NMR Studies of Field-Induced Phases and Phase Transitions

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

non-technical abstract Superconductivity is probably the most widely known example of a quantum mechanical phenomenon manifested at a macroscopic level. The physical properties attributed to the superconducting state are spectacular and include the flow of electrical current without loss and the expulsion of magnetic flux from the interior of a superconducting sample. These same properties make superconductivity uniquely useful for many applications. Of important scientific and technological importance is to understand how superconductivity survives to high magnetic fields. Many years ago, it was predicted that this is possible through a mechanism producing 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 complemented by investigations of new forms of field-induced magnetic states in a class of magnetic insulators. Magnetic resonance is suitable for this type of problem because it is sensitive to the environment locally proximate to the nuclear spins being probed. Graduate students will engineer and carry out the experimental program under a range of extreme conditions, including very low temperatures and high magnetic fields. Undergraduate students will assist in the design and construction of specific components. technical abstract The nature of magnetic field-induced phases in superconductors and quantum spin systems, stabilized by large magnetic fields, will be studied using nuclear magnetic resonance techniques. In the case of the superconductors, the materials are organic superconductors with highly anisotropic superconducting properties which weakens orbital pair-breaking. Evidence exists for distinct high-field phases in each case, making them excellent candidates for inhomogeneous superconductivity in a form predicted forty years ago but not yet confirmed, and for which its properties remain untested experimentally. Magnetic resonance accommodates easily the necessary extreme conditions, and it is a local probe sensitive to inhomogeneities. The quantum spin dimer system is an S=1 system in which the dimers sit on a layered hexagonal lattice, and for which thermodynamic measurements give evidence for multiple phase transitions as the field is varied at temperatures T<1K. Graduate students will design and carry out the experimental program, and are assisted by undergraduates who will learn how to engineer specific components. Evidence for inhomogeneous states will be looked for, from which new quantum effects are expected to emerge.