Dipolar excitons

The first section recalls the Frenkel-Davydov model in terms of a set of electromagnetically coupled point dipoles. A compact version of Tyablikov s quantum-mechanical solution is displayed and found equivalent to the usual semiclassical theory. The general solution is then applied to a 3D lattice. Ewald summation and nonanalyticity at the zone center are discussed.14 Separating short and long-range terms in the equations allows us to introduce Coulomb (dipolar) excitons and polaritons.15,16 Lastly, the finite extent of actual molecules is considered, and consequent modifications of the above theory qualitatively discussed.14-22... 

Section IV is devoted to excitons in a disordered lattice. In the first subsection, restricted to the 2D radiant exciton, we study how the coherent emission is hampered by such disorder as thermal fluctuation, static disorder, or surface annihilation by surface-molecule photodimerization. A sharp transition is shown to take place between coherent emission at low temperature (or weak extended disorder) and incoherent emission of small excitonic coherence domains at high temperature (strong extended disorder). Whereas a mean-field theory correctly deals with the long-range forces involved in emission, these approximations are reviewed and tested on a simple model case the nondipolar triplet naphthalene exciton. The very strong disorder then makes the inclusion of aggregates in the theory compulsory. From all this study, our conclusion is that an effective-medium theory needs an effective interaction as well as an effective potential, as shown by the comparison of our theoretical results with exact numerical calculations, with very satisfactory agreement at all concentrations. Lastly, the 3D case of a dipolar exciton with disorder is discussed qualitatively. 

In the first part of this introductory section, we summarize the main collective phenomena acquired by the dipolar exciton from the lattice-symmetry collectivization of molecular properties. The crystal is considered as an assembly of electrically neutral systems, the molecules, physically separated from each other and in electromagnetic interaction. This /V-body problem will be treated quantum-mechanically in the limit of low exciton densities. We redemonstrate the complete equivalence of this treatment with the theories of Lorentz and Ewald, as well as with the semiclassical approximation. In Section I.A, in a more compact but still gradual way, we establish the model of the rigid lattice of dipoles and the general theory of low-exciton-density systems in interaction with the radiation field. Coulombic excitons, photons,... 

For K — 0, the two energies [solutions of (1.71)] are those of the Davydov components of the dipolar exciton, their difference being the Davydov... 

This expression means both a differential shift and coupling of the vibronic states. In dipolar excitons the D term is very important, and (2.89) explains why the calculations reported in refs.59,63 clearly overestimate the collective coupling effect Their D term is comparable to J (500cm" ) and adds to it. 

In the last two sections we analyzed spectral and relaxation properties of 3D and 2D strong dipolar excitons in high-quality crystals at low temperatures in terms of the strong excitonic coherence of band width 500 cm l, preserving the properties of the quasi-ideal crystal structure (what we called the intrinsic surface-bulk system) in the presence of weak disorder A... 

In what follows, we present in Section IV.A a theory of the effects of weak disorder on the retarded interactions of 2D strong dipolar excitons, and in Section IV.B we analyze the effects of stronger disorders on the coulombic interactions, calculating the density of states and absorption spectra in 2D lattices, in the framework of various approximations of the mean-field theory. 

C. Nicolas, R. Spezia, A. Boutin, and R. Vuilleumier (2003) Molecular Dynamics simulations of a silver atom in water dipolar excitonic state evidence. Phys. Rev. Lett. 91, p. 208304... 

Bouvier B, Gustavsson T, Markovitsi D, Millie P (2002) Dipolar coupling between electronic transitions of the DNA bases and its relevance to exciton states in double helices. Chem Phys 275 75—92... 

Let us assume that the coulombic branch is resolved, for K = 0 and for a given direction K, by the diagonalization of (1.70). This means that we know, for each direction K, the eigenenergies a>e(K) for each excitonic mode, as well as the eigenvectors e>, which are linear functions by the transformation (1.32), (1.51) of the creation and annihilation operators of molecular states. Furthermore, let us define the excitonic dipolar moment by the same linear transformation on the molecular dipoles,... 

A theory of 2D excitons and polaritons is presented for this type of surfaces, with continuity conditions matching 2D states their 3D counterparts in the bulk substrate, investigated in Sections I and II. This leads to a satisfactory description of the excitations (polaritons, excitons, phonons) and their theoretical interactions in a general type of real finite crystals A crystal of layered structure (easy cleavage) with strong dipolar transitions (triplet states do not build up long-lived polaritons). 

On the other hand, molecular crystals are characterized by the existence of strongly bound (Frenkel type) excitons, and it has been shown that the lower-energy part of the absorption spectrum (say, the first 2 eV) is completely dominated by these excitons [168], even to the extent that the absorption corresponding to electron-hole pair generation is completely hidden in the exciton spectrum [128] and is revealed only by such methods as modulated electrorefletance [169]. The only states in the exciton bands that are accessible by photon absorption are those at the center of the Brillouin zone, so the absorption is not a continuous band as for semiconductors, but a sharp line. The existence of this sharp line therefore does not mean that the exciton band is narrow (i.e., that its dispersion relation in the Brillouin zone is flat). On the contrary, since that dispersion is caused by dipolar interactions, exciton bandwidths can be several eV [168,170] the total bandwidth is four times the coupling term. This will be particularly... 

A separate development of the TRMC techniques was their application to the study of dipolar and excitonic species formed on flash-photolysis of dilute solutions and, more recently, to charge transport and charge separation in thin (aligned) solid films. In the present review we restrict ourselves to results that we have obtained on pulse-irradiated materials, for which the method has become known as the pulse-radiolysis time-resolved microwave conductivity or PR-TBAIC technique. 

In Rb+TCNQ and TMB /tri-methyl-benzim-idazolium/TCNQ, the former having a dimerized chain-type TCNQ"-structure, the latter having isolated dimers, such triplet excitons were studied. The observed spin-dipolar interaction gives information on the electron-hole separation, the behavior of the linewidths as a function of temperature and angle of the magnetic field yields accurate values of the anisotropy of the exciton motion and shows that the excitons are self-trapped by a lattice distortion. 

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