NMR Studies of Field-Induced Phases and Phase Transitions

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non-technical abstractSuperconductivity is probably the most widely known example of aquantum mechanical phenomenon manifested at a macroscopic level.The physical properties attributed to the superconducting stateare spectacular and include the flow of electrical current withoutloss and the expulsion of magnetic flux from the interior of asuperconducting sample. These same properties makesuperconductivity uniquely useful for many applications. Ofimportant scientific and technological importance is to understandhow superconductivity survives to high magnetic fields. Many yearsago, it was predicted that this is possible through a mechanismproducing a state of matter inhomogeneous on a microscopic level.In this project, magnetic resonance techniques will be used to study superconducting systems for which there is evidence for formation of a new phase at high fields, including an exploration of their physical properties, with the aim of identifying the inhomogeneous phase and characterizing its properties. This work will be complementedby investigations of new forms of field-induced magnetic states ina class of magnetic insulators. Magnetic resonance is suitable forthis type of problem because it is sensitive to the environmentlocally proximate to the nuclear spins being probed. Graduatestudents will engineer and carry out the experimental programunder a range of extreme conditions, including very lowtemperatures and high magnetic fields. Undergraduate students willassist in the design and construction of specific components.technical abstractThe nature of magnetic field-induced phases in superconductors andquantum spin systems, stabilized by large magnetic fields, will bestudied using nuclear magnetic resonance techniques. In the caseof the superconductors, the materials are organic superconductorswith highly anisotropic superconducting properties which weakensorbital pair-breaking. Evidence exists for distinct high-fieldphases in each case, making them excellent candidates forinhomogeneous superconductivity in a form predicted forty yearsago but not yet confirmed, and for which its properties remainuntested experimentally. Magnetic resonance accommodates easilythe necessary extreme conditions, and it is a local probesensitive to inhomogeneities. The quantum spin dimer system is anS=1 system in which the dimers sit on a layered hexagonal lattice,and for which thermodynamic measurements give evidence formultiple phase transitions as the field is varied at temperaturesT1K. Graduate students will design and carry out the experimentalprogram, and are assisted by undergraduates who will learn how toengineer specific components. Evidence for inhomogeneous stateswill be looked for, from which new quantum effects are expected toemerge.