NMR in Organic Superconductors

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

****NON-TECHNICAL ABSTRACT**** Organic conductors are carbon-based materials that conduct electricity, become magnetic and even superconducting, depending upon pressure and temperature. They exhibit striking similarities to the high temperature superconductors discovered in 1986, despite having entirely different chemical compositions. The aim of this project is to discover what general principles govern the behavior of these two classes of materials. Nuclear magnetic resonance (NMR), the same physical phenomenon at work in MRI scanners, will be used as an experimental tool to determine whether the electrons in organic conductors arrange themselves into two distinct classes, with different properties. Further NMR experiments will test whether electrons in these materials form ordered arrangements that have magnetic character. Still other experiments will search for vestiges of superconductivity that may appear at high temperatures where the resistance to electrical current is not yet zero. If successful, these experiments will lead to a deeper understanding of superconductivity, a phenomenon with immense technological significance. Knowledge from these experiments will also help to engineer future compounds at the molecular level with precisely defined electrical and magnetic properties. This project will also involve the development of new techniques to enable NMR measurements of extremely small amounts of material, organic and otherwise. By performing these experiments, both undergraduate and PhD students will become proficient in electronic instrumentation, cryogenics, materials analysis and NMR. The experiments proposed here will enhance the applicability of NMR to materials science and carry its technological development further. ****TECHNICAL ABSTRACT**** Organic conductors based on the BEDT-TTF donor molecule are strongly correlated metals that exhibit magnetic, superconducting and anomalous metallic states. They show striking similarities to high temperature superconductors, but with an energy scale one tenth as large. This project will utilize nuclear magnetic resonance to examine the normal state of these materials. Knight shift measurements will be used to look for evidence of a two-component electronic fluid. Rotating frame relaxation measurements will be taken to search for slowly varying magnetic fluctuations such as spin density waves. The anomalously large Nernst coefficient of certain organic conductors indicates the presence of magnetic vortices well above the superconducting transition temperature. NMR will measure the magnetic fluctuations arising from possible normal-state vortices. Spin lattice relaxation in the organic conductors reveals an apparent energy gap in the spectrum of magnetic excitations. This "pseudogap" is not understood and may be a clue to the origin of superconductivity in both the organic and copper oxide materials. NMR measurements of nuclei in the insulating layers will test the possibility that the pseudogap effect is due to a change in valence state. The project will train PhD students in the areas of NMR and condensed matter physics. It will also fund undergraduate research to develop new techniques for performing NMR measurements on single crystals of sub-millimeter scale.