Investigation On Phase Transition Properties Of Ferroelectric Thin Films With The Higher-order Green's Function Technique

Ferroelectric thin films have a series of excellent typical characteristics compared with the corresponding bulk materials, for example: ferroelectricity, piezoelectricity, dielectricity, pyroelectricity, thermo-optic effect, electro-optic effect, acousto-optic effect, photorefractive effect and nonlinear optical effect, etc. As a result, ferroelectric thin films have very important or potential applications in modern optoelectronics or microelectronics so that ferroelectric thin films have attracted material scientists and physicists extensive attention in the whole world. The physical properties of ferroelectric thin films are different from those of the corresponding bulk materials. Sometime, even materials and components as well as the preparation process detail are exactly the same, but all the results indicate that there are still some individual differences. In the fabrication process of ferroelectric thin films, for example the influence of impurities, defects, surface stress and interfacial stress as well as interfacial effect factors or interchange interactions between different structure layers of ferroelectric thin films may be the primary cause of that the physical properties of ferroelectric thin films are different from those of the corresponding bulk materials. Therefore, the investigation of phase transition properties for ferroelectric thin films is really valuable. Based on the above reason, phase transition properties for ferroelectric thin film have been studied in detail by the Fermi-type Green's function technique within the framework of the transverse Ising model.The main work and results are introduced as follows:(1) In order to inspect the influence of interchange interactions and transverse fields as well as the ferroelectric thin film layer number on phase diagrams of ferroelectric thin films, especially the crossover value characteristics of all interaction parameters from the ferroelectric-dominant phase diagram (FPD) to the paraelectric-dominant phase diagram (PPD). All interation crossover values [ J bc ,?sc ,?bc] for a ferroelectric thin film with a certain layer number N are displayed as a curved surface in the three-dimensional parameter space with respect to [ J b ,?s ,?b]. Meanwhile, the results are compared with the usual mean-field approximation as well as the effective-field with correlations.(2) Within the framework of the effective-field theory with correlations, the differential operator technique has been extended to study phase transition properties in ferroelectric thin films with two-surface-layer. The general analytical equation for phase diagrams of two-surface-layer ferroelectric thin films with arbitrary interchange interactions and transverse fields as well as layer number is derived, and then the effects of interchange interactions and transverse fields as well as the layer number N on phase diagrams of two-surface-layer ferroelectric thin films are discussed in detail. Meanwhile, the crossover characteristics for all interaction parameters are calculated. The results are also compared with the usual mean-field approximation and Fermi-type Green's function technique. It indicates that the effective-field theory with correlations may reduce the ferroelectric features of ferroelectric thin films to some extent.(3) The effects of surface layer number on phase diagrams are studied by comparing the N-independent common intersection point coordinates and various interaction parameters crossover values in phase diagrams of one-surface-layer ferroelectric thin films with those of two-surface-layer ferroelectric thin films in the three theoretical approximations. The results show that the surface layer number has an important effect on the N-independent common intersection point coordinates. However, the surface layer number has no effect on crossover values of all interaction parameters of ferroelectric thin films when the bulk layer number is uniform. The ferroelectric phase transition region range for two-surface-layer ferroelectric thin films is larger than those of one-surface-layer ferroelectric thin films.(4) By introducing the four-spin exchange interaction, the Fermi-type Green's technique is used to study the first-order phase transition properties in ferroelectric thin films. The effects of interchange interactions and transverse fields as well the layer number N on the first-order phase transition in ferroelectric thin films with an epitaxy surface layer are analysed in detail. The results show that changes of interchange interaction and transverse field parameters as well as layer number N on the epitaxy surface layer will influence significantly the first-order phase transition properties.