86 research outputs found
Fast energy transfer mediated by multi-quanta bound states in a nonlinear quantum lattice
By using a Generalized Hubbard model for bosons, the energy transfer in a
nonlinear quantum lattice is studied, with special emphasis on the interplay
between local and nonlocal nonlinearity. For a strong local nonlinearity, it is
shown that the creation of v quanta on one site excites a soliton band formed
by bound states involving v quanta trapped on the same site. The energy is
first localized on the excited site over a significant timescale and then
slowly delocalizes along the lattice. As when increasing the nonlocal
nonlinearity, a faster dynamics occurs and the energy propagates more rapidly
along the lattice. Nevertheless, the larger is the number of quanta, the slower
is the dynamics. However, it is shown that when the nonlocal nonlinearity
reaches a critical value, the lattice suddenly supports a very fast energy
propagation whose dynamics is almost independent on the number of quanta. The
energy is transfered by specific bound states formed by the superimposition of
states involving v-p quanta trapped on one site and p quanta trapped on the
nearest neighbour sites, with p=0,..,v-1. These bound states behave as
independent quanta and they exhibit a dynamics which is insensitive to the
nonlinearity and controlled by the single quantum hopping constant.Comment: 28 pages, 8 figure
Energy transfer in finite-size exciton-phonon systems : confinement-enhanced quantum decoherence
Based on the operatorial formulation of the perturbation theory, the
exciton-phonon problem is revisited for investigating exciton-mediated energy
flow in a finite-size lattice. Within this method, the exciton-phonon
entanglement is taken into account through a dual dressing mechanism so that
exciton and phonons are treated on an equal footing. In a marked contrast with
what happens in an infinite lattice, it is shown that the dynamics of the
exciton density is governed by several time scales. The density evolves
coherently in the short-time limit whereas a relaxation mechanism occurs over
intermediated time scales. Consequently, in the long-time limit, the density
converges toward a nearly uniform distributed equilibrium distribution. Such a
behavior results from quantum decoherence that originates in the fact that the
phonons evolve differently depending on the path followed by the exciton to
tunnel along the lattice. Although the relaxation rate increases with the
temperature and with the coupling, it decreases with the lattice size,
suggesting that the decoherence is inherent to the confinement
Two-vibron bound states in alpha-helix proteins : the interplay between the intramolecular anharmonicity and the strong vibron-phonon coupling
The influence of the intramolecular anharmonicity and the strong
vibron-phonon coupling on the two-vibron dynamics in an -helix protein
is studied within a modified Davydov model. The intramolecular anharmonicity of
each amide-I vibration is considered and the vibron dynamics is described
according to the small polaron approach. A unitary transformation is performed
to remove the intramolecular anharmonicity and a modified Lang-Firsov
transformation is applied to renormalize the vibron-phonon interaction. Then, a
mean field procedure is realized to obtain the dressed anharmonic vibron
Hamiltonian. It is shown that the anharmonicity modifies the vibron-phonon
interaction which results in an enhancement of the dressing effect. In
addition, both the anharmonicity and the dressing favor the occurrence of two
different bound states which the properties strongly depend on the interplay
between the anharmonicity and the dressing. Such a dependence was summarized in
a phase diagram which characterizes the number and the nature of the bound
states as a function of the relevant parameters of the problem. For a
significant anharmonicity, the low frequency bound states describe two vibrons
trapped onto the same amide-I vibration whereas the high frequency bound states
refer to the trapping of the two vibrons onto nearest neighbor amide-I
vibrations.Comment: may 2003 submitted to Phys. Rev.
Dynamical structure factor of a nonlinear Klein-Gordon lattice
The quantum modes of a nonlinear Klein-Gordon lattice have been computed
numerically [L. Proville, Phys. Rev. B 71, 104306 (2005)]. The on-site
nonlinearity has been found to lead to phonon bound states. In the present
paper, we compute numerically the dynamical structure factor so as to simulate
the coherent scattering cross section at low temperature. The inelastic
contribution is studied as a function of the on-site anharmonicity.
Interestingly, our numerical method is not limited to the weak anharmonicity
and permits one to study thoroughly the spectra of nonlinear phonons
Discrete-row growth of xenon adsorbed on the vicinal Pt(997) surface: Comparison between theory and experiment
Xe exhibits a discrete-row growth mode on the vicinal Pt(997) surface by sequential attachment to the substrate steps. In order to interpret experimental results obtained by grazing incidence helium scattering, potential calculations are performed. A mean-field Hamiltonian within the two-dimensional Ising model is shown to explain the sequential-row growth observed in helium-atom diffraction studies. More specifically, the calculated temperatures for the occurrence of each row depend mainly on the shape of the potential increment due to the steps and countersteps. They are in good agreement with the experimental values associated with maxima in the scattered He intensity versus coverage curves
Relaxation channels of two-vibron bound states in \alpha-helix proteins
Relaxation channels for two-vibron bound states in an anharmonic alpha-helix
protein are studied. It is pointed out that the relaxation originates in the
interaction between the dressed anharmonic vibrons and the remaining phonons.
This interaction is responsible for the occurrence of transitions between
two-vibron eigenstates mediated by both phonon absorption and phonon emission.
At biological temperature, it is shown that the relaxation rate does not
significantly depends on the nature of the two-vibron state involved in the
process. Therefore, the lifetime for both bound and free states is of the same
order of magnitude and ranges between 0.1 and 1.0 ps for realistic parameters.
By contrast, the relaxation channels strongly depend on the nature of the
two-vibron states which is a consequence of the breather-like behavior of the
two-vibron bound states.Comment: octobre 2003 - soumis Phys. Rev.
On the low-temperature lattice thermal transport in nanowires
We propose a theory of low temperature thermal transport in nano-wires in the
regime where a competition between phonon and flexural modes governs the
relaxation processes. Starting with the standard kinetic equations for two
different types of quasiparticles we derive a general expression for the
coefficient of thermal conductivity. The underlying physics of thermal
conductance is completely determined by the corresponding relaxation times,
which can be calculated directly for any dispersion of quasiparticles depending
on the size of a system. We show that if the considered relaxation mechanism is
dominant, then at small wire diameters the temperature dependence of thermal
conductivity experiences a crossover from to -dependence.
Quantitative analysis shows reasonable agreement with resent experimental
results.Comment: 12 pages, 3 eps figure
Defect-induced perturbations of atomic monolayers on solid surfaces
We study long-range morphological changes in atomic monolayers on solid
substrates induced by different types of defects; e.g., by monoatomic steps in
the surface, or by the tip of an atomic force microscope (AFM), placed at some
distance above the substrate. Representing the monolayer in terms of a suitably
extended Frenkel-Kontorova-type model, we calculate the defect-induced density
profiles for several possible geometries. In case of an AFM tip, we also
determine the extra force exerted on the tip due to the tip-induced
de-homogenization of the monolayer.Comment: 4 pages, 2 figure
Net Charge on a Noble Gas Atom Adsorbed on a Metallic Surface
Adsorbed noble gas atoms donate (on the average) a fraction of an electronic
charge to the substrate metal. The effect has been experimentally observed as
an adsorptive change in the electronic work function. The connection between
the effective net atomic charge and the binding energy of the atom to the metal
is theoretically explored.Comment: ReVvTeX 3.1 format, Two Figures, Three Table
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