Superconductivity excitonic

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]. 

In Science, every concept, question, conclusion, experimental result, method, theory or relationship is always open to reexamination. Molecules do exist Nevertheless, there are serious questions about precise definition. Some of these questions lie at the foundations of modem physics, and some involve states of aggregation or extreme conditions such as intense radiation fields or the region of the continuum. There are some molecular properties that are definable only within limits, for example, the geometrical stmcture of non-rigid molecules, properties consistent with the uncertainty principle, or those limited by the negleet of quantum-field, relativistic or other effects. And there are properties which depend specifically on a state of aggregation, such as superconductivity, ferroelectric (and anti), ferromagnetic (and anti), superfluidity, excitons. polarons, etc. Thus, any molecular definition may need to be extended in a more complex situation. 

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. 

A being the value of the vector potential at the n-th layer. This results in a (small) diamagnetic susceptibility, j(, which is however strongly H-dependent in a manner similar to the superconducting case. The response of an exciton-paired p-n junction to the transverse field (V) is insulating, provided that V satisfies condition... 

For possible electronic phase transitions the ordinary nonmagnetic impurities at small concentrations have a small influence on the superconducting transition (see e.g. ) but can considerably suppress the excitonic (Peierls) transition (see e.g. [2] ). 

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,... 

In Chaps. 5-10, we treated primarily individual, intrinsic properties of organic solids phonons, excitons, spin excitations, semiconducting properties, metallic conductivity, and superconductivity. Our main interest was directed in particular to excitations in the bulk of the organic solids. 

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. " ... 

This trend goes further away from the peculiar physical properties which were predicted for 1-D electronic systems. There are the Peierls-Frdhlich CDW collective transport mechanism [30], and Little s excitonic mechanism for high-T superconductivity [33], in which an electronically polarizable entity is used instead of the ionic lattice. It is necessary to conclude that there is a divorce between the experimental results and these initial attractive concepts. 

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