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.
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.
Intellectual merit. In the course of this project, we executed an experimental program examining novel quantum states of matter. More specifically, we used the application of magnetic fields at low temperature, to stabilize new forms of order and to study their properties. These types of efforts are intended to address fundamental questions on the nature and properties of matter when interactions are important. The phenomena studied were observed in layered superconductors in which correlations are important, and non-magnetic insulators. In both cases, the application of a magnetic field stabilizes new phases, but with unknown and unexplored properties. The principal tool of our investigations was magnetic resonance, which is useful because of the experimenters' ability to manipulate the spin states of nuclei embedded in the material, and because the measured response resulting from the manipulations is influenced by the details of the interactions with its surroundings. Thus, for example, encountering a transition to a new phase of matter is evident in the data, which is then interpreted. The majority of the research was carried out at UCLA, and at the National High Magnetic Field Laboratory (NHMFL) in the case that higher magnetic fields were necessary and helpful. Broader impact. The research program provided many scientific and educational opportunities for students and scholars at various levels. For example, the project involved collaborations with scientists from other institutions within the United States, and from Europe and Japan. In addition to the transfer of hardware and technology between research groups initiated by these collaborations, the students were exposed to complementary expertise by way of extended visits to facilities such as the NHMFL. Two graduate students carried out and completed their Ph. D. research under this project. They went on to take technical positions elsewhere. UCLA undergraduates were integrated into the research program with appropriate projects, supported either directly by this project, NSF REU funding, or through course credit. All of the undergraduates associated with this project went on to enroll in Physics Ph. D. programs. Research-related activities impact the educational experience of undergraduates more broadly. For example, in the laboratory setting, I developed a one quarter course aimed at simple instrument-building. It was offered for the first time in Spring, 2012, with full enrollment. The purpose is to introduce undergraduates to basic measurement and control techniques as used in condensed matter physics and in other areas. The students spend the entire quarter on a specific project, for which they develop a proposal and design, followed by fabrication, testing, and dissemination through a written report and an oral presentation. Results. The layered superconductors we studied are from a class of compounds known as organic or molecular superconductors. The most significant aspect to note here is that because the interlayer coupling is so weak, the superconducting state is particularly robust against magnetic fields. The experiments confirmed the transition to a new high field phase consistent with the inhomogeneous state we were looking for. Moreover, the response to low magnetic fields is like what might be found in a textbook for the case of the magnetic field coupling to the electronic spin degrees of freedom only. The quantum magnet system we studied was Ba3Mn2O8. This material has a non-magnetic ground state, meaning that there is negligible magnetic response at low fields. At sufficiently low temperatures, the application of a magnetic field beyond a threshold stabilizes new phases with finite magnetization. Of interest was the nature of the phases, and what generic aspects of the behavior around the phase transition can be inferred, both in static properties and the fluctuations that break up the phase at the boundary. NMR was applied to both, with good agreement between experiment and theory in the former, but not the latter. We used NMR in a number of other problems where a local probe could be useful. These problems included the study of the dynamical properties of a molecular rotor, and the identification of a generic phase diagram for a novel class of insulator when the interactions are controlled and manipulated. Regarding the rotor, the design and synthesis of artificial molecular machines have potential uses in biotechnology and information storage. Our colleague in UCLA's Department of Chemistry and Biochemistry, Professor Miguel Garcia-Garibay, has taken the path of synthesizing molecular rotors in a solid state matrix. We worked on a couple of systems with his group, one of which is known as BiBco. In that compound, we used the sensitivity of NMR to low frequency fluctuations to study rotor dynamics over a wide temperature range. Very low rotational barriers were identified, which presumably result from a rotor configuration that is structurally frustrated. Low potential barriers are but one part of the problem since free rotation depends on it; new directions are focused on the functionality of the rotors.
