When 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.
Vortices 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.
As modern society continues to expand its use of increasingly advanced materials there is also an evolving need for a fundamental understand of these, especially since the most significant advances are expected to be in precisely those materials that are the most complex to understand and control. Among materials with remarkable electronic and magnetic properties, few are more extraordinary than the superconductors. One of the grand challenges in this field is to understand the microscopic details and the possible ways in which the superconducting state can emerge. When a superconductor is placed in a magnetic field, it is threaded by whirlpools of electric current known as vortices. The vortices behave like massive entities, and provide a unique probe into the nature of the host superconductor. This research supported by this award focused on detailed studies of materials where superconductivity and local magnetism moments (in addition to the vortices) coexist. While magnetism is generally considered to be detrimental to the superconducting state, a number of material exist where new and interesting physics emerge as a result of the competition between the two phases. The have been two main outcomes as a result of the award, both obtained by studying the vortex phase in a number of different superconducting materials. The first is a better microscopic understanding of the electronic state within the vortex cores in materials where the superconductivity is especially sensitive to applied magnetic fields (Pauli limiting). A considerable effort was also directed towards the search for an intrinsically inhomogeneous superconducting phase that was theoretically predicted to be more resilient to magnetic fields. However, evidence for this phase obtained by bulk probes was found to be due to an unconventional magnetic ordering within the superconducting state instead. The second outcome was motivated by the discovery of the iron-based (pnictide) superconductors during the award period. Here exploratory studies showed the vortices to be strongly pinned, making these materials interesting from an applications standpoint since moving vortices will cause dissipation even within the superconducting state. To further the broader understanding and appreciation of science, a demonstration experiment using superconducting levitation was built as part of the award. This has been used extensively, both at Notre Dame as well as at extramural outreach events.
