Interaction Parameters Change With The Number Of Layers The Phase Transition Properties Of Ferroelectric Thin Films,

The ferroelectricity means the spontaneous polarization that exists in some materials, and the spontaneous polarization can change its direction when the external electric field alters. The ferroelectric materials are characterized by the ferroelectricity, namely, there is a ferroelectric hysteresis loop relation between the polarization and the external electric field. The ultrathin materials which show ferroelectricity and the thickness of which is from several ten nanometers to a few microns are called ferroelectric thin film. The ferroelectric thin film is an important functional thin film material, and is striking the scientific researchers more and more who devote themselves in the field of material science or condensed matter physics. Compared with the body materials, the ferroelectric thin film has many merits: (1) It promise to integrate with the microelectronic devices or photoelectronic devices, which results monolithical integrated devices (2) The big difference of the refractive index between the thin film and the substrate renders the thin film and the substrate themselves to be a light waveguide, thus makes the device have high power density, which will realize the miniaturization of the integrated light waveguide devices. (3) The ability of resistance to light-damage is better than body materials of the same type. Compared with the light waveguide of other materials, the advantages of the light waveguide made from ferroelectric thin film lie in: low in price, easily made, compact structure and high reliability. Therefore, the investiment on the thin film plays an important role both in the theory and in the engineering technology, especially in industry production, aerospace, computer and information technology, and the military field.In the first chapter, the concept of the ferroelectric thin film, many kinds of the application, and the recent development situation are introduced.In the second chapter are some primary theory about the ferroelectric thin film described by the transverse field Ising model: the phase transition theory, the Ising model of the ferroelectrics.In the third chapter, we show how to deal with the ferroelectric thin film: it is analyzed with the Landau theory (in the macro-area), or statistical physics theory (in the microscopic field) which begins with the Hamiltonian of the system based on the transverse field Ising model and includes the Green function approach and the mean field approach.In the fourth chapter, we introduce how to study the phase transition property of the ferroelectric thin film described by the transverse field Ising model in the framework of the mean field approach. In the past, the pioneers assumed that the thin film material was an uniform material. Different from the predecessors, my innovation lies in: the phase transition properties are investigated while the interaction parameters are changed with the layers, which means that the whole material is an uneven material. To be convenient, the interaction parameters are assumed to be changed with a certain function. Meanwhile, I present two functions in order to find out what effect of the changing tendency of the parameters on the phase transition properties. I perfect my results by contrast to the investigation results of the pioneers.In summary, what I have done in this field can be generalized:1. I have studied the effects of the exchange interaction and transverse field parameters on the ferroelectric region and paraelectric region while they are changed with the layers.2. I have studied the effects of the exchange interaction and transverse field parameters on the Curie temperature while they are changed with the layers.3. While the exchange interaction and transverse field parameters are the values of the parameters of the first layer and of the mid-layer (now the whole material is an uniform material, and its exchange interaction and transverse field parameters are the values of the parameters of the first layer and of the mid-layer), the phase diagrams and polarizations are compared with those corresponding to the layer-dependent parameters. 4. While the exchange interactions decrease and the transverse field parameters increase, or while the exchange interactions increase and the transverse field parameters decrease, the phase diagrams and polarizations have also been studied.The results show that various layer-dependent parameters have sensitive effects on the phase transition property. The range of the ferroelectric phase in the phase diagrams and the value of the Curie temperature of the n-layer ferroelectric thin film are determined by whether the interactions or the transverse field parameters being dominant. If the interactions in the material are dominant over the transverse field parameters, the faster the parameters decrease, the smaller the range of the ferroelectric phase and the Curie temperature are. The faster the parameters increase, the larger the range of the ferroelectric phase and the Curie temperature are. While the transverse field parameters are dominant over interactions in the material, contrary situation will result. It indicates that when the properties of phase transition of the ferroelectric thin film are investigated in the future, the layer-dependent parameters should be involved carefully and some experiments may be explained reasonably.