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Discrete nonlinear Schrödinger approximation of a mixed Klein-Gordon/Fermi-Pasta-Ulam chain: Modulational instability and a statistical condition for creation of thermodynamic breathers
Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .ORCID iD: 0000-0001-6708-1560
2006 (English)In: Physica D: Non-linear phenomena, ISSN 0167-2789, Vol. 216, no 1 SPEC. ISS., 62-70 p.Article in journal (Refereed) Published
Abstract [en]

We analyze certain aspects of the classical dynamics of a one-dimensional discrete nonlinear Schrödinger model with inter-site as well as on-site nonlinearities. The equation is derived from a mixed Klein-Gordon/Fermi-Pasta-Ulam chain of anharmonic oscillators coupled with anharmonic inter-site potentials, and approximates the slow dynamics of the fundamental harmonic of discrete small-amplitude modulational waves. We give explicit analytical conditions for modulational instability of travelling plane waves, and find in particular that sufficiently strong inter-site nonlinearities may change the nature of the instabilities from long-wavelength to short-wavelength perturbations. Further, we describe thermodynamic properties of the model using the grand-canonical ensemble to account for two conserved quantities: norm and Hamiltonian. The available phase space is divided into two separated parts with qualitatively different properties in thermal equilibrium: one part corresponding to a normal thermalized state with exponentially small probabilities for large-amplitude excitations, and another part typically associated with the formation of high-amplitude localized excitations, interacting with an infinite-temperature phonon bath. A modulationally unstable travelling wave may exhibit a transition from one region to the other as its amplitude is varied, and thus modulational instability is not a sufficient criterion for the creation of persistent localized modes in thermal equilibrium. For pure on-site nonlinearities the created localized excitations are typically pinned to particular lattice sites, while for significant inter-site nonlinearities they become mobile, in agreement with well-known properties of localized excitations in Fermi-Pasta-Ulam-type chains. © 2006 Elsevier Ltd. All rights reserved.

Place, publisher, year, edition, pages
2006. Vol. 216, no 1 SPEC. ISS., 62-70 p.
National Category
Natural Sciences
URN: urn:nbn:se:liu:diva-37681DOI: 10.1016/j.physd.2005.12.021Local ID: 37325OAI: diva2:258530
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2014-01-13

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Johansson, Magnus
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