****NON-TECHNICAL ABSTRACT**** An important scientific idea is that of energy or the equivalence of heat and work; a concept central to the industrial revolution. At present solar radiation can be converted to many forms of energy by means of technology; but, to make energy produced this way an economical contribution to our national energy supply requires understanding the long-term stability of materials, especially those at high temperatures. Preliminary neutron scattering experiments on a crystal of salt at high temperatures by the Cornell-Lawrence Livermore National Laboratory collaboration have identified unexpected energy localization or hot spots in the crystal. These findings indicate a high temperature complexity for such a simple ionic lattice and point to the need for new understanding about the way energy is distributed and transported amongst atoms in real materials. This individual investigator program will make use of sophisticated measurement tools located at a variety of national facilities to examine and characterize in a systematic manner the hot spot behavior in high temperature crystals. The knowledge about such intrinsic material defects may be expected to ultimately impact the science of high temperature materials. Both graduate and undergraduate students will develop expertise in research methods and a variety of experimental techniques appropriate to this new interdisciplinary field.
A long standing question in condensed-matter sciences and nonlinear dynamics is whether or not intrinsic localized vibrational modes (ILMs), which rely on nonlinearity and lattice discreteness and which have been explored in detail in macroscopic 1-D lattices, can appear in a 3-D atomic lattice in thermal equilibrium. Preliminary experiments using inelastic neutron scattering from crystalline solids by the Cornell-Lawrence Livermore National Laboratory collaboration have identified dynamical localization with well-defined discrete energies as a new, necessary component of high temperature lattice dynamics. Since high temperature materials now appear to be a natural habitat for dynamical localization and since such excitations are localized hot spots in the material they represent intrinsic material defects that may be expected to influence high temperature diffusion and ultimately the strength of materials. This individual investigator award supports a project to measure, with inelastic neutron scattering, the vibrational localization that appears in crystals at high temperatures. A goal of the project is to understand the properties and ramifications of ILMs in a variety of high temperature crystals. The experimental program will involve a postdoc, a graduate and undergraduate students, and will employ a suite of scattering measurement techniques located at national facilities, ranging from spallation source neutron scattering to reactor source thermal neutron scattering, to expand the knowledge about such excitations in dielectric crystals.
It had been known for some time that nonlinearity and discreteness play important roles in many branches of condensed matter physics as evidenced by the appearance of domain walls, kinks and solitons. A more recent discovery is that localized dynamical energy in a perfect nonlinear lattice can be stabilized by lattice discreteness. Intrinsic localized modes are the resulting feature. Their energy profiles resemble those of localized modes at defects in a harmonic lattice but, like solitons, they can propagate; however, in contrast with solitons, collisions between such excitations result in energy transfer between them with the more localized excitations stealing energy from the less localized ones. The appearance of such localized excitations, which can focus energy in a single lattice site, has been predicted for both macroscopic objects and atomic crystal lattices. We have made measurements on the creation and destruction of these localized excitations for a number of micromechanical arrays. Some dynamical features agree with theoretical predictions while others do not. At the same time we have also completed our study and analysis of the reported appearance of localized vibrational excitations in crystals at high temperatures. We have found that the creation of a vibrational localized mode requires almost as much energy as to generate a crystal vacancy. This finding demonstrates that the concentration of such localized modes will be very small, even near the crystal melting point. Our conclusion is that a crystal lattice at low temperatures, with the optically active plane wave mode driven by laser radiation, is a more appropriate way to study these new kinds excitations in atomic systems.
