808 research outputs found
Microscopic derivation of Frenkel excitons in second quantization
Starting from the microscopic hamiltonian describing free electrons in a
periodic lattice, we derive the hamiltonian appropriate to Frenkel excitons.
This is done through a grouping of terms different from the one leading to
Wannier excitons. This grouping makes appearing the atomic states as a relevant
basis to describe Frenkel excitons in the second quantization. Using them, we
derive the Frenkel exciton creation operators as well as the commutators which
rule these operators and which make the Frenkel excitons differing from
elementary bosons. The main goal of the present paper is to provide the
necessary grounds for future works on Frenkel exciton many-body effects, with
the composite nature of these particles treated exactly through a procedure
similar to the one we have recently developed for Wannier excitons.Comment: 16 pages, 4 figure
Role of anisotropy in the F\"orster energy transfer from a semiconductor quantum well to an organic crystalline overlayer
We consider the non-radiative resonant energy transfer from a two-dimensional
Wannier exciton (donor) to a Frenkel exciton of a molecular crystal overlayer
(acceptor). We characterize the effect of the optical anisotropy of the organic
subsystem on this process. Using realistic values of material parameters, we
show that it is possible to change the transfer rate within typically a factor
of two depending on the orientation of the crystalline overlayer. The resonant
matching of donor and acceptor energies is also partly tunable via the organic
crystal orientation.Comment: 6 pages, 8 figure
Biphonons in the Klein-Gordon lattice
A numerical approach is proposed for studying the quantum optical modes in
the Klein-Gordon lattices where the energy contribution of the atomic
displacements is non-quadratic. The features of the biphonon excitations are
investigated in detail for different non-quadratic contributions to the
Hamiltonian. The results are extended to multi-phonon bound states.Comment: Comments and suggestions are welcom
Comment on "Effects of spatial dispersion on electromagnetic surface modes and on modes associated with a gap between two half spaces"
Recently Bo E. Sernelius [Phys. Rev. B {\bf 71}, 235114 (2005)] investigated
the effects of spatial dispersion on the thermal Casimir force between two
metal half spaces. He claims that incorporating spatial dispersion results in a
negligible contribution from the transverse electric mode at zero frequency as
compared to the transverse magnetic mode. We demonstrate that this conclusion
is not reliable because, when applied to the Casimir effect, the approximate
description of spatial dispersion used is unjustified.Comment: 9 pages, minor corrections in accordance with the journal publication
have been mad
High temperature phase transition in the coupled atom-light system in the presence of optical collisions
The problem of photonic phase transition for the system of a two-level atomic
ensemble interacting with a quantized single-mode electromagnetic field in the
presence of optical collisions (OC) is considered. We have shown that for large
and negative atom-field detuning a photonic field exhibits high temperature
second order phase transition to superradiant state under thermalization
condition for coupled atom-light states. Such a transition can be connected
with superfluid (coherent) properties of photon-like low branch (LB)
polaritons. We discuss the application of metallic cylindrical waveguide for
observing predicted effects.Comment: 8 pages, 2 figure
Coherent responses of resonance atom layer to short optical pulse excitation
Coherent responses of resonance atom layer to short optical pulse excitation
are numerically considered. The inhomogeneous broadening of one-photon
transition, the local field effect, and the substrate dispersion are involved
into analysis. For a certain intensity of incident pulses a strong coherent
interaction in the form of sharp spikes of superradiation is observed in
transmitted radiation. The Lorentz field correction and the substrate
dispersion weaken the effect, providing additional spectral shifts. Specific
features of photon echo in the form of multiple responses to a double or triple
pulse excitation is discussed.Comment: only PDF,15 page
q-Functional Wick's theorems for particles with exotic statistics
In the paper we begin a description of functional methods of quantum field
theory for systems of interacting q-particles. These particles obey exotic
statistics and are the q-generalization of the colored particles which appear
in many problems of condensed matter physics, magnetism and quantum optics.
Motivated by the general ideas of standard field theory we prove the
q-functional analogues of Hori's formulation of Wick's theorems for the
different ordered q-particle creation and annihilation operators. The formulae
have the same formal expressions as fermionic and bosonic ones but differ by a
nature of fields. This allows us to derive the perturbation series for the
theory and develop analogues of standard quantum field theory constructions in
q-functional form.Comment: 15 pages, LaTeX, submitted to J.Phys.
Tunable exciton interactions in optical lattices with polar molecules
Rotational excitation of polar molecules trapped in an optical lattice gives
rise to rotational excitons. Here we show that non-linear interactions of such
excitons can be controlled by an electric field. The exciton--exciton
interactions can be tuned to induce exciton pairing, leading to the formation
of biexcitons. Tunable non-linear interactions between excitons can be used for
many applications ranging from the controlled preparation of entangled
quasiparticles to the study of polaron interactions and the effects of
non-linear interactions on quantum energy transport in molecular aggregates.Comment: Some typos have been corrected in this versio
Vibrational Tamm states at the edges of graphene nanoribbons
We study vibrational states localized at the edges of graphene nanoribbons.
Such surface oscillations can be considered as a phonon analog of Tamm states
well known in the electronic theory. We consider both armchair and zigzag
graphene stripes and demonstrate that surface modes correspond to phonons
localized at the edges of the graphene nanoribbon, and they can be classified
as in-plane and out-of-plane modes. In addition, in armchair nanoribbons
anharmonic edge modes can experience longitudinal localization in the form of
self-localized nonlinear modes, or surface breather solitons.Comment: 10 pages, 10 figure
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
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