Collaborative Research: Collective Mode Spectroscopy in Unconventional Superconductors

合作研究：非常规超导体的集体模式光谱学

• 批准号: 0509357
• 负责人: John Ketterson
• 金额: --
• 依托单位: Northwestern University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2005
• 资助国家: 美国
• 起止时间: 2005-11-01 至 2009-04-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0509357&HistoricalAwards=false
• 关键词: Collaborative; Research; Collective; Mode; Spectroscopy

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.

自量子力学发现以来，对固体性质的理解稳步发展，有许多重要应用的着名例子。 一类被证明特别难以处理的材料是所谓的强相关金属，它们通常是涉及元素周期表中稀土和锕系（铀）族元素的金属间化合物。 在这些元素中，有些电子（用d和f表示）与它们各自的原子核密切相关，但并不紧密到不参与原子的结合和电输运。 在传导过程中，这些电子从一个原子移动到另一个原子，它们花了相当长的时间绕d和f原子轨道运行，当另一个电子试图在已经存在的电子附近移动时，这两个电子必须调整（关联）它们的运动，以使静电排斥最小化;这种关联很难在数学上处理，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，有人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序参量，如果受到适当的激励，这个序参量会振荡（振动）。 普通超导体和非常规超导体的振动性质截然不同，描述这种差异应该可以明确地描述新超导体的关键特性。 具有适当频率的电磁波（微波）将激发这些振动并允许这样的表征;使用微波探针建立超导性的性质是本研究的目标。 这项研究的教育组成部分在于未来的高科技劳动力的培训，该计划将配备博士后和研究生的重要实验室技能，涉及材料制备，先进的微波技术，和cryogenics.TechnicalThis建议是针对强相关的金属间化合物，同时超导序参数集体模式的检测和表征。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，并且在那些超导材料中，配对被认为是由非常规机制产生的。 传统的超导体（大多数超导体）在零角动量的状态下配对，并且压倒性地通过电子-声子相互作用产生的吸引力。那些以非零角动量配对（或在时间反演下的反对称行为）的超导体被称为非常规的，导致配对的吸引力被怀疑是电子性质的。 集体模式可以被看作是有限频率的“振动”的相关序参数（差距功能）;非常规的顺序参数的模式通常是各向异性和多个模式存在。 目前建立非常规配对的存在依赖于间接的热力学或运输证据，大多数报告都受到一些怀疑，这种情况持续存在（在某些材料中超过十年），在某种意义上是一种危机;然而，微波集体模式研究预计将有必要的选择性，以消除歧义。本提案的目标是利用微波吸收探测集体模式在一些强关联材料的非常规配对的间接证据似乎是令人信服的，但其中的确切形式的序参数仍然存在争议。