个极好的动手培训平台。该项目的基本概念和结果将被纳入一门高级物理课程，以突出量子临界在现代凝聚态物理中的地位。这项提议是系统地研究电荷密度波(CDW)和超导电性(SC)之间的相互作用，而不是在研究得很好的单一化合物NbSe_2中，而是在新的3-4-13锡化物(Ca，Sr)_3(Ir_(1-x)Rh_x)_4Sn13中。在这些化合物中，通过改变组成，连续地将CDW调谐到结构量子临界点(QCP)。与自旋密度波(SDW)和超导共存的材料，特别是(Ba，K)(Fe，T)2(As，P)2(T=Co，Ni，Rh)进行了比较。在其中一些系统中，超导穹顶下的QCP已经被建立起来。然而，人们对它们的结构和对无序的反应的了解有限。具体地说，目前尚不清楚QCP对结构点状无序的健壮性。理论上的预言从QCP完全消失，到强健甚至被无序稳定。这项研究涉及结构、热力学和运输测量。特别是，x射线散射、Muon自旋旋转光谱、电和热传输、伦敦穿透深度、磁化和弹性电阻。测量是通过温度-组成相图进行的，强调了正常相和超导相中QCP附近的量子临界行为。MeV范围电子辐照引入的受控点状无序将在固定的化学、电子和磁性组态下扰动所研究的系统。正在讨论的具体问题是：(1)SC是否削弱或保护QCP？(2)SC状态下的QCP是否比正常金属中的QCP更抗无序？(3)SC相中QCP的普适性类别是否与正常状态下相同？(4)是否存在与无序相关的新的紧急现象，如大而罕见的区域导致QCP附近的Griffiths奇点？该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.