Vortices and the Interplay between Superconductivity and Magnetism

University Of Notre Dame, Notre Dame IN

Investigators

Abstract

NON-TECHNICAL ABSTRACT 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. TECHNICAL ABSTRACT 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.