Study On The Characteristics Of Magnetic Relaxation In High Tc Superconductivity YBCO And The Characteristics Of Lower Temperature Liquid HeII

It's very important to investigate the magnetic relaxation of high Tc superconductivity in the study of physics because its relations with temperature and magnetic field lead to yield new conceptions such as collection creeping and quantum tunneling of high Tc superconductivity et al. On the other hand, from the aims of application, the magnetic relaxation of high Tc superconductivities changed their I-V properties and determined the fact that the sustained current density relies on the temperature. In a word, it's very important to study the magnetic relaxation of high Tc superconductivity no mater in the study of physics or in the aims of application. As high Tc superconductivity YBCO with the same structure of Ca-Ti layer as other copper oxide high Tc superconductivities and possess higher critical temperature (more than 90K), it's with the most comprehensive representative. Thus it was selected for the study of the magnetic relaxation in this thesis.The magnetic relaxation of YBCO in two temperature intervals 0～20K and 20～80K was investigated. In the interval of 0～20K, a quantum tunneling model was established based on the conception of quantum tunneling that proposed by predecessors, and then induced S-T expression between magnetic relaxation rate and temperature by quantum calculating. Then combining the S-T expression that induced from the Anderson-Kim thermal activation model, we fitted the former experimental results and found that the model and the experimental curve are fitted very well if the combined action of quantum tunneling and thermal activation were considered. The phenomenon indicated thermal activation and quantum are concurrent in this temperature scope. In the interval of 20～80K, we induced S-T expressions of vortex glass phase and reverse thermal activation's, and got the graphs of these S-T expressions in this temperature range by fitting these expressions. At last, the magnetic relaxation phenomenon in this temperature range was discussed and explained.Fully understanding the properties of liquid HeII can promote the development of low temperature technology and other technologies such as superconductive technology et al, the properties of low temperature liquid HeII was also studied.In the study of low temperature liquid HeII, we mainly studied the relationships between the temperature and free energy,entropy and specific heat. By fitting the dispersion relations of the low temperature liquid HeII, we got an uniform analytical formula for it in 0～1.6K interval, and induced the relationships between the temperature and free energy,entropy and specific heat. Based on these studies, we calculated the specific heat expression of liquid HeII, and found there exists a minor error between the specific heat expression and the experiment results. At last, we discussed and explained the reason of this error.