MS&E Colloquium - Dr. Mike Manley
Date: December 04, 2009 from 2:00 pm to 3:00 pm EST
Location: 214 S. W. Mudd
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Intrinsic localized modes of atomic motion and their impact on materials properties

Michael E. Manley
Lawrence Livermore National Laboratory

Convention wisdom says that a local vibrational disturbance in a crystal should spread out, much the way water waves spread across the surface of a pond when it is disturbed. This familiar image is a natural consequence of linear force-displacement behavior. The natural frequency of the disturbed atoms matches that of surrounding atoms, so they resonate with them, quickly transferring energy in the form of waves. About two decades ago, however, it was theorized that in the presence of nonlinear forces a very different result is possible, one in which the energy remains localized [1]. For a nonlinear force-displacement relationship the frequency of the vibrating atoms depends on amplitude. In this situation the disturbed atoms can vibrate at a frequency distinct from the other atoms. As a result the vibration does not couple well to its surroundings and the energy remains trapped locally. In thermal equilibrium configurational entropy can stabilize a random distribution of these intrinsic localized modes (ILMs) at high temperatures.

In 2006 we reported the first experimental observation of ILMs in a conventional solid; metallic uranium [2]. These modes were observed forming at high temperatures by exciting vibrational quanta of the modes using inelastic neutron and x-ray scattering. Furthermore, nonequilibrium experiments were used to show that the modes are created by amplitude fluctuations that mirror the modes themselves, demonstrating their intrinsic nature [3]. More recently we observed ILMs in a simple ionic crystal, sodium iodide [4], using the same methods.

A comparison of the ILM activation temperatures with properties from the literature shows that they impact a wide variety of materials properties. Thermal expansion is enhanced, made more anisotropic, and, at a microscopic level, becomes inhomogeneous. Interstitial diffusion, ionic conductivity, the annealing rate of radiation damage, and void growth are all influenced by ILMs. For uranium the lattice thermal conductivity is suppressed at the ILM activation temperature while no impact is observed in the electrical conductivity. This complement of transport properties suggests that ILMs could improve thermoelectric performance. Ramifications also include a thermal ratcheting that helped redirect a nuclear industry, a transition from brittle to ductile fracture, and possibly a phase transformation in uranium [5].

[1] A. J. Sievers & S. Takeno, Phys. Rev. Lett. 61, 970 (1988).
[2] M. E. Manley et al., Phys. Rev. Lett. 96 , 125501 (2006).
[3] M. E. Manley et al., Phys. Rev. B 77, 214305 (2008).
[4] M. E. Manley et al., Phys. Rev. B, 79, 134304 (2009).
[5] M. E. Manley et al., Phys. Rev. B 77, 052301 (2008).