Intrinsic Localized Lattice Modes and Thermal Transport: Potential Application in a Thermal Rectifier
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1172-T09-02
Intrinsic Localized Lattice Modes and Thermal Transport: Potential Application in a Thermal Rectifier Michael E. Manley Condensed Matter and Materials Division, Lawrence Livermore National Laboratory, Livermore, CA 94551, U.S.A. ABSTRACT Recent experiments provide evidence of intrinsic localized modes (ILMs) in the lattice dynamics of conventional 3D materials. Here evidence that ILMs in uranium metal enhance the thermal conductivity is presented along with speculation on how thermal transport by ILMs might be used to improve a reported design for a solid-state thermal rectifier. INTRODUCTION In materials the occurrence of nonlinear forces between atoms, or anharmonicity, causes the vibrational frequencies of the lattice vibrations to depend on amplitude. A well known manifestation is the decrease in phonon frequencies or lattice “softening” with increasing temperature. The softening of the lattice results in excess vibrational entropy which tends to stabilize an increased volume with increasing temperature and this can be associated with the well-known anharmonic thermal expansion effect [1]. Another manifestation is the phononphonon scattering processes that are responsible for the finite thermal conductivities of solids [1]. Both of these effects can be understood in the context of perturbation theory, where the phonons are treated as quasi-harmonic modes with softening and scattering (or lifetime) effects added on. Beyond perturbation effects, strong nonlinearity in a lattice can also give rise to new types of spatially localized vibrational modes, called an intrinsically localized modes (ILMs) [2] or discrete breathers [3]. These modes form when large-amplitude local fluctuations develop frequencies that do not resonate with the normal modes, trapping energy in dynamic “hotspots”. At high temperatures these ILMs are expected to form randomly on the lattice stabilized by configurational entropy, much like vacancies [2]. Although known about for more than 20 years [1] only recently has evidence of the existence of ILMs in conventional 3D materials emerged. In particular, using inelastic neutron and x-ray scattering new thermally activated localized modes have been observed forming at elevated temperatures in a metallic crystal, uranium [4], and an ionic crystal, sodium iodide [5]. Experiments have also created non-equilibrium ILMs from the ground state in cold (RT) crystals [5, 6]. These experiments demonstrate that these localized modes are created by amplitude fluctuations that resemble the modes themselves, which demonstrates the nonlinear formation mechanism of an ILM [6]. Supporting data also shows that these ILMs strongly influence a surprisingly wide variety of properties; including heat capacity [4], thermal conductivity [6], thermal expansion [7], and mechanical deformation [7]. Furthermore, evidence suggests that ILMs in uranium act as an incipient driver for a solid-state phase transition [8]. Here we focus on the influence of ILMs on thermal conductivity and how this concept of dynamic no
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