Collaborative Research: Collective Mode Spectroscopy in Unconventional Superconductors

自量子力学发现以来，对固体性质的理解稳步发展，有许多著名的重要应用实例。有一类被证明特别难以处理的材料是所谓的强相关金属，这是典型的金属间化合物，涉及元素周期表中的稀土元素和锕系元素（铀）。在这些元素中，一些电子（称为d和f）与各自的原子核紧密相连，但没有紧密到不参与原子的结合和电传递。在传导过程中，这些电子从一个原子移动到另一个原子，它们花相当长的时间绕d和f原子运行，当另一个电子试图在已经存在的一个原子附近移动时，这两个电子必须调整（关联）它们的运动，使静电排斥最小化；事实证明，这种相关性很难用数学方法来处理，因此提出了一个挑战。这些材料通常是超导的，在这些材料中，人们认为有些材料可能会表现出一种新的超导性，这种超导性被称为非常规超导性。所有超导体的特征都是所谓的有序参数，如果适当地激发，这个有序参数可以振荡。普通超导体和非常规超导体的振动性质有根本的不同，对这种差异的描述应该允许对新型超导体的关键特性进行明确的描述。适当频率的电磁波（微波）将激发这些振动并允许这样的表征；利用微波探针确定超导性的性质是本研究的目的。这项研究的教育部分在于培养未来的高科技劳动力；该项目将为博士后和研究生提供重要的实验室技能，包括材料制备、先进的微波技术和低温技术。本提案旨在检测和表征同时具有超导性的强相关金属间化合物中的序参数集体模式。强相关材料之所以令人感兴趣，是因为单个电子的运动涉及剩余电子的高度协调响应，而在那些超导材料中，这种配对被认为是由一种非常规机制产生的。传统超导体（大多数超导体）在零角动量的状态下配对，并且绝大多数是通过电子-声子相互作用产生的吸引力。正是那些具有非零角动量配对的超导体（或时间反转下的反对称行为）被称为非常规的，并且导致配对的吸引力被怀疑是电子性质的。集体模态可以可视化为相关阶数参数（间隙函数）的有限频率“振动”；对于非常规阶参量，模态一般为各向异性，存在多模态。非常规配对的存在目前依赖于间接的热力学或输运证据，大多数报告都受到一些怀疑，这种情况持续存在（在一些材料中超过十年），在某种意义上是一种危机；然而，微波集体模式研究期望有必要的选择性，以消除歧义。本提案的目标是利用微波吸收来探测一些强相关材料中的集体模式，在这些材料中，非常规配对的间接证据似乎是令人信服的，但其顺序参数的精确形式仍然存在争议。

Non - Technical Since the discovery of quantum mechanics, understanding the properties of solids has progressed steadily, with many well-known examples of important applications. One class of materials that has proved especially difficult to treat is the so-called strongly correlated metals, which are typically intermetallic compounds involving elements from the rare earth and actinide (uranium) groups of the periodic table. In these elements some of the electrons (designated d and f) are closely associated with their respective nuclei but not so closely that they do not participate in binding the atoms together and electrical transport. During conduction, in which such electrons move from atom to atom, they spend a considerable time orbiting the d and f atoms and when another electron attempts to move in the immediate vicinity of one that is already present the two must adjust (correlate) their motion so that the electrostatic repulsion is minimized; this correlation turns out to be difficult to treat mathematically and hence presents a challenge. These materials are often superconducting and among those that are it is thought that some may exhibit a new kind of superconductivity, which has been termed unconventional superconductivity. All superconductors are characterized by what is called an order parameter and, if suitably excited, this order parameter can oscillate (vibrate). The nature of the vibrations for ordinary and unconventional superconductors differs radically, and characterizing this difference should permit an unambiguous characterization of the key characteristic of the new superconductors. Electromagnetic waves (microwaves) with appropriate frequencies will excite these vibrations and allow such a characterization; establishing the nature of the superconductivity using the microwave probe is the goal of this research. The educational component of this research lies in the training of a future high-tech work force; the program will equip post docs and graduate students with important laboratory skills involving materials preparation, advanced microwave techniques, and cryogenics.TechnicalThis proposal is directed at the detection and characterization of order-parameter collective modes in strongly correlated inter-metallic compounds that are simultaneously superconducting. The strongly correlated materials are of interest because the motion of the individual electrons involves a highly-coordinated response of the remaining electrons, and in those materials that are superconducting the pairing is thought to arise from an unconventional mechanism. Conventional superconductors (most superconductors) pair in a state with zero angular momentum and overwhelmingly via an attraction originating from electron-phonon interactions. It is those superconductors that pair with non-zero angular momentum (or an antisymmetric behavior under time inversion) that are termed unconventional, and the attraction leading to the pairing is suspected to be electronic in character. Collective modes can be visualized as finite frequency "vibrations" of the associated order parameter (the gap function); for unconventional order parameters the modes are generally anisotropic and multiple modes exist. Establishing the presence of unconventional pairing presently rests on indirect thermodynamic or transport evidence and most reports are greeted with some skepticism, a situation that persists (in some materials for more than a decade), and is in some sense a crisis; however microwave collective mode studies are expected to have the required selectivity to remove ambiguity. The goal of the present proposal is to use microwave absorption to probe for collective modes in some strongly correlated materials where the indirect evidence for unconventional pairing appears to be compelling, but for which the precise form of the order parameter remains controversial.