NMR Studies of Organic and Inorganic Frustrated Quantum Magnets

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Non-technical abstractThis project, supported by the Condensed Matter Physics Program in the Division of Materials Research, is in the area of magnetic materials. Disordered ground states are postulated to arise in magnetic systems where the spins are arranged in specific frustrated geometries. Long sought-after, these so-called spin liquid phases have the potential for use in quantum computation technology and are proposed as a route to high temperature superconductivity. However, candidate materials for these were experimentally realized only recently. Competing interactions, determined by geometry, are understood as an important component for their stability, but how the real systems are dependent on the details remains unclear. This research project aims to explore the circumstances for stability of disordered and ordered ground states of frustrated quantum magnets by exploiting the delicate balance of interactions linked to the geometry. Tuning is accomplished by means of varying strain, pressure, and magnetic field strength. The nature of the accessed phases and associated excitations are determined by a combination of measurements, with considerable reliance on solid state magnetic resonance techniques. The project goals are achieved in tandem with the science education and training of graduate students and undergraduate students in nationally important technical fields, including high frequency and magnetic resonance techniques, as well as high pressure and cryogenic instrumentation.Technical abstractThis project entails the use of solid state magnetic resonance techniques to probe the properties of frustrated quantum magnets and field-induced inhomogeneous superconductivity. With respect to the quantum magnets, the contrast between long-range ordered and quantum disordered systems is of particular interest, as are the nature of the ground states, the spectrum of excitations, and the effects of tuning by magnetic field or strain. The materials selected for study can be classified into two groups. The first includes two structurally similar inorganic compounds, which exhibit antiferromagnetic (AF) and disordered ground states. The second group includes two organic anisotropic triangular systems. The ground states of both are continuously tunable by the application of uniaxial strain or pressure. The focus is on the evolution of physical properties associated with the suppression of magnetically ordered or spin-gapped phases. Signatures for phase transitions, and the nature of excitations, will be inferred from NMR spectroscopy and relaxation measurements. Clean and layered superconductors are ideal candidates for field-induced superconducting phases. The system chosen for study is known for weakly-coupled layers, and the appropriate field range is easily accessed. Magnetic resonance is often an ideal probe for these problems, because of its compatibility to the required extreme conditions, and because it is a local probe sensitive to the electronic environment through the hyperfine interaction. The research will be conducted in the laboratory of the P.I. at UCLA, and at the National High Magnetic Field Laboratory (NHMFL) for fields larger than what is available locally.