Spin Waves in Disordered Potentials: Interplay between Disorder, Nonlinearity, and Incoherence

Colorado State University, Fort Collins CO

Investigators

Abstract

Non-Technical Abstract Wave theory is fundamental to physics, underlying everything from radar to fiber optics to cell phone communications. Waves occur in magnetic materials, where the magnetic spin precesses in a wave pattern, propagating energy and information. Magnetic materials are important in many technological contexts, including solid-state hard drives. In a perfectly ordered magnetic material a wave would propagate smoothly; however, real materials have disorder, imperfections that lead to localization of waves, so that information or energy gets trapped in a certain location in the material. This research seeks to understand many unanswered questions about localization of waves in disordered magnetic materials. In particular, three main questions are addressed: (1) Does coherence matter? Lasers, for example, are coherent, and spin waves can be too. (2) How does nonlinearity destroy localization? Nonlinearity means the whole is not the sum of the parts. In spin waves the strength of this effect can be controlled, and its impact on localization explored. (3) Can chaos cause delocalization? Real materials often exhibit chaos. Does chaos allow a localized wave to escape? To support this research and create tomorrow's scientists, the project provides extensive training opportunities for students at undergraduate and graduate levels, especially interdisciplinary cross-training between experimental and theoretical physics. Outreach to high schools in Colorado is accomplished through the Colorado State University "Little Shop of Physics" program, focusing on those in disadvantaged areas. Outreach to the broader scientific community occurs via organizing of scientific conferences, workshops, and symposia. Technical Abstract Anderson localization and Aubry-André localization have both attracted rather considerable interest across a number of disciplines in recent years due to their ubiquitous nature. Theoretical studies have yielded many predictions about the effects of coherence and nonlinearity on localization that are of great fundamental importance but are often controversial or debatable. The research in this project not only settles several current debates on localization, but also provides first experimental justifications to a number of theoretical predictions. As such, the project deepens the understanding of the interplay between disorder, nonlinearity, and coherence in general. Furthermore, the research enhances the understanding of spin-wave dynamics in magnetic thin films and damping processes in magnetic materials with disordered defects. Specifically, the studies make use of spin waves in yttrium iron garnet (YIG) thin film strips. Disordered potentials for spin waves are developed by two approaches: (1) the fabrication of disordered grooves on the surfaces of YIG strips and (2) the development of disordered local field variations by depositing meander lines on the YIG strips and passing electric currents through the lines. Two types of disordered potentials are considered: random potentials and quasi-periodic potentials. The former is used to study Anderson localization, while the latter is used for the study of Aubry-André localization. The research consists of both experimental and theoretical efforts. It is carried out through integral collaborations between Mingzhong Wu's experimental group at Colorado State University and Lincoln Carr's theoretical group at Colorado School of Mines. The new program has transformative impacts in view of promising potential applications of localization effects. For example, the energy density within a localized mode can be several orders of magnitude larger than that of the incident wave, and this huge field enhancement has potential applications for energy harvest, storage, and conversion. In addition to mentorship at undergraduate to graduate levels and outreach as described above, the principal investigators are jointly engaged in curriculum development, focusing on bringing up-to-date applications and experimental demonstrations into graduate core courses in electrodynamics and classical dynamical systems.