Excitonic superconductivity model

As can be seen by this paper the quasiparticle called an "exciton" is a very useful construct in describing the physics and chemistry of organic solids. It is not only a good definition to some excitations of the N-body system but can be used in describing the polarization field as was the case for the superconductivity model. 

Hamiltonians equivalent to (1) have been used by many authors for the consideration of a wide variety of problems which relate to the interaction of electrons or excitons with the locaJ environment in solids [22-25]. The model with a Hamiltonian containing the terms describing the interaction between excitons or electrons also allows for the use of NDCPA. For example, the Hamiltonian (1) in which the electron-electron interaction terms axe taken into account becomes equivalent to the Hamiltonians (for instance, of Holstein type) of some theories of superconductivity [26-28]. 

THE EXCITON MODEL OF SUPERCONDUCTIVITY IN LINEAR CHAINS - REVISITED... 

We present a detailed calculation of the transition temperature of a model, filamentary excitonic superconductor. The proposed structure consists of a linear chain of transition-metal atoms to which is complexed a ligand system of highly polarizable dye molecules. The model is discussed in the light of recent developments in our understanding of one-dimensional metals. We show that for the structure proposed, the momentum dependence of the exciton interaction results in the superconducting state being favoured over the Peierls state, and in vertex corrections to the electron-exciton interaction which are small. The calculation of the transition temperature is based on what we believe to be reasonable estimates of the strength of the excitonic interaction, Coulomb repulsion and band structure. 

For the particular model proposed, transition temperature of several hundred degrees are calculated. However, we find superconductivity only in systems in which the excitonic medium is within a covalent bond length of, and completely surrounds the conductive spine. This imposes severe constraints on the structure of any excitonic superconductor. 

Many authors have discussed the excitonic mechanism (36-37.) of superconductivity, in which the effective attractive interaction between conduction electrons originates from virtual excitations of excltons rather than phonons. The basic idea of the models proposed is that conduction electrons residing on the conducting filament (or plane) induce electronic transitions on nearby easily polarizable molecules (or complexes), which result in an effective attractive interaction between conduction electrons. As perhaps a striking realization of the excitonic mechanism of superconductivity,... 

The BCS and Little models for superconductivity are both based on the formation of pairs of electrons with an effective attractive interaction due to phonons or excitons respectively. Recently, J. Bardeen (8,28) revived a model, originally presented by Frohlich in 1954 (152), as a possible explanation of the reported anomalous conductivity behavior of (TTF)(TCNQ) (97). This model predates the BCS theory and relies on the direct interaction between electrons and the one-dimensional lattice resulting in the formation of charge density waves. The model has also been applied to the one-dimensional metal K2Pt(CN)4Bro.3o(H20)s (72, 457). 

The present model for nerve impulse resembles closely the exciton mechanism of high-temperature superconductivity, as put forward by Little and Ginzburg. Little s polymer consists of a polyene spine with polarizable dye side chains, the latter forming the exciton band. Ginzburg proposes high-temperature superconductivity to be found in thin metallic films placed between highly dielectric layers. " ... 

---