****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.
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.
A superconductor conducts electricity without generating any heat. Superconductors are used in MRI machines to provide large magnetic fields for imaging. In principle, superconducting cables could become crucial elements of power grid. The drawback is that most materials do not become superconducting until the temperature is very low, therefore requiring liquid helium as a coolant. As yet, no one understands how to synthesize materials that would become superconducting at room temperature. The goal of our research is to understand the connection between superconductivity and magnetism, in hopes of eventually synthesizing new materials that will be technologically useful. Our project focused on superconductors made from organic molecules. To explore the properties of these compounds we used nuclear magnetic resonance (NMR), a technique that forms the basis for MRI. NMR allowed us to explore the behavior of the electrons in the organic material as they transition from a state of ordinary electrical conduction at high temperatures to a state of superconductivity at low temperatures. Two separate experiments were performed, each providing the basis for a PhD thesis by a graduate student. The first experiment studied the NMR signal from carbon atoms in the organic molecules. That experiment showed that at temperatures well above the transition to superconductivity the rotational motion of the organic molecules begins to slow down, which in turn affects the behavior of the electrons. The carbon NMR signal becomes spread over a wide range of energies. The second experiment studied the NMR of hydrogen atoms in the organic molecules. We found that just before the transition to superconductivity, the electrons undergo yet another change, into a peculiar form of disordered magnetism. This magnetic phase persists as the system is cooled into the superconducting phase. In effect, electrons in the superconducting phase of the organic materials appear to exist in a mixture of two states, one of which is magnetic and one of which is superconducting. This same kind of coexistence has been observed in higher temperature superconductors based on copper and oxygen compounds. Our experiments showed that despite the very different chemical compositions of these two classes of materials, they have some very basic things in common. This result may point the way toward a unified theory of superconductivity and magnetism that can eventually be used to design new superconductors for technological applications.
