Study On The Electrical Transport Properties Of Three-dimensional Dielectric Materials CCTO And Low-dimensional Materials

Semiconductor materials are the cornerstone of the development of modern human civilization.The research on the electrical transport behaviors of semiconductor materials has always been a hot topic in condensed matter physics.In this dissertation,three-dimensional CCTO materials and low-dimensional materials such as pseudo-quantum dots,asymmetric Gauss potential quantum wells and graphene are selected as the research objects.The dielectric properties of CCTO are studied by the method of in-situ high pressure impedance measurement.Moreover,the polaron states of low-dimensional materials such as pseudo-quantum dots,asymmetric Gauss potential quantum wells and graphene are studied.The purpose of the dissertation is to understand the stability of dielectric properties of CCTO materials under extreme conditions,and investigate the effects of external temperature,electron-phonon coupling strength,Coulomb potential field,and electromagnetic field on the polaron states so as to fully understand the properties of pseudo-quantum dots,asymmetric Gauss potential quantum wells,polaron states in graphene and qubit.The main results are listed as follows:1.By analyzing the impedance spectra of CaCu₃Ti₄O₁₂ sample,it is found that both the resistance and relaxation frequency of CaCu₃Ti₄O₁₂ sample change discontinuously at 7.4 and 14.1 GPa,indicating that CaCu₃Ti₄O₁₂ undergoes two phase transitions at those pressure points.Electrons are the main charge carrier of CaCu₃Ti₄O₁₂.In the pressure range of 0⁷.4 GPa,the pressure suppresses the relaxation process of the dipole relaxation and promotes carriers local transmission.While in the pressure range of 7.4³0.0 GPa,the pressure has little effect on the long-range transport and short-range transport of carriers in CaCu₃Ti₄O₁₂.In addition,study on the pressure and frequency dependencies of the real part of the material dielectric constant?',imaginary part?''and dissipative factor tandshows that the dielectric properties of CaCu₃Ti₄O₁₂ are optimal at 5.4 GPa from the energy efficiency point of view.In the pressure range from 7.4 GPa to 30.0 GPa,the pressure has little effect on the long-range and short-range transport of carriers in CaCu₃Ti₄O₁₂.The dielectric properties of CaCu₃Ti₄O₁₂ reach the energy efficiency optimum state at 7.4GPa.Activation energy is closely related to the band gap of materials.The change rate of activation energy with the pressure shows that dEa/dP>0 within 3¹4.1 GPa.This indicates that the pressure in this region widens the band gap of the material;while in14.1³0.0 GPa,dEa/dP<0,it shows that the pressure narrows the band gap of the material in this pressure region,which is consistent with the calculation results.2.Compared with CaCu₃Ti₄O₁₂-xNiO?x=0.003?samples,we find that doping changes the phase transition points from 7.4 and 14.1 GPa to 7.1 and 17.6 GPa.In addition,in the range of 0⁷.1 GPa and 17.3²5.1 GPa,the position of the relaxation peak shifts to high frequency with the increase of pressure,indicating that the relaxation frequency increases gradually with the increase of pressure,but it shows the opposite trend in 7.1¹7.3 GPa.We analyze the electrical transport behavior of the sample under high pressure,and find that the dielectric properties of CaCu₃Ti₄O₁₂-xNiO?x=0.003?remain stable under high pressure.3.Based on the Pekar type variational method and quantum statistical theory,the effects of electric field and temperature on the energy of Strong Coupling Polaron States and excitation energy in an asymmetric Gaussian potential quantum well are studied.The results show that?i?the state energy and the excitation energy functions are increasing functions of temperature;?ii?the state energy decreases with the increase of the electric field,but,on the contrary,the excitation energy increases with the electric field increasing;?iii?the state energy is a decreasing function of the asymmetric Gauss potential quantum well and the electron-phonon coupling strength,and the excitation energy and the transition frequency are their increasing functions in the low temperature limit.The results lay a foundation for further understanding the properties of polarons in asymmetric Gauss potential quantum wells.4.The temperature effect of the polaron electron probability density for the RbCl pseudo-quantum dot is studied by using the Pekar variational method and the quantum statistical theory.The ground state energy and the first excited state energy of the pseudo-quantum dot system can form an independent qubit.When electrons are in a superposition state,the probability density of electrons oscillates periodically in RbCl pseudo-quantum dots.Moreover,the temperature and two-dimensional electron gasification potential dependence of electron probability density are calculated.And we find a way to adjust the probability density and oscillation period of electrons:changing the temperature of the system and adjusting the two-dimensional electron gas chemical potential of the pseudo-quantum dots.5.The temperature effect of monolayer graphene bound magnetopolaron has been studied by LLP transformation,linear combination operator and quantum statistical theory.Neglecting the Coulomb interaction,the band gaps of the strong coupling bound magnetopolaron in single-layer graphene at finite temperature are calculated.Meanwhile,the band gap of single-layer graphene at finite temperature is also calculated under considering the Coulomb interaction.By comparing the above two cases,the significance of the existence of Coulomb impurities can be demonstrated.Our results show that the statistical average phonon number N increases with the ascending Debye cut-off wavenumber,and finally reaches a stable value.Under the same conditions,the Coulomb binding potential can promote the energy gap value of monolayer graphene,so the Coulomb binding potential is another important factor to adjust the energy gap of monolayer graphene.The energy gap of monolayer graphene increases with rising temperature T and tends to reach a stable value gradually.The increase of magnetic field B on the single layer graphene will boost the energy gap value of graphene,and the increase of Coulomb binding potential will also lead to the increase of energy gap of the single layer graphene.The change of magnetic field intensity B has no effect on the band gap growth rate with increasing temperature in single layer graphene.Therefore,the band gap of single-layer graphene can be effectively controlled by external temperature,Coulomb binding parameter g and magnetic field B.This discovery will provide new insights in designing the band gap value of graphene.