Vortices and the Interplay between Superconductivity and Magnetism

当超导体被置于磁场中时，它就会被称为涡流或通量线的电流漩涡所缠绕。除了在超导体的实际应用中是一个基本的限制因素外，涡旋也为宿主超导体的微观特性提供了一个独特的探针。虽然我们知道，超导性总是由于电子结合成所谓的库珀对而产生的，但在许多种类的材料中，包括最著名的高温氧化物，这种结合发生的方式仍然基本上是未知的。目前项目的主要目标是研究富尔德-费雷尔-拉金-奥夫钦尼科夫（FFLO）态中的一种特殊电子对，这种电子对在40多年前就被理论上预测到了，但迄今为止还没有直接观察到。虽然对FFLO相本身的研究本质上是基础的，但磁性和超导性之间相互作用的更大框架与超导性的实际应用有关。参与该项目的学生将接受使用中子散射的培训，因此该项目将增加美国中子源的未来用户基础，例如橡树岭国家实验室新建的散裂中子源。为了提高人们对超导潜在应用的认识，我们将构建一个超导悬浮的演示实验。这将允许参与者铺设磁性轨道，并使包含用液氮冷却的高温超导体的模型火车悬浮起来。技术摘要：在外加磁场作用下，超导体中产生的涡流可以作为主体材料中超导状态详细性质的微观探针。这个项目的主要重点是使用涡旋研究来研究顺磁性和超导性之间的相互作用，无论是在具有局域磁矩的材料中，还是在重费米子超导体CeCoIn5中。特别令人感兴趣的是电子配对可能发生的方式，以及在上临界场是强泡利限制的系统中所谓的富尔德-费雷尔-拉金-奥夫尼尼科夫（FFLO）超导态的非均匀理论预测。最近有越来越多的实验证据表明在CeCoIn5中存在这样一个FFLO相，但到目前为止，在这个或任何其他超导体中没有观察到这种状态的结构证据；FFLO状态的详细属性实际上仍然是未知的。本项目的主要目的是通过涡旋晶格的小角中子散射与扫描隧道光谱相结合，为CeCoIn5（如果存在的话）中的FFLO状态提供直接证据。该项目包括本科生和研究生的大量参与，并使他们有机会在大型国际机构进行研究。因此，该项目有助于教育和培训在高需求领域具有专业知识的新一代科学家。最后，将开发一个以高温超导悬浮为特色的示范实验，用于外展活动。

NON-TECHNICAL ABSTRACTWhen a superconductor is placed in a magnetic field, it is threaded by swirling whirlpools of electric current known as vortices or flux-lines. Besides being a fundamental limiting factor in the practical applications of superconductors, the vortices also provide a unique probe into the microscopic properties of the host superconductor. While it is know that superconductivity always arises due to the binding of electrons into so-called Cooper pairs, the way in which this binding occurs is still essentially unknown in many classes of materials including most notably the high-temperature oxides. The main goal of the present project is to investigate a particular electron pairing in what is known as the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state which was predicted theoretically more than 40 years ago but has so far eluded direct observation. While studies of the FFLO phase in itself are fundamental in nature, the larger framework of interplay between magnetism and superconductivity is of relevance for the practical applications of superconductivity. Students participating in the project will be trained in the use of neutron scattering and thereby the project will increase the future user base for US neutron sources such as the new constructed spallation neutron source at the Oak Ridge National Laboratory. To enhance broader awareness of the potential applications of superconductivity a demonstration experiment featuring superconducting levitation will be constructed. This will allow participants to lay down a magnetic track, and levitate a model train containing a high-temperature superconductor cooled with liquid nitrogen.TECHNICAL ABSTRACTVortices induced in a superconductor by an applied magnetic field can serve as microscopic probes of the detailed nature of the superconducting state in the host material. The main focus of this project is to use vortex studies to investigate the interplay between paramagnetism and superconductivity, both in materials with localized magnetic moments as well as in the heavy-fermion superconductor CeCoIn5. Of particular interest is the possible ways in which pairing of electrons can occur, and the theoretical prediction of a non uniform so-called Fulde-Ferrell-Larkin-Ovcninnikov (FFLO) superconducting state in systems where the upper critical field is strongly Pauli limited. Recently there has been mounting experimental evidence for the existence of such a FFLO phase in CeCoIn5, but to date no structural evidence for this state has been observed in this or any other superconductor; the detailed properties of the FFLO state are still effectively unknown. The principal aim of this project is to provide such direct evidence for the FFLO state in CeCoIn5 (if it exists) by small-angle neutron scattering of the vortex lattice complemented with scanning tunnelling spectroscopy. The project incorporates substantial participation by undergraduate and graduate students and exposes them to research at large international facilities. Hence the project contributes to the education and training of a new generation of scientists with expertise in fields of high demand. Finally, a demonstration experiment featuring high-temperature superconducting levitation will be developed for use in outreach activities.