Study On Phonon Model Of Conducting Molecules

As a new kind of functional material, conjugated polymers or small oligomers have been attracting much theoretical and experimental interest both because of the unique eleectric, magnetic and optical properties that occur in these materials and because of the technological potentical of electronic devices fabricates from them. In particular, they are already the basis of effective organic light-emitting diodes(OLEDS) and a wider range of devices and applications such as organic spin valves and magnetic resistance. On the other hand, conjugated polymers have common characteristic of the sofe condensed matters and can serve as a medium to understand other organic even biological molecular molecules. Up to now, there have been extensive studies on organic and DNA molecular electronics. The single molecular switch and wire have been successfully fabricated and can be used to design the devices focusing on the logical computation in the scale of a single molecule. In addition, DNA molecular exist in a form of the dynamic random state ,so that they can behave as a metallic conductor, a semiconductor, or an insulator, according to different contacts, molecular lengths, and ambient surroundings. In conclusion, all the studies indicate that the organic materials have abundant physical and functional pproperties and we have to investigate such materials using a comprehensive method which may be related with physics,chemistry and even biology.Because organic conducting molecules have the rich features and good prospects for the development, it has attracted a large number of theoretical and experimental workers on its electrical properties, electronic structures and so on. There are strong el-ph interaction in the organic conducting molecules. It is a fundamental difference between organic conducting moleculars and the traditional inorganic semiconductors. So depth understanding of the organic conductive polymer in the el-ph interaction for the study of organic conducting molecules, such as the nature of the electronic structure, has important significance.At present, the organic conducting molecules for the research model has three majors, respectively, the coordinates for the real-space model; tight-binding model; continuum model. It is most frequently used tight-binding model. This model is Su and others early in 1978 for trans-polyacetylene proposed, which successfully solved the problem of soliton. Later, people develop this model and give the ground state and excited state of the polymers. It is widely used to study various properties of polymers. For example, Based on this model people credits on Brazovskii-Kirova-type for researching non-degenerate ground state system. But SSH model also has some limitations, such as the atomic lattice classical treatment for low-temperature environment. When lattice vibration will lead to the emergence of a large number of phonons at high temperatures.,classical treatment will be a great error. To overcome the limitation, the introduction of phonon is an important method, the establishment of the phonon model of organic conductive polymer has a lot of significances.In recent years, the phonon model of organic molecular materials attract a lot of theoretical workers. At present, the theoretical reseach of the el-ph interaction model has mainly two types. The one is Su-Schrieffer-Heeger (SSH) model based on tight-binding approximation. It takes full account of electron - lattice interaction and give the result of the polymer ground state and excited state. These results are widely used to study various properties of polymers. In fact, SSH model considers the nonlocal interaction. The coupling with acoustical phonons has been studied in order to explain the anomalous transport properties of nonlocal excitations, such as solitions and polarons, in various one dimensional systems. The tinght-binding Su-Schrieffer-Heeger(SSH) model was introduced to explain of alternating double and single bonds; The another is Holstein model which is represented by full-quantum model. The Holstein model, where quantum vibrations interact locally with the electrons, the crossover from large to small polaron has been extensively studied by several numerical techniques and variation approach. Quantization of electron and lattice is very complicated, so we only receive some special solution. Because local and nonlocal interactions are simultaneously existing in the actual materials, Based on the quantum electron-phonon interaction model, by means of a coherent state expansion, we get the lattice displacement and the total energy of a conjugated molecular chain. Effect of electron-phonon interaction on the system is discussed. The detailed research and main results are given as follows:1. The characteristics of the organic conducting molecularsWe make further elaborated on the characteristics of organic semiconductors and the properties of its light, electricity, magnetic. We introduce its application in various fields and reseash progress. At present, The research of the organic conductive polymer often use a typical model.2. Phonon and phonon modelThe definition of phonon and electron-phonon are given simply. This paper introduces two types of electron - phonon interaction: electron and acoustic phonon interaction; electron and optical phonon interaction. At present, The three phonon-models are introduced.3 .The nonlinear Schrodinger equation in Organic conductive polymerBased on the quantum electron-phonon interaction model, by means of a coherent state expansion, we get the lattice displacement and the total energy of a conjugated molecular chain. Effect of electron-phonon interaction on the system is discussed. We get the nonlinear Schrodinger equation and the expression of lattice displacement expression.4. the ground state properties of materials in the different electron - phonon interactionThere are two forms in the study of electron-phonon interaction. They are local coupling model and nonlocal coupling model. If we consider the actual structure of the materials and the importance of the two kinds of el-ph coupling, the nonlocal electron-phonon interaction is more important for the conducting polymers. For DNA molecules, the local electron-phonon interaction dominated. Compared to the organic polymers, the two kinds of electron-phonon interaction of the rigid semiconductor can be neglected. In this paper, starting from the nonlinear Schrodinger equation,we study on the nature of the organic conductive polymer in the different electron-phonon interaction.