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