【作者】 张中卫；

【导师】 林德华；

【作者基本信息】 重庆大学 ， 凝聚态物理， 2007， 硕士

【摘要】 在物理研究中,高温超导体中的磁弛豫与温度、磁场等参数间的关系致使许多新概念的提出,如集体蠕动、量子隧穿等,对其进行研究显得非常重要。同时,从技术应用的角度来看,高温超导体中的磁弛豫改变了高温超导体的V-I特性,决定了持续电流密度对温度的依赖关系,对技术应用可产生重大影响。可见,不管在物理研究还是技术应用方面对高温超导体中的磁弛豫现象进行研究均具有重要意义。由于高Tc超导体YBCO和其它高Tc氧化物超导体一样都是层状钙钛矿结构、且具有大于90K的超导转变温度,在所有的高Tc超导体中最具有代表性,因此本文以它为对象对高Tc超导体的磁弛豫现象进行了研究。本文在磁弛豫特性研究过程中,对YBCO在0～20K和20～80K两个温区内的磁弛豫性质分别进行了研究。对0～20K温区内的YBCO磁弛豫进行研究时,在前人所提出量子隧穿概念基础上,通过建立量子隧穿模型和量子计算导出了YBCO在量子隧穿情况下磁弛豫率与温度之间的S-T关系式。随后,用所导出的关系式与AK模型热激活理论下的S-T关系式结合对过去的实验结果进行拟合,结果发现,若考虑热激活与量子隧穿两者的共同作用,在0～20K整个温区内的拟合结果和实验结果符合得很好,从而证明在这一温区内磁通线的热激活运动和量子隧穿运动混合并存。在对20-80K温区内的YBCO磁弛豫特性进行研究时,导出了涡旋玻璃态以及反向热激活跳跃下的S-T关系式,根据这些关系式分别拟合得到了在这一温区内的S-T图像。最后,结合这些图像对这一温区内的磁弛豫现象进行了解释。进一步加深对液体HeII性质的有关规律的认识,可更好的为低温技术以及其它技术(如超导等技术)的进一步发展提供服务,因此本文还对低温液体HeII的特性进行了研究。本文对低温液体HeII进行的研究,主要针对低温液体HeII的自由能、熵、比热与温度的关系进行了深入研究。通过对液体HeII的色散关系进行的拟合,得到了0～1.6K温区内色散关系的统一解析表达式,并进而导出了自由能、熵、比热与温度的关系。在此基础上,进一步对所导出的液体HeII的比热表达式进行计算并进行拟合,发现所导出的比热表达式和实验结果相比存在较小的误差。最后,对这一微小误差产生的原因进行了分析和解释。更多还原

【Abstract】 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.更多还原