Low Energy Spectroscopy of Complex, Geometrically Frustrated and Strongly Correlated Matter

University Of California-Santa Cruz, Santa Cruz CA

Investigators

Abstract

Non-Technical: Spectroscopy is an enhanced mode of "seeing" in which one separates photons according to their energy or wavelength. This project centers on the use of spectroscopy to explore, elucidate and understand the fundamental nature and application potential of materials influenced by correlation, frustration and complexity. The nanoscale structure of zirconium tungstate includes surprisingly large open spaces and a sense of atomic freedom know as "underconstraint". Zirconium tungstate also contracts when it is heated, a phenomenon known as negative thermal expansion. Spectroscopic measurements will be utilized to elucidate the relationship between the unusual microscopic structure of this complex compound and the macroscopic phenomenon of negative thermal expansion. This will center on the study of far-infrared phonons. Sodium-cobalt-oxide is similar to cuprate high-temperature superconductors in that it has two-dimensional conducting sheets, however, it is different in that the atomic structure within these sheets is based on triangles rather than squares. The resulting difference in the high-frequency Hall response, i.e., the reaction of electrons in the presence of a magnetic field perpendicular to the sheets, has been predicted to be significant due to the "frustrated" nature of the triangular lattice. This project will seek to measure this effect. Electrons normally repel each other, however, in the context of Tl-doped and In-doped PbTe it has been suggested that they may attract one another. Unexpected changes in the electrical resistance of this unusual doped semiconductor, including superconductivity, have been attributed to such a "negative-U scattering" mechanism. This project will use infrared reflectivity measurements explore and elucidate the fundamental nature of the electron interactions and transport in these exotic materials. This multifaceted project integrates research, education and outreach. Students will be prepared to work in the complex and changing world of modern materials relevant for both their fundamental and applied aspects. Students will also be taught to communicate the importance and excitement of their work to broad audiences through our ongoing outreach efforts in4th, 5th and 6th grade classrooms. Technical: This individual investigator award supports spectroscopic studies of materials in which correlation effects, structural complexity and/or frustration influence macroscopic phenomena. Studies of far-infrared reflectivity in zirconium tungstate and related materials will examine relationships between structural underconstraint, unusual phonon phenomena and negative thermal expansion, as well as possible ice-like behavior and manifestations of geometrical frustration in these complex materials. Measurements of finite-frequency Hall response in layered cobalt oxides will investigate the influence of strong correlations in the context of a triangular lattice for which 3-sided flux-enclosing loops may dominate the response. Studies of the optical and far-infrared reflectivity of Tl- and In-doped PbTe will seek to elucidate the origins of novel transport phenomena observed in these systems where anomalous doping characteristics and transport have been associated with negative-U scattering. This multifaceted project integrates research, education and outreach. Students will be prepared to work in the complex and changing world of modern materials relevant for both fundamental and applied aspects, and to communicate the importance and excitement of their work to broad audiences.